Cupertino nav bar transitions between routes (#20322)

examples/flutter_gallery/lib/demo/cupertino/cupertino_refresh_demo.dart

@@ -55,7 +55,11 @@ class _CupertinoRefreshControlDemoState extends State<CupertinoRefreshControlDem
               new CupertinoSliverRefreshControl(
                 onRefresh: () {
                   return new Future<void>.delayed(const Duration(seconds: 2))
-                      ..then((_) => setState(() => repopulateList()));
+                      ..then((_) {
+                        if (mounted) {
+                          setState(() => repopulateList());
+                        }
+                      });
                 },
               ),
               new SliverSafeArea(

packages/flutter/lib/src/cupertino/app.dart

@@ -320,13 +320,10 @@ class _AlwaysCupertinoScrollBehavior extends ScrollBehavior {
 }
 
 class _CupertinoAppState extends State<CupertinoApp> {
-  HeroController _heroController;
-  List<NavigatorObserver> _navigatorObservers;
 
   @override
   void initState() {
     super.initState();
-    _heroController = new HeroController(); // Linear tweening.
     _updateNavigator();
   }
 
@@ -342,9 +339,6 @@ class _CupertinoAppState extends State<CupertinoApp> {
                      widget.routes.isNotEmpty ||
                      widget.onGenerateRoute != null ||
                      widget.onUnknownRoute != null;
-    _navigatorObservers =
-        new List<NavigatorObserver>.from(widget.navigatorObservers)
-          ..add(_heroController);
   }
 
   Widget defaultBuilder(BuildContext context, Widget child) {
@@ -361,7 +355,7 @@ class _CupertinoAppState extends State<CupertinoApp> {
         routes: widget.routes,
         onGenerateRoute: widget.onGenerateRoute,
         onUnknownRoute: widget.onUnknownRoute,
-        navigatorObservers: _navigatorObservers,
+        navigatorObservers: widget.navigatorObservers,
       );
       if (widget.builder != null) {
         return widget.builder(context, navigator);

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -2,9 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
 import 'dart:ui' show ImageFilter;
 
 import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
@@ -15,6 +17,8 @@ import 'page_scaffold.dart';
 import 'route.dart';
 
 /// Standard iOS navigation bar height without the status bar.
+///
+/// This height is constant and independent of accessibility as it is in iOS.
 const double _kNavBarPersistentHeight = 44.0;
 
 /// Size increase from expanding the navigation bar into an iOS-11-style large title
@@ -43,6 +47,14 @@ const Border _kDefaultNavBarBorder = Border(
   ),
 );
 
+const TextStyle _kMiddleTitleTextStyle = TextStyle(
+  fontFamily: '.SF UI Text',
+  fontSize: 17.0,
+  fontWeight: FontWeight.w600,
+  letterSpacing: -0.08,
+  color: CupertinoColors.black,
+);
+
 const TextStyle _kLargeTitleTextStyle = TextStyle(
   fontFamily: '.SF Pro Display',
   fontSize: 34.0,
@@ -51,6 +63,78 @@ const TextStyle _kLargeTitleTextStyle = TextStyle(
   color: CupertinoColors.black,
 );
 
+// There's a single tag for all instances of navigation bars because they can
+// all transition between each other (per Navigator) via Hero transitions.
+const _HeroTag _defaultHeroTag = _HeroTag();
+
+class _HeroTag {
+  const _HeroTag();
+  // Let the Hero tag be described in tree dumps.
+  @override
+  String toString() => 'Default Hero tag for Cupertino navigation bars';
+}
+
+TextStyle _navBarItemStyle(Color color) {
+  return new TextStyle(
+    fontFamily: '.SF UI Text',
+    fontSize: 17.0,
+    letterSpacing: -0.24,
+    color: color,
+  );
+}
+
+/// Returns `child` wrapped with background and a bottom border if background color
+/// is opaque. Otherwise, also blur with [BackdropFilter].
+///
+/// When `updateSystemUiOverlay` is true, the nav bar will update the OS
+/// status bar's color theme based on the background color of the nav bar.
+Widget _wrapWithBackground({
+  Border border,
+  Color backgroundColor,
+  Widget child,
+  bool updateSystemUiOverlay = true,
+}) {
+  Widget result = child;
+  if (updateSystemUiOverlay) {
+    final bool darkBackground = backgroundColor.computeLuminance() < 0.179;
+    final SystemUiOverlayStyle overlayStyle = darkBackground
+        ? SystemUiOverlayStyle.light
+        : SystemUiOverlayStyle.dark;
+    result = new AnnotatedRegion<SystemUiOverlayStyle>(
+      value: overlayStyle,
+      sized: true,
+      child: result,
+    );
+  }
+  final DecoratedBox childWithBackground = new DecoratedBox(
+    decoration: new BoxDecoration(
+      border: border,
+      color: backgroundColor,
+    ),
+    child: result,
+  );
+
+  if (backgroundColor.alpha == 0xFF)
+    return childWithBackground;
+
+  return new ClipRect(
+    child: new BackdropFilter(
+      filter: new ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),
+      child: childWithBackground,
+    ),
+  );
+}
+
+// Whether the current route supports nav bar hero transitions from or to.
+bool _isTransitionable(BuildContext context) {
+  final ModalRoute<dynamic> route = ModalRoute.of(context);
+
+  // Fullscreen dialogs never transitions their nav bar with other push-style
+  // pages' nav bars or with other fullscreen dialog pages on the way in or on
+  // the way out.
+  return route is PageRoute && !route.fullscreenDialog;
+}
+
 /// An iOS-styled navigation bar.
 ///
 /// The navigation bar is a toolbar that minimally consists of a widget, normally
@@ -73,13 +157,20 @@ const TextStyle _kLargeTitleTextStyle = TextStyle(
 /// If the given [backgroundColor]'s opacity is not 1.0 (which is the case by
 /// default), it will produce a blurring effect to the content behind it.
 ///
+/// When [transitionBetweenRoutes] is true, this navigation bar will transition
+/// on top of the routes instead of inside it if the route being transitioned
+/// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
+/// with [transitionBetweenRoutes] set to true. If [transitionBetweenRoutes] is
+/// true, none of the [Widget] parameters can contain a key in its subtree since
+/// that widget will exist in multiple places in the tree simultaneously.
+///
 /// See also:
 ///
 ///  * [CupertinoPageScaffold], a page layout helper typically hosting the
 ///    [CupertinoNavigationBar].
 ///  * [CupertinoSliverNavigationBar] for a navigation bar to be placed in a
 ///    scrolling list and that supports iOS-11-style large titles.
-class CupertinoNavigationBar extends StatelessWidget implements ObstructingPreferredSizeWidget {
+class CupertinoNavigationBar extends StatefulWidget implements ObstructingPreferredSizeWidget {
   /// Creates a navigation bar in the iOS style.
   const CupertinoNavigationBar({
     Key key,
@@ -93,8 +184,21 @@ class CupertinoNavigationBar extends StatelessWidget implements ObstructingPrefe
     this.backgroundColor = _kDefaultNavBarBackgroundColor,
     this.padding,
     this.actionsForegroundColor = CupertinoColors.activeBlue,
+    this.transitionBetweenRoutes = true,
+    this.heroTag = _defaultHeroTag,
   }) : assert(automaticallyImplyLeading != null),
        assert(automaticallyImplyMiddle != null),
+       assert(transitionBetweenRoutes != null),
+       assert(
+         heroTag != null,
+         'heroTag cannot be null. Use transitionBetweenRoutes = false to '
+         'disable Hero transition on this navigation bar.'
+       ),
+       assert(
+         !transitionBetweenRoutes || identical(heroTag, _defaultHeroTag),
+         'Cannot specify a heroTag override if this navigation bar does not '
+         'transition due to transitionBetweenRoutes = false.'
+       ),
        super(key: key);
 
   /// {@template flutter.cupertino.navBar.leading}
@@ -199,6 +303,34 @@ class CupertinoNavigationBar extends StatelessWidget implements ObstructingPrefe
   /// iOS standard design.
   final Color actionsForegroundColor;
 
+  /// {@template flutter.cupertino.navBar.transitionBetweenRoutes}
+  /// Whether to transition between navigation bars.
+  ///
+  /// When [transitionBetweenRoutes] is true, this navigation bar will transition
+  /// on top of the routes instead of inside it if the route being transitioned
+  /// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
+  /// with [transitionBetweenRoutes] set to true.
+  ///
+  /// When set to true, only one navigation bar can be present per route unless
+  /// [heroTag] is also set.
+  ///
+  /// This value defaults to true and cannot be null.
+  /// {@endtemplate}
+  final bool transitionBetweenRoutes;
+
+  /// {@template flutter.cupertino.navBar.heroTag}
+  /// Tag for the navigation bar's Hero widget if [transitionBetweenRoutes] is true.
+  ///
+  /// Defaults to a common tag between all [CupertinoNavigationBar] and
+  /// [CupertinoSliverNavigationBar] instances so they can all transition
+  /// between each other as long as there's only one per route. Use this tag
+  /// override with different tags to have multiple navigation bars per route.
+  ///
+  /// Cannot be null. To disable Hero transitions for this navigation bar,
+  /// set [transitionBetweenRoutes] to false.
+  /// {@endtemplate}
+  final Object heroTag;
+
   /// True if the navigation bar's background color has no transparency.
   @override
   bool get fullObstruction => backgroundColor.alpha == 0xFF;
@@ -208,25 +340,67 @@ class CupertinoNavigationBar extends StatelessWidget implements ObstructingPrefe
     return const Size.fromHeight(_kNavBarPersistentHeight);
   }
 
+  @override
+  _CupertinoNavigationBarState createState() {
+    return new _CupertinoNavigationBarState();
+  }
+}
+
+// A state class exists for the nav bar so that the keys of its sub-components
+// don't change when rebuilding the nav bar, causing the sub-components to
+// lose their own states.
+class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
+  _NavigationBarStaticComponentsKeys keys;
+
+  @override
+  void initState() {
+    super.initState();
+    keys = new _NavigationBarStaticComponentsKeys();
+  }
+
   @override
   Widget build(BuildContext context) {
-    final Widget effectiveMiddle = _effectiveTitle(
-      title: middle,
-      automaticallyImplyTitle: automaticallyImplyMiddle,
-      currentRoute: ModalRoute.of(context),
+    final _NavigationBarStaticComponents components = new _NavigationBarStaticComponents(
+      keys: keys,
+      route: ModalRoute.of(context),
+      userLeading: widget.leading,
+      automaticallyImplyLeading: widget.automaticallyImplyLeading,
+      automaticallyImplyTitle: widget.automaticallyImplyMiddle,
+      previousPageTitle: widget.previousPageTitle,
+      userMiddle: widget.middle,
+      userTrailing: widget.trailing,
+      padding: widget.padding,
+      actionsForegroundColor: widget.actionsForegroundColor,
+      userLargeTitle: null,
+      large: false,
     );
 
-    return _wrapWithBackground(
-      border: border,
-      backgroundColor: backgroundColor,
-      child: new _CupertinoPersistentNavigationBar(
-        leading: leading,
-        automaticallyImplyLeading: automaticallyImplyLeading,
-        previousPageTitle: previousPageTitle,
-        middle: effectiveMiddle,
-        trailing: trailing,
-        padding: padding,
-        actionsForegroundColor: actionsForegroundColor,
+    final Widget navBar = _wrapWithBackground(
+      border: widget.border,
+      backgroundColor: widget.backgroundColor,
+      child: new _PersistentNavigationBar(
+        components: components,
+        padding: widget.padding,
+      ),
+    );
+
+    if (!widget.transitionBetweenRoutes || !_isTransitionable(context)) {
+      return navBar;
+    }
+
+    return new Hero(
+      tag: widget.heroTag,
+      createRectTween: _linearTranslateWithLargestRectSizeTween,
+      placeholderBuilder: _navBarHeroLaunchPadBuilder,
+      flightShuttleBuilder: _navBarHeroFlightShuttleBuilder,
+      child: new _TransitionableNavigationBar(
+        componentsKeys: keys,
+        backgroundColor: widget.backgroundColor,
+        actionsForegroundColor: widget.actionsForegroundColor,
+        border: widget.border,
+        hasUserMiddle: widget.middle != null,
+        largeExpanded: false,
+        child: navBar,
       ),
     );
   }
@@ -261,11 +435,19 @@ class CupertinoNavigationBar extends StatelessWidget implements ObstructingPrefe
 /// route if none is provided and [automaticallyImplyTitle] is true (true by
 /// default).
 ///
+/// When [transitionBetweenRoutes] is true, this navigation bar will transition
+/// on top of the routes instead of inside it if the route being transitioned
+/// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
+/// with [transitionBetweenRoutes] set to true. If [transitionBetweenRoutes] is
+/// true, none of the [Widget] parameters can contain any [GlobalKey]s in their
+/// subtrees since those widgets will exist in multiple places in the tree
+/// simultaneously.
+///
 /// See also:
 ///
 ///  * [CupertinoNavigationBar], an iOS navigation bar for use on non-scrolling
 ///    pages.
-class CupertinoSliverNavigationBar extends StatelessWidget {
+class CupertinoSliverNavigationBar extends StatefulWidget {
   /// Creates a navigation bar for scrolling lists.
   ///
   /// The [largeTitle] argument is required and must not be null.
@@ -282,8 +464,16 @@ class CupertinoSliverNavigationBar extends StatelessWidget {
     this.backgroundColor = _kDefaultNavBarBackgroundColor,
     this.padding,
     this.actionsForegroundColor = CupertinoColors.activeBlue,
+    this.transitionBetweenRoutes = true,
+    this.heroTag = _defaultHeroTag,
   }) : assert(automaticallyImplyLeading != null),
        assert(automaticallyImplyTitle != null),
+       assert(
+         automaticallyImplyTitle == true || largeTitle != null,
+         'No largeTitle has been provided but automaticallyImplyTitle is also '
+         'false. Either provide a largeTitle or set automaticallyImplyTitle to '
+         'true.'
+       ),
        super(key: key);
 
   /// The navigation bar's title.
@@ -304,6 +494,9 @@ class CupertinoSliverNavigationBar extends StatelessWidget {
   /// If null and [automaticallyImplyTitle] is true, an appropriate [Text]
   /// title will be created if the current route is a [CupertinoPageRoute] and
   /// has a `title`.
+  ///
+  /// This parameter must either be non-null or the route must have a title
+  /// ([CupertinoPageRoute.title]) and [automaticallyImplyTitle] must be true.
   final Widget largeTitle;
 
   /// {@macro flutter.cupertino.navBar.leading}
@@ -355,73 +548,96 @@ class CupertinoSliverNavigationBar extends StatelessWidget {
   /// iOS standard design.
   final Color actionsForegroundColor;
 
+  /// {@macro flutter.cupertino.navBar.transitionBetweenRoutes}
+  final bool transitionBetweenRoutes;
+
+  /// {@macro flutter.cupertino.navBar.heroTag}
+  final Object heroTag;
+
   /// True if the navigation bar's background color has no transparency.
   bool get opaque => backgroundColor.alpha == 0xFF;
 
+  @override
+  _CupertinoSliverNavigationBarState createState() => new _CupertinoSliverNavigationBarState();
+}
+
+// A state class exists for the nav bar so that the keys of its sub-components
+// don't change when rebuilding the nav bar, causing the sub-components to
+// lose their own states.
+class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigationBar> {
+  _NavigationBarStaticComponentsKeys keys;
+
+  @override
+  void initState() {
+    super.initState();
+    keys = new _NavigationBarStaticComponentsKeys();
+  }
+
   @override
   Widget build(BuildContext context) {
-    final Widget effectiveTitle = _effectiveTitle(
-      title: largeTitle,
-      automaticallyImplyTitle: automaticallyImplyTitle,
-      currentRoute: ModalRoute.of(context),
+    final _NavigationBarStaticComponents components = new _NavigationBarStaticComponents(
+      keys: keys,
+      route: ModalRoute.of(context),
+      userLeading: widget.leading,
+      automaticallyImplyLeading: widget.automaticallyImplyLeading,
+      automaticallyImplyTitle: widget.automaticallyImplyTitle,
+      previousPageTitle: widget.previousPageTitle,
+      userMiddle: widget.middle,
+      userTrailing: widget.trailing,
+      userLargeTitle: widget.largeTitle,
+      padding: widget.padding,
+      actionsForegroundColor: widget.actionsForegroundColor,
+      large: true,
     );
 
     return new SliverPersistentHeader(
       pinned: true, // iOS navigation bars are always pinned.
-      delegate: new _CupertinoLargeTitleNavigationBarSliverDelegate(
+      delegate: new _LargeTitleNavigationBarSliverDelegate(
+        keys: keys,
+        components: components,
+        userMiddle: widget.middle,
+        backgroundColor: widget.backgroundColor,
+        border: widget.border,
+        padding: widget.padding,
+        actionsForegroundColor: widget.actionsForegroundColor,
+        transitionBetweenRoutes: widget.transitionBetweenRoutes,
+        heroTag: widget.heroTag,
         persistentHeight: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
-        largeTitle: effectiveTitle,
-        leading: leading,
-        automaticallyImplyLeading: automaticallyImplyLeading,
-        previousPageTitle: previousPageTitle,
-        middle: middle,
-        trailing: trailing,
-        padding: padding,
-        border: border,
-        backgroundColor: backgroundColor,
-        actionsForegroundColor: actionsForegroundColor,
+        alwaysShowMiddle: widget.middle != null,
       ),
     );
   }
 }
 
-class _CupertinoLargeTitleNavigationBarSliverDelegate
+class _LargeTitleNavigationBarSliverDelegate
     extends SliverPersistentHeaderDelegate with DiagnosticableTreeMixin {
-  _CupertinoLargeTitleNavigationBarSliverDelegate({
+  _LargeTitleNavigationBarSliverDelegate({
+    @required this.keys,
+    @required this.components,
+    @required this.userMiddle,
+    @required this.backgroundColor,
+    @required this.border,
+    @required this.padding,
+    @required this.actionsForegroundColor,
+    @required this.transitionBetweenRoutes,
+    @required this.heroTag,
     @required this.persistentHeight,
-    @required this.largeTitle,
-    this.leading,
-    this.automaticallyImplyLeading,
-    this.previousPageTitle,
-    this.middle,
-    this.trailing,
-    this.padding,
-    this.border,
-    this.backgroundColor,
-    this.actionsForegroundColor,
-  }) : assert(persistentHeight != null);
-
-  final double persistentHeight;
-
-  final Widget largeTitle;
-
-  final Widget leading;
-
-  final bool automaticallyImplyLeading;
-
-  final String previousPageTitle;
-
-  final Widget middle;
-
-  final Widget trailing;
-
-  final EdgeInsetsDirectional padding;
-
+    @required this.alwaysShowMiddle,
+  }) : assert(persistentHeight != null),
+       assert(alwaysShowMiddle != null),
+       assert(transitionBetweenRoutes != null);
+
+  final _NavigationBarStaticComponentsKeys keys;
+  final _NavigationBarStaticComponents components;
+  final Widget userMiddle;
   final Color backgroundColor;
-
   final Border border;
-
+  final EdgeInsetsDirectional padding;
   final Color actionsForegroundColor;
+  final bool transitionBetweenRoutes;
+  final Object heroTag;
+  final double persistentHeight;
+  final bool alwaysShowMiddle;
 
   @override
   double get minExtent => persistentHeight;
@@ -433,21 +649,16 @@ class _CupertinoLargeTitleNavigationBarSliverDelegate
   Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
     final bool showLargeTitle = shrinkOffset < maxExtent - minExtent - _kNavBarShowLargeTitleThreshold;
 
-    final _CupertinoPersistentNavigationBar persistentNavigationBar =
-        new _CupertinoPersistentNavigationBar(
-      leading: leading,
-      automaticallyImplyLeading: automaticallyImplyLeading,
-      previousPageTitle: previousPageTitle,
-      middle: middle ?? largeTitle,
-      trailing: trailing,
-      // If middle widget exists, always show it. Otherwise, show title
-      // when collapsed.
-      middleVisible: middle != null ? null : !showLargeTitle,
+    final _PersistentNavigationBar persistentNavigationBar =
+        new _PersistentNavigationBar(
+      components: components,
       padding: padding,
-      actionsForegroundColor: actionsForegroundColor,
+      // If a user specified middle exists, always show it. Otherwise, show
+      // title when sliver is collapsed.
+      middleVisible: alwaysShowMiddle ? null : !showLargeTitle,
     );
 
-    return _wrapWithBackground(
+    final Widget navBar = _wrapWithBackground(
       border: border,
       backgroundColor: backgroundColor,
       child: new Stack(
@@ -471,19 +682,19 @@ class _CupertinoLargeTitleNavigationBarSliverDelegate
                     start: _kNavBarEdgePadding,
                     bottom: 8.0, // Bottom has a different padding.
                   ),
-                  child: new DefaultTextStyle(
-                    style: _kLargeTitleTextStyle,
-                    maxLines: 1,
-                    overflow: TextOverflow.ellipsis,
+                  child: new SafeArea(
+                    top: false,
+                    bottom: false,
                     child: new AnimatedOpacity(
                       opacity: showLargeTitle ? 1.0 : 0.0,
                       duration: _kNavBarTitleFadeDuration,
-                      child: new SafeArea(
-                        top: false,
-                        bottom: false,
-                        child: new Semantics(
-                          header: true,
-                          child: largeTitle,
+                      child: new Semantics(
+                        header: true,
+                        child: new DefaultTextStyle(
+                          style: _kLargeTitleTextStyle,
+                          maxLines: 1,
+                          overflow: TextOverflow.ellipsis,
+                          child: components.largeTitle,
                         ),
                       ),
                     ),
@@ -501,70 +712,44 @@ class _CupertinoLargeTitleNavigationBarSliverDelegate
         ],
       ),
     );
+
+    if (!transitionBetweenRoutes || !_isTransitionable(context)) {
+      return navBar;
+    }
+
+    return new Hero(
+      tag: heroTag,
+      createRectTween: _linearTranslateWithLargestRectSizeTween,
+      flightShuttleBuilder: _navBarHeroFlightShuttleBuilder,
+      placeholderBuilder: _navBarHeroLaunchPadBuilder,
+      // This is all the way down here instead of being at the top level of
+      // CupertinoSliverNavigationBar like CupertinoNavigationBar because it
+      // needs to wrap the top level RenderBox rather than a RenderSliver.
+      child: new _TransitionableNavigationBar(
+        componentsKeys: keys,
+        backgroundColor: backgroundColor,
+        actionsForegroundColor: actionsForegroundColor,
+        border: border,
+        hasUserMiddle: userMiddle != null,
+        largeExpanded: showLargeTitle,
+        child: navBar,
+      ),
+    );
   }
 
   @override
-  bool shouldRebuild(_CupertinoLargeTitleNavigationBarSliverDelegate oldDelegate) {
-    return persistentHeight != oldDelegate.persistentHeight
-        || largeTitle != oldDelegate.largeTitle
-        || leading != oldDelegate.leading
-        || middle != oldDelegate.middle
-        || trailing != oldDelegate.trailing
-        || border != oldDelegate.border
+  bool shouldRebuild(_LargeTitleNavigationBarSliverDelegate oldDelegate) {
+       return components != oldDelegate.components
+        || userMiddle != oldDelegate.userMiddle
         || backgroundColor != oldDelegate.backgroundColor
-        || actionsForegroundColor != oldDelegate.actionsForegroundColor;
-  }
-}
-
-/// Returns `child` wrapped with background and a bottom border if background color
-/// is opaque. Otherwise, also blur with [BackdropFilter].
-Widget _wrapWithBackground({
-  Border border,
-  Color backgroundColor,
-  Widget child,
-}) {
-
-  final bool darkBackground = backgroundColor.computeLuminance() < 0.179;
-  final SystemUiOverlayStyle overlayStyle = darkBackground
-      ? SystemUiOverlayStyle.light
-      : SystemUiOverlayStyle.dark;
-  final DecoratedBox childWithBackground = new DecoratedBox(
-    decoration: new BoxDecoration(
-      border: border,
-      color: backgroundColor,
-    ),
-    child: new AnnotatedRegion<SystemUiOverlayStyle>(
-      value: overlayStyle,
-      sized: true,
-      child: child,
-    ),
-  );
-
-  if (backgroundColor.alpha == 0xFF)
-    return childWithBackground;
-
-  return new ClipRect(
-    child: new BackdropFilter(
-      filter: new ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),
-      child: childWithBackground,
-    ),
-  );
-}
-
-Widget _effectiveTitle({
-  Widget title,
-  bool automaticallyImplyTitle,
-  ModalRoute<dynamic> currentRoute,
-}) {
-  // Auto use the CupertinoPageRoute's title if middle not provided.
-  if (title == null &&
-      automaticallyImplyTitle &&
-      currentRoute is CupertinoPageRoute &&
-      currentRoute.title != null) {
-    return new Text(currentRoute.title);
+        || border != oldDelegate.border
+        || padding != oldDelegate.padding
+        || actionsForegroundColor != oldDelegate.actionsForegroundColor
+        || transitionBetweenRoutes != oldDelegate.transitionBetweenRoutes
+        || persistentHeight != oldDelegate.persistentHeight
+        || alwaysShowMiddle != oldDelegate.alwaysShowMiddle
+        || heroTag != oldDelegate.heroTag;
   }
-
-  return title;
 }
 
 /// The top part of the navigation bar that's never scrolled away.
@@ -572,123 +757,57 @@ Widget _effectiveTitle({
 /// Consists of the entire navigation bar without background and border when used
 /// without large titles. With large titles, it's the top static half that
 /// doesn't scroll.
-class _CupertinoPersistentNavigationBar extends StatelessWidget implements PreferredSizeWidget {
-  const _CupertinoPersistentNavigationBar({
+class _PersistentNavigationBar extends StatelessWidget {
+  const _PersistentNavigationBar({
     Key key,
-    this.leading,
-    this.automaticallyImplyLeading,
-    this.previousPageTitle,
-    this.middle,
-    this.trailing,
+    this.components,
     this.padding,
-    this.actionsForegroundColor,
     this.middleVisible,
   }) : super(key: key);
 
-  final Widget leading;
-
-  final bool automaticallyImplyLeading;
-
-  final String previousPageTitle;
-
-  final Widget middle;
-
-  final Widget trailing;
+  final _NavigationBarStaticComponents components;
 
   final EdgeInsetsDirectional padding;
-
-  final Color actionsForegroundColor;
-
   /// Whether the middle widget has a visible animated opacity. A null value
   /// means the middle opacity will not be animated.
   final bool middleVisible;
 
-  @override
-  Size get preferredSize => const Size.fromHeight(_kNavBarPersistentHeight);
-
   @override
   Widget build(BuildContext context) {
-    final TextStyle actionsStyle = new TextStyle(
-      fontFamily: '.SF UI Text',
-      fontSize: 17.0,
-      letterSpacing: -0.24,
-      color: actionsForegroundColor,
-    );
-
-    final Widget styledLeading = leading == null
-        ? null
-        : new Padding(
-          padding: new EdgeInsetsDirectional.only(
-            start: padding?.start ?? _kNavBarEdgePadding,
-          ),
-          child: new DefaultTextStyle(
-            style: actionsStyle,
-            child: leading,
-          ),
-        );
+    Widget middle = components.middle;
 
-    final Widget styledTrailing = trailing == null
-        ? null
-        : Padding(
-          padding: new EdgeInsetsDirectional.only(
-            end: padding?.end ?? _kNavBarEdgePadding,
-          ),
-          child: new DefaultTextStyle(
-            style: actionsStyle,
-            child: trailing,
-          ),
+    if (middle != null) {
+      middle = new DefaultTextStyle(
+        style: _kMiddleTitleTextStyle,
+        child: new Semantics(header: true, child: middle),
+      );
+      // When the middle's visibility can change on the fly like with large title
+      // slivers, wrap with animated opacity.
+      middle = middleVisible == null
+        ? middle
+        : new AnimatedOpacity(
+          opacity: middleVisible ? 1.0 : 0.0,
+          duration: _kNavBarTitleFadeDuration,
+          child: middle,
         );
+    }
 
-    // Let the middle be black rather than `actionsForegroundColor` in case
-    // it's a plain text title.
-    final Widget styledMiddle = middle == null
-        ? null
-        : new DefaultTextStyle(
-          style: actionsStyle.copyWith(
-            fontWeight: FontWeight.w600,
-            letterSpacing: -0.08,
-            color: CupertinoColors.black,
-          ),
-          child: new Semantics(child: middle, header: true),
-        );
+    Widget leading = components.leading;
+    final Widget backChevron = components.backChevron;
+    final Widget backLabel = components.backLabel;
 
-    final Widget animatedStyledMiddle = middleVisible == null
-      ? styledMiddle
-      : new AnimatedOpacity(
-        opacity: middleVisible ? 1.0 : 0.0,
-        duration: _kNavBarTitleFadeDuration,
-        child: styledMiddle,
+    if (leading == null && backChevron != null && backLabel != null) {
+      leading = new CupertinoNavigationBarBackButton._assemble(
+        backChevron,
+        backLabel,
+        components.actionsForegroundColor,
       );
-
-    // Auto add back button if leading not provided.
-    Widget backOrCloseButton;
-    if (styledLeading == null && automaticallyImplyLeading) {
-      final ModalRoute<dynamic> currentRoute = ModalRoute.of(context);
-      if (currentRoute?.canPop == true) {
-        if (currentRoute is PageRoute && currentRoute?.fullscreenDialog == true) {
-          backOrCloseButton = new CupertinoButton(
-            child: const Padding(
-              padding: EdgeInsetsDirectional.only(
-                start: _kNavBarEdgePadding,
-              ),
-              child: Text('Close'),
-            ),
-            padding: EdgeInsets.zero,
-            onPressed: () { Navigator.maybePop(context); },
-          );
-        } else {
-          backOrCloseButton = new CupertinoNavigationBarBackButton(
-            color: actionsForegroundColor,
-            previousPageTitle: previousPageTitle,
-          );
-        }
-      }
     }
 
     Widget paddedToolbar = new NavigationToolbar(
-      leading: styledLeading ?? backOrCloseButton,
-      middle: animatedStyledMiddle,
-      trailing: styledTrailing,
+      leading: leading,
+      middle: middle,
+      trailing: components.trailing,
       centerMiddle: true,
       middleSpacing: 6.0,
     );
@@ -713,6 +832,303 @@ class _CupertinoPersistentNavigationBar extends StatelessWidget implements Prefe
   }
 }
 
+// A collection of keys always used when building static routes' nav bars's
+// components with _NavigationBarStaticComponents and read in
+// _NavigationBarTransition in Hero flights in order to reference the components'
+// RenderBoxes for their positions.
+//
+// These keys should never re-appear inside the Hero flights.
+@immutable
+class _NavigationBarStaticComponentsKeys {
+  _NavigationBarStaticComponentsKeys()
+      : navBarBoxKey = new GlobalKey(debugLabel: 'Navigation bar render box'),
+        leadingKey = new GlobalKey(debugLabel: 'Leading'),
+        backChevronKey = new GlobalKey(debugLabel: 'Back chevron'),
+        backLabelKey = new GlobalKey(debugLabel: 'Back label'),
+        middleKey = new GlobalKey(debugLabel: 'Middle'),
+        trailingKey = new GlobalKey(debugLabel: 'Trailing'),
+        largeTitleKey = new GlobalKey(debugLabel: 'Large title');
+
+  final GlobalKey navBarBoxKey;
+  final GlobalKey leadingKey;
+  final GlobalKey backChevronKey;
+  final GlobalKey backLabelKey;
+  final GlobalKey middleKey;
+  final GlobalKey trailingKey;
+  final GlobalKey largeTitleKey;
+}
+
+// Based on various user Widgets and other parameters, construct KeyedSubtree
+// components that are used in common by the CupertinoNavigationBar and
+// CupertinoSliverNavigationBar. The KeyedSubtrees are inserted into static
+// routes and the KeyedSubtrees' child are reused in the Hero flights.
+@immutable
+class _NavigationBarStaticComponents {
+  _NavigationBarStaticComponents({
+    @required _NavigationBarStaticComponentsKeys keys,
+    @required ModalRoute<dynamic> route,
+    @required Widget userLeading,
+    @required bool automaticallyImplyLeading,
+    @required bool automaticallyImplyTitle,
+    @required String previousPageTitle,
+    @required Widget userMiddle,
+    @required Widget userTrailing,
+    @required Widget userLargeTitle,
+    @required EdgeInsetsDirectional padding,
+    @required this.actionsForegroundColor,
+    @required bool large,
+  }) : leading = createLeading(
+         leadingKey: keys.leadingKey,
+         userLeading: userLeading,
+         route: route,
+         automaticallyImplyLeading: automaticallyImplyLeading,
+         padding: padding,
+         actionsForegroundColor: actionsForegroundColor,
+       ),
+       backChevron = createBackChevron(
+         backChevronKey: keys.backChevronKey,
+         userLeading: userLeading,
+         route: route,
+         automaticallyImplyLeading: automaticallyImplyLeading,
+       ),
+       backLabel = createBackLabel(
+         backLabelKey: keys.backLabelKey,
+         userLeading: userLeading,
+         route: route,
+         previousPageTitle: previousPageTitle,
+         automaticallyImplyLeading: automaticallyImplyLeading,
+       ),
+       middle = createMiddle(
+         middleKey: keys.middleKey,
+         userMiddle: userMiddle,
+         userLargeTitle: userLargeTitle,
+         route: route,
+         automaticallyImplyTitle: automaticallyImplyTitle,
+         large: large,
+       ),
+       trailing = createTrailing(
+         trailingKey: keys.trailingKey,
+         userTrailing: userTrailing,
+         padding: padding,
+         actionsForegroundColor: actionsForegroundColor,
+       ),
+       largeTitle = createLargeTitle(
+         largeTitleKey: keys.largeTitleKey,
+         userLargeTitle: userLargeTitle,
+         route: route,
+         automaticImplyTitle: automaticallyImplyTitle,
+         large: large,
+       );
+
+  static Widget _derivedTitle({
+    bool automaticallyImplyTitle,
+    ModalRoute<dynamic> currentRoute,
+  }) {
+    // Auto use the CupertinoPageRoute's title if middle not provided.
+    if (automaticallyImplyTitle &&
+        currentRoute is CupertinoPageRoute &&
+        currentRoute.title != null) {
+      return new Text(currentRoute.title);
+    }
+
+    return null;
+  }
+
+  final Color actionsForegroundColor;
+
+  final KeyedSubtree leading;
+  static KeyedSubtree createLeading({
+    @required GlobalKey leadingKey,
+    @required Widget userLeading,
+    @required ModalRoute<dynamic> route,
+    @required bool automaticallyImplyLeading,
+    @required EdgeInsetsDirectional padding,
+    @required Color actionsForegroundColor
+  }) {
+    Widget leadingContent;
+
+    if (userLeading != null) {
+      leadingContent = userLeading;
+    } else if (
+      automaticallyImplyLeading &&
+      route is PageRoute &&
+      route.canPop &&
+      route.fullscreenDialog
+    ) {
+      leadingContent = new CupertinoButton(
+        child: const Text('Close'),
+        padding: EdgeInsets.zero,
+        onPressed: () { route.navigator.maybePop(); },
+      );
+    }
+
+    if (leadingContent == null) {
+      return null;
+    }
+
+    return new KeyedSubtree(
+      key: leadingKey,
+      child: new Padding(
+        padding: new EdgeInsetsDirectional.only(
+          start: padding?.start ?? _kNavBarEdgePadding,
+        ),
+        child: new DefaultTextStyle(
+          style: _navBarItemStyle(actionsForegroundColor),
+          child: IconTheme.merge(
+            data: new IconThemeData(
+              color: actionsForegroundColor,
+              size: 32.0,
+            ),
+            child: leadingContent,
+          ),
+        ),
+      ),
+    );
+  }
+
+  final KeyedSubtree backChevron;
+  static KeyedSubtree createBackChevron({
+    @required GlobalKey backChevronKey,
+    @required Widget userLeading,
+    @required ModalRoute<dynamic> route,
+    @required bool automaticallyImplyLeading,
+  }) {
+    if (
+      userLeading != null ||
+      !automaticallyImplyLeading ||
+      route == null ||
+      !route.canPop ||
+      (route is PageRoute && route.fullscreenDialog)
+    ) {
+      return null;
+    }
+
+    return new KeyedSubtree(key: backChevronKey, child: const _BackChevron());
+  }
+
+  /// This widget is not decorated with a font since the font style could
+  /// animate during transitions.
+  final KeyedSubtree backLabel;
+  static KeyedSubtree createBackLabel({
+    @required GlobalKey backLabelKey,
+    @required Widget userLeading,
+    @required ModalRoute<dynamic> route,
+    @required bool automaticallyImplyLeading,
+    @required String previousPageTitle,
+  }) {
+    if (
+      userLeading != null ||
+      !automaticallyImplyLeading ||
+      route == null ||
+      !route.canPop ||
+      (route is PageRoute && route.fullscreenDialog)
+    ) {
+      return null;
+    }
+
+    return new KeyedSubtree(
+      key: backLabelKey,
+      child: new _BackLabel(
+        specifiedPreviousTitle: previousPageTitle,
+        route: route,
+      ),
+    );
+  }
+
+  /// This widget is not decorated with a font since the font style could
+  /// animate during transitions.
+  final KeyedSubtree middle;
+  static KeyedSubtree createMiddle({
+    @required GlobalKey middleKey,
+    @required Widget userMiddle,
+    @required Widget userLargeTitle,
+    @required bool large,
+    @required bool automaticallyImplyTitle,
+    @required ModalRoute<dynamic> route,
+  }) {
+    Widget middleContent = userMiddle;
+
+    if (large) {
+      middleContent ??= userLargeTitle;
+    }
+
+    middleContent ??= _derivedTitle(
+      automaticallyImplyTitle: automaticallyImplyTitle,
+      currentRoute: route,
+    );
+
+    if (middleContent == null) {
+      return null;
+    }
+
+    return new KeyedSubtree(
+      key: middleKey,
+      child: middleContent,
+    );
+  }
+
+  final KeyedSubtree trailing;
+  static KeyedSubtree createTrailing({
+    @required GlobalKey trailingKey,
+    @required Widget userTrailing,
+    @required EdgeInsetsDirectional padding,
+    @required Color actionsForegroundColor,
+  }) {
+    if (userTrailing == null) {
+      return null;
+    }
+
+    return new KeyedSubtree(
+      key: trailingKey,
+      child: new Padding(
+        padding: new EdgeInsetsDirectional.only(
+          end: padding?.end ?? _kNavBarEdgePadding,
+        ),
+        child: new DefaultTextStyle(
+          style: _navBarItemStyle(actionsForegroundColor),
+          child: IconTheme.merge(
+            data: new IconThemeData(
+              color: actionsForegroundColor,
+              size: 32.0,
+            ),
+            child: userTrailing,
+          ),
+        ),
+      ),
+    );
+  }
+
+  /// This widget is not decorated with a font since the font style could
+  /// animate during transitions.
+  final KeyedSubtree largeTitle;
+  static KeyedSubtree createLargeTitle({
+    @required GlobalKey largeTitleKey,
+    @required Widget userLargeTitle,
+    @required bool large,
+    @required bool automaticImplyTitle,
+    @required ModalRoute<dynamic> route,
+  }) {
+    if (!large) {
+      return null;
+    }
+
+    final Widget largeTitleContent = userLargeTitle ?? _derivedTitle(
+      automaticallyImplyTitle: automaticImplyTitle,
+      currentRoute: route,
+    );
+
+    assert(
+      largeTitleContent != null,
+      'largeTitle was not provided and there was no title from the route.',
+    );
+
+    return new KeyedSubtree(
+      key: largeTitleKey,
+      child: largeTitleContent,
+    );
+  }
+}
+
 /// A nav bar back button typically used in [CupertinoNavigationBar].
 ///
 /// This is automatically inserted into [CupertinoNavigationBar] and
@@ -730,9 +1146,20 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   const CupertinoNavigationBarBackButton({
     @required this.color,
     this.previousPageTitle,
-  }) : assert(color != null);
-
-  /// The [Color] of the back chevron.
+  }) : _backChevron = null,
+       _backLabel = null,
+       assert(color != null);
+
+  // Allow the back chevron and label to be separately created (and keyed)
+  // because they animate separately during page transitions.
+  const CupertinoNavigationBarBackButton._assemble(
+    this._backChevron,
+    this._backLabel,
+    this.color,
+  ) : previousPageTitle = null,
+      assert(color != null);
+
+  /// The [Color] of the back button.
   ///
   /// Must not be null.
   final Color color;
@@ -742,6 +1169,10 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   /// previous routes are both [CupertinoPageRoute]s.
   final String previousPageTitle;
 
+  final Widget _backChevron;
+
+  final Widget _backLabel;
+
   @override
   Widget build(BuildContext context) {
     final ModalRoute<dynamic> currentRoute = ModalRoute.of(context);
@@ -758,20 +1189,23 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
         button: true,
         child: ConstrainedBox(
           constraints: const BoxConstraints(minWidth: _kNavBarBackButtonTapWidth),
-          child: new Row(
-            mainAxisSize: MainAxisSize.min,
-            mainAxisAlignment: MainAxisAlignment.start,
-            children: <Widget>[
-              const Padding(padding: EdgeInsetsDirectional.only(start: 8.0)),
-              new _BackChevron(color: color),
-              const Padding(padding: EdgeInsetsDirectional.only(start: 6.0)),
-              new Flexible(
-                child: new _BackLabel(
-                  specifiedPreviousTitle: previousPageTitle,
-                  route: currentRoute,
+          child: new DefaultTextStyle(
+            style: _navBarItemStyle(color),
+            child: new Row(
+              mainAxisSize: MainAxisSize.min,
+              mainAxisAlignment: MainAxisAlignment.start,
+              children: <Widget>[
+                const Padding(padding: EdgeInsetsDirectional.only(start: 8.0)),
+                _backChevron ?? const _BackChevron(),
+                const Padding(padding: EdgeInsetsDirectional.only(start: 6.0)),
+                new Flexible(
+                  child: _backLabel ?? new _BackLabel(
+                    specifiedPreviousTitle: previousPageTitle,
+                    route: currentRoute,
+                  ),
                 ),
-              ),
-            ],
+              ],
+            ),
           ),
         ),
       ),
@@ -781,16 +1215,14 @@ class CupertinoNavigationBarBackButton extends StatelessWidget {
   }
 }
 
-class _BackChevron extends StatelessWidget {
-  const _BackChevron({
-    @required this.color,
-  }) : assert(color != null);
 
-  final Color color;
+class _BackChevron extends StatelessWidget {
+  const _BackChevron({ Key key }) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
     final TextDirection textDirection = Directionality.of(context);
+    final TextStyle textStyle = DefaultTextStyle.of(context).style;
 
     // Replicate the Icon logic here to get a tightly sized icon and add
     // custom non-square padding.
@@ -799,7 +1231,7 @@ class _BackChevron extends StatelessWidget {
         text: new String.fromCharCode(CupertinoIcons.back.codePoint),
         style: new TextStyle(
           inherit: false,
-          color: color,
+          color: textStyle.color,
           fontSize: 34.0,
           fontFamily: CupertinoIcons.back.fontFamily,
           package: CupertinoIcons.back.fontPackage,
@@ -827,9 +1259,11 @@ class _BackChevron extends StatelessWidget {
 /// is true.
 class _BackLabel extends StatelessWidget {
   const _BackLabel({
+    Key key,
     @required this.specifiedPreviousTitle,
     @required this.route,
-  }) : assert(route != null);
+  }) : assert(route != null),
+       super(key: key);
 
   final String specifiedPreviousTitle;
   final ModalRoute<dynamic> route;
@@ -841,11 +1275,21 @@ class _BackLabel extends StatelessWidget {
       return const SizedBox(height: 0.0, width: 0.0);
     }
 
-    if (previousTitle.length > 10) {
-      return const Text('Back');
+    Text textWidget = new Text(
+      previousTitle,
+      maxLines: 1,
+      overflow: TextOverflow.ellipsis,
+    );
+
+    if (previousTitle.length > 12) {
+      textWidget = const Text('Back');
     }
 
-    return new Text(previousTitle, maxLines: 1);
+    return new Align(
+      alignment: AlignmentDirectional.centerStart,
+      widthFactor: 1.0,
+      child: textWidget,
+    );
   }
 
   @override
@@ -866,3 +1310,802 @@ class _BackLabel extends StatelessWidget {
     }
   }
 }
+
+/// This should always be the first child of Hero widgets.
+///
+/// This class helps each Hero transition obtain the start or end navigation
+/// bar's box size and the inner components of the navigation bar that will
+/// move around.
+///
+/// It should be wrapped around the biggest [RenderBox] of the static
+/// navigation bar in each route.
+class _TransitionableNavigationBar extends StatelessWidget {
+  _TransitionableNavigationBar({
+    @required this.componentsKeys,
+    @required this.backgroundColor,
+    @required this.actionsForegroundColor,
+    @required this.border,
+    @required this.hasUserMiddle,
+    @required this.largeExpanded,
+    @required this.child,
+  }) : assert(componentsKeys != null),
+       assert(largeExpanded != null),
+       super(key: componentsKeys.navBarBoxKey);
+
+  final _NavigationBarStaticComponentsKeys componentsKeys;
+  final Color backgroundColor;
+  final Color actionsForegroundColor;
+  final Border border;
+  final bool hasUserMiddle;
+  final bool largeExpanded;
+  final Widget child;
+
+  RenderBox get renderBox {
+    final RenderBox box = componentsKeys.navBarBoxKey.currentContext.findRenderObject();
+    assert(
+      box.attached,
+      '_TransitionableNavigationBar.renderBox should be called when building '
+      'hero flight shuttles when the from and the to nav bar boxes are already '
+      'laid out and painted.',
+    );
+    return box;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    assert(() {
+      bool inHero;
+      context.visitAncestorElements((Element ancestor) {
+        if (ancestor is ComponentElement) {
+          assert(
+            ancestor.widget.runtimeType != _NavigationBarTransition,
+            '_TransitionableNavigationBar should never re-appear inside '
+            '_NavigationBarTransition. Keyed _TransitionableNavigationBar should '
+            'only serve as anchor points in routes rather than appearing inside '
+            'Hero flights themselves.',
+          );
+          if (ancestor.widget.runtimeType == Hero) {
+            inHero = true;
+          }
+        }
+        inHero ??= false;
+        return true;
+      });
+      assert(
+        inHero == true,
+        '_TransitionableNavigationBar should only be added as the immediate '
+        'child of Hero widgets.',
+      );
+      return true;
+    }());
+    return child;
+  }
+}
+
+/// This class represents the widget that will be in the Hero flight instead of
+/// the 2 static navigation bars by taking inner components from both.
+///
+/// The `topNavBar` parameter is the nav bar that was on top regardless of
+/// push/pop direction.
+///
+/// Similarly, the `bottomNavBar` parameter is the nav bar that was at the
+/// bottom regardless of the push/pop direction.
+///
+/// If [MediaQuery.padding] is still present in this widget's [BuildContext],
+/// that padding will become part of the transitional navigation bar as well.
+///
+/// [MediaQuery.padding] should be consistent between the from/to routes and
+/// the Hero overlay. Inconsistent [MediaQuery.padding] will produce undetermined
+/// results.
+class _NavigationBarTransition extends StatelessWidget {
+  _NavigationBarTransition({
+    @required this.animation,
+    @required _TransitionableNavigationBar topNavBar,
+    @required _TransitionableNavigationBar bottomNavBar,
+  }) : heightTween = new Tween<double>(
+         begin: bottomNavBar.renderBox.size.height,
+         end: topNavBar.renderBox.size.height,
+       ),
+       backgroundTween = new ColorTween(
+         begin: bottomNavBar.backgroundColor,
+         end: topNavBar.backgroundColor,
+       ),
+       borderTween = new BorderTween(
+         begin: bottomNavBar.border,
+         end: topNavBar.border,
+       ),
+       componentsTransition = new _NavigationBarComponentsTransition(
+         animation: animation,
+         bottomNavBar: bottomNavBar,
+         topNavBar: topNavBar,
+       );
+
+  final Animation<double> animation;
+  final _NavigationBarComponentsTransition componentsTransition;
+
+  final Tween<double> heightTween;
+  final ColorTween backgroundTween;
+  final BorderTween borderTween;
+
+  @override
+  Widget build(BuildContext context) {
+    final List<Widget> children = <Widget>[
+      // Draw an empty navigation bar box with changing shape behind all the
+      // moving components without any components inside it itself.
+      AnimatedBuilder(
+        animation: animation,
+        builder: (BuildContext context, Widget child) {
+          return _wrapWithBackground(
+            // Don't update the system status bar color mid-flight.
+            updateSystemUiOverlay: false,
+            backgroundColor: backgroundTween.evaluate(animation),
+            border: borderTween.evaluate(animation),
+            child: new SizedBox(
+              height: heightTween.evaluate(animation),
+              width: double.infinity,
+            ),
+          );
+        },
+      ),
+      // Draw all the components on top of the empty bar box.
+      componentsTransition.bottomBackChevron,
+      componentsTransition.bottomBackLabel,
+      componentsTransition.bottomLeading,
+      componentsTransition.bottomMiddle,
+      componentsTransition.bottomLargeTitle,
+      componentsTransition.bottomTrailing,
+      // Draw top components on top of the bottom components.
+      componentsTransition.topLeading,
+      componentsTransition.topBackChevron,
+      componentsTransition.topBackLabel,
+      componentsTransition.topMiddle,
+      componentsTransition.topLargeTitle,
+      componentsTransition.topTrailing,
+    ];
+
+    children.removeWhere((Widget child) => child == null);
+
+    // The actual outer box is big enough to contain both the bottom and top
+    // navigation bars. It's not a direct Rect lerp because some components
+    // can actually be outside the linearly lerp'ed Rect in the middle of
+    // the animation, such as the topLargeTitle.
+    return new SizedBox(
+      height: math.max(heightTween.begin, heightTween.end) + MediaQuery.of(context).padding.top,
+      width: double.infinity,
+      child: new Stack(
+        children: children,
+      ),
+    );
+  }
+}
+
+/// This class helps create widgets that are in transition based on static
+/// components from the bottom and top navigation bars.
+///
+/// It animates these transitional components both in terms of position and
+/// their appearance.
+///
+/// Instead of running the transitional components through their normal static
+/// navigation bar layout logic, this creates transitional widgets that are based
+/// on these widgets' existing render objects' layout and position.
+///
+/// This is possible because this widget is only used during Hero transitions
+/// where both the from and to routes are already built and laid out.
+///
+/// The components' existing layout constraints and positions are then
+/// replicated using [Positioned] or [PositionedTransition] wrappers.
+///
+/// This class should never return [KeyedSubtree]s created by
+/// _NavigationBarStaticComponents directly. Since widgets from
+/// _NavigationBarStaticComponents are still present in the widget tree during the
+/// hero transitions, it would cause global key duplications. Instead, return
+/// only the [KeyedSubtree]s' child.
+@immutable
+class _NavigationBarComponentsTransition {
+  _NavigationBarComponentsTransition({
+    @required this.animation,
+    @required _TransitionableNavigationBar bottomNavBar,
+    @required _TransitionableNavigationBar topNavBar,
+  }) : bottomComponents = bottomNavBar.componentsKeys,
+       topComponents = topNavBar.componentsKeys,
+       bottomNavBarBox = bottomNavBar.renderBox,
+       topNavBarBox = topNavBar.renderBox,
+       bottomActionsStyle = _navBarItemStyle(bottomNavBar.actionsForegroundColor),
+       topActionsStyle = _navBarItemStyle(topNavBar.actionsForegroundColor),
+       bottomHasUserMiddle = bottomNavBar.hasUserMiddle,
+       topHasUserMiddle = topNavBar.hasUserMiddle,
+       bottomLargeExpanded = bottomNavBar.largeExpanded,
+       topLargeExpanded = topNavBar.largeExpanded,
+       transitionBox =
+           // paintBounds are based on offset zero so it's ok to expand the Rects.
+           bottomNavBar.renderBox.paintBounds.expandToInclude(topNavBar.renderBox.paintBounds);
+
+  static final Tween<double> fadeOut = new Tween<double>(
+    begin: 1.0,
+    end: 0.0,
+  );
+  static final Tween<double> fadeIn = new Tween<double>(
+    begin: 0.0,
+    end: 1.0,
+  );
+
+  final Animation<double> animation;
+  final _NavigationBarStaticComponentsKeys bottomComponents;
+  final _NavigationBarStaticComponentsKeys topComponents;
+
+  // These render boxes that are the ancestors of all the bottom and top
+  // components are used to determine the components' relative positions inside
+  // their respective navigation bars.
+  final RenderBox bottomNavBarBox;
+  final RenderBox topNavBarBox;
+
+  final TextStyle bottomActionsStyle;
+  final TextStyle topActionsStyle;
+  final bool bottomHasUserMiddle;
+  final bool topHasUserMiddle;
+  final bool bottomLargeExpanded;
+  final bool topLargeExpanded;
+
+  // This is the outer box in which all the components will be fitted. The
+  // sizing component of RelativeRects will be based on this rect's size.
+  final Rect transitionBox;
+
+  // Take a widget it its original ancestor navigation bar render box and
+  // translate it into a RelativeBox in the transition navigation bar box.
+  RelativeRect positionInTransitionBox(
+    GlobalKey key, {
+    @required RenderBox from,
+  }) {
+    final RenderBox componentBox = key.currentContext.findRenderObject();
+    assert(componentBox.attached);
+
+    return new RelativeRect.fromRect(
+      componentBox.localToGlobal(Offset.zero, ancestor: from) & componentBox.size,
+      transitionBox,
+    );
+  }
+
+  // Create a Tween that moves a widget between its original position in its
+  // ancestor navigation bar to another widget's position in that widget's
+  // navigation bar.
+  //
+  // Anchor their positions based on the center of their respective render
+  // boxes' leading edge.
+  //
+  // Also produce RelativeRects with sizes that would preserve the constant
+  // BoxConstraints of the 'from' widget so that animating font sizes etc don't
+  // produce rounding error artifacts with a linearly resizing rect.
+  RelativeRectTween slideFromLeadingEdge({
+    @required GlobalKey fromKey,
+    @required RenderBox fromNavBarBox,
+    @required GlobalKey toKey,
+    @required RenderBox toNavBarBox,
+  }) {
+    final RelativeRect fromRect = positionInTransitionBox(fromKey, from: fromNavBarBox);
+
+    final RenderBox fromBox = fromKey.currentContext.findRenderObject();
+    final RenderBox toBox = toKey.currentContext.findRenderObject();
+    final Rect toRect =
+        toBox.localToGlobal(
+          Offset.zero,
+          ancestor: toNavBarBox,
+        ).translate(
+          0.0,
+          - fromBox.size.height / 2 + toBox.size.height / 2
+        ) & fromBox.size; // Keep the from render object's size.
+
+    return new RelativeRectTween(
+        begin: fromRect,
+        end: new RelativeRect.fromRect(toRect, transitionBox),
+      );
+  }
+
+  Animation<double> fadeInFrom(double t, { Curve curve = Curves.easeIn }) {
+    return fadeIn.animate(
+      new CurvedAnimation(curve: new Interval(t, 1.0, curve: curve), parent: animation),
+    );
+  }
+
+  Animation<double> fadeOutBy(double t, { Curve curve = Curves.easeOut }) {
+    return fadeOut.animate(
+      new CurvedAnimation(curve: new Interval(0.0, t, curve: curve), parent: animation),
+    );
+  }
+
+  Widget get bottomLeading {
+    final KeyedSubtree bottomLeading = bottomComponents.leadingKey.currentWidget;
+
+    if (bottomLeading == null) {
+      return null;
+    }
+
+    return new Positioned.fromRelativeRect(
+      rect: positionInTransitionBox(bottomComponents.leadingKey, from: bottomNavBarBox),
+      child: new FadeTransition(
+        opacity: fadeOutBy(0.4),
+        child: bottomLeading.child,
+      ),
+    );
+  }
+
+  Widget get bottomBackChevron {
+    final KeyedSubtree bottomBackChevron = bottomComponents.backChevronKey.currentWidget;
+
+    if (bottomBackChevron == null) {
+      return null;
+    }
+
+    return new Positioned.fromRelativeRect(
+      rect: positionInTransitionBox(bottomComponents.backChevronKey, from: bottomNavBarBox),
+      child: new FadeTransition(
+        opacity: fadeOutBy(0.6),
+        child: new DefaultTextStyle(
+          style: bottomActionsStyle,
+          child: bottomBackChevron.child,
+        ),
+      ),
+    );
+  }
+
+  Widget get bottomBackLabel {
+    final KeyedSubtree bottomBackLabel = bottomComponents.backLabelKey.currentWidget;
+
+    if (bottomBackLabel == null) {
+      return null;
+    }
+
+    final RelativeRect from = positionInTransitionBox(bottomComponents.backLabelKey, from: bottomNavBarBox);
+
+    // Transition away by sliding horizontally to the left off of the screen.
+    final RelativeRectTween positionTween = new RelativeRectTween(
+      begin: from,
+      end: from.shift(new Offset(-bottomNavBarBox.size.width / 2.0, 0.0)),
+    );
+
+    return new PositionedTransition(
+      rect: positionTween.animate(animation),
+      child: new FadeTransition(
+        opacity: fadeOutBy(0.2),
+        child: new DefaultTextStyle(
+          style: bottomActionsStyle,
+          child: bottomBackLabel.child,
+        ),
+      ),
+    );
+  }
+
+  Widget get bottomMiddle {
+    final KeyedSubtree bottomMiddle = bottomComponents.middleKey.currentWidget;
+    final KeyedSubtree topBackLabel = topComponents.backLabelKey.currentWidget;
+    final KeyedSubtree topLeading = topComponents.leadingKey.currentWidget;
+
+    // The middle component is non-null when the nav bar is a large title
+    // nav bar but would be invisible when expanded, therefore don't show it here.
+    if (!bottomHasUserMiddle && bottomLargeExpanded) {
+      return null;
+    }
+
+    if (bottomMiddle != null && topBackLabel != null) {
+      return new PositionedTransition(
+        rect: slideFromLeadingEdge(
+          fromKey: bottomComponents.middleKey,
+          fromNavBarBox: bottomNavBarBox,
+          toKey: topComponents.backLabelKey,
+          toNavBarBox: topNavBarBox,
+        ).animate(animation),
+        child: new FadeTransition(
+          // A custom middle widget like a segmented control fades away faster.
+          opacity: fadeOutBy(bottomHasUserMiddle ? 0.4 : 0.7),
+          child: new Align(
+            // As the text shrinks, make sure it's still anchored to the leading
+            // edge of a constantly sized outer box.
+            alignment: AlignmentDirectional.centerStart,
+            child: new DefaultTextStyleTransition(
+              style: TextStyleTween(
+                begin: _kMiddleTitleTextStyle,
+                end: topActionsStyle,
+              ).animate(animation),
+              child: bottomMiddle.child,
+            ),
+          ),
+        ),
+      );
+    }
+
+    // When the top page has a leading widget override, don't move the bottom
+    // middle widget.
+    if (bottomMiddle != null && topLeading != null) {
+      return new Positioned.fromRelativeRect(
+        rect: positionInTransitionBox(bottomComponents.middleKey, from: bottomNavBarBox),
+        child: new FadeTransition(
+          opacity: fadeOutBy(bottomHasUserMiddle ? 0.4 : 0.7),
+          // Keep the font when transitioning into a non-back label leading.
+          child: new DefaultTextStyle(
+            style: _kMiddleTitleTextStyle,
+            child: bottomMiddle.child,
+          ),
+        ),
+      );
+    }
+
+    return null;
+  }
+
+  Widget get bottomLargeTitle {
+    final KeyedSubtree bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget;
+    final KeyedSubtree topBackLabel = topComponents.backLabelKey.currentWidget;
+    final KeyedSubtree topLeading = topComponents.leadingKey.currentWidget;
+
+    if (bottomLargeTitle == null || !bottomLargeExpanded) {
+      return null;
+    }
+
+    if (bottomLargeTitle != null && topBackLabel != null) {
+      return new PositionedTransition(
+        rect: slideFromLeadingEdge(
+          fromKey: bottomComponents.largeTitleKey,
+          fromNavBarBox: bottomNavBarBox,
+          toKey: topComponents.backLabelKey,
+          toNavBarBox: topNavBarBox,
+        ).animate(animation),
+        child: new FadeTransition(
+          opacity: fadeOutBy(0.6),
+          child: new Align(
+            // As the text shrinks, make sure it's still anchored to the leading
+            // edge of a constantly sized outer box.
+            alignment: AlignmentDirectional.centerStart,
+            child: new DefaultTextStyleTransition(
+              style: TextStyleTween(
+                begin: _kLargeTitleTextStyle,
+                end: topActionsStyle,
+              ).animate(animation),
+              maxLines: 1,
+              overflow: TextOverflow.ellipsis,
+              child: bottomLargeTitle.child,
+            ),
+          ),
+        ),
+      );
+    }
+
+    if (bottomLargeTitle != null && topLeading != null) {
+      final RelativeRect from = positionInTransitionBox(bottomComponents.largeTitleKey, from: bottomNavBarBox);
+
+      final RelativeRectTween positionTween = new RelativeRectTween(
+        begin: from,
+        end: from.shift(new Offset(bottomNavBarBox.size.width / 4.0, 0.0)),
+      );
+
+      // Just shift slightly towards the right instead of moving to the back
+      // label position.
+      return new PositionedTransition(
+        rect: positionTween.animate(animation),
+        child: new FadeTransition(
+          opacity: fadeOutBy(0.4),
+          // Keep the font when transitioning into a non-back-label leading.
+          child: new DefaultTextStyle(
+            style: _kLargeTitleTextStyle,
+            child: bottomLargeTitle.child,
+          ),
+        ),
+      );
+    }
+
+    return null;
+  }
+
+  Widget get bottomTrailing {
+    final KeyedSubtree bottomTrailing = bottomComponents.trailingKey.currentWidget;
+
+    if (bottomTrailing == null) {
+      return null;
+    }
+
+    return new Positioned.fromRelativeRect(
+      rect: positionInTransitionBox(bottomComponents.trailingKey, from: bottomNavBarBox),
+      child: new FadeTransition(
+        opacity: fadeOutBy(0.6),
+        child: bottomTrailing.child,
+      ),
+    );
+  }
+
+  Widget get topLeading {
+    final KeyedSubtree topLeading = topComponents.leadingKey.currentWidget;
+
+    if (topLeading == null) {
+      return null;
+    }
+
+    return new Positioned.fromRelativeRect(
+      rect: positionInTransitionBox(topComponents.leadingKey, from: topNavBarBox),
+      child: new FadeTransition(
+        opacity: fadeInFrom(0.6),
+        child: topLeading.child,
+      ),
+    );
+  }
+
+  Widget get topBackChevron {
+    final KeyedSubtree topBackChevron = topComponents.backChevronKey.currentWidget;
+    final KeyedSubtree bottomBackChevron = bottomComponents.backChevronKey.currentWidget;
+
+    if (topBackChevron == null) {
+      return null;
+    }
+
+    final RelativeRect to = positionInTransitionBox(topComponents.backChevronKey, from: topNavBarBox);
+    RelativeRect from = to;
+
+    // If it's the first page with a back chevron, shift in slightly from the
+    // right.
+    if (bottomBackChevron == null) {
+      final RenderBox topBackChevronBox = topComponents.backChevronKey.currentContext.findRenderObject();
+      from = to.shift(new Offset(topBackChevronBox.size.width * 2.0, 0.0));
+    }
+
+    final RelativeRectTween positionTween = new RelativeRectTween(
+      begin: from,
+      end: to,
+    );
+
+    return new PositionedTransition(
+      rect: positionTween.animate(animation),
+      child: new FadeTransition(
+        opacity: fadeInFrom(bottomBackChevron == null ? 0.7 : 0.4),
+        child: new DefaultTextStyle(
+          style: topActionsStyle,
+          child: topBackChevron.child,
+        ),
+      ),
+    );
+  }
+
+  Widget get topBackLabel {
+    final KeyedSubtree bottomMiddle = bottomComponents.middleKey.currentWidget;
+    final KeyedSubtree bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget;
+    final KeyedSubtree topBackLabel = topComponents.backLabelKey.currentWidget;
+
+    if (topBackLabel == null) {
+      return null;
+    }
+
+    final RenderAnimatedOpacity topBackLabelOpacity =
+        topComponents.backLabelKey.currentContext?.ancestorRenderObjectOfType(
+          const TypeMatcher<RenderAnimatedOpacity>()
+        );
+
+    Animation<double> midClickOpacity;
+    if (topBackLabelOpacity != null && topBackLabelOpacity.opacity.value < 1.0) {
+      midClickOpacity = new Tween<double>(
+        begin: 0.0,
+        end: topBackLabelOpacity.opacity.value,
+      ).animate(animation);
+    }
+
+    // Pick up from an incoming transition from the large title. This is
+    // duplicated here from the bottomLargeTitle transition widget because the
+    // content text might be different. For instance, if the bottomLargeTitle
+    // text is too long, the topBackLabel will say 'Back' instead of the original
+    // text.
+    if (bottomLargeTitle != null &&
+        topBackLabel != null &&
+        bottomLargeExpanded
+    ) {
+      return new PositionedTransition(
+        rect: slideFromLeadingEdge(
+          fromKey: bottomComponents.largeTitleKey,
+          fromNavBarBox: bottomNavBarBox,
+          toKey: topComponents.backLabelKey,
+          toNavBarBox: topNavBarBox,
+        ).animate(animation),
+        child: new FadeTransition(
+          opacity: midClickOpacity ?? fadeInFrom(0.4),
+          child: new DefaultTextStyleTransition(
+            style: TextStyleTween(
+              begin: _kLargeTitleTextStyle,
+              end: topActionsStyle,
+            ).animate(animation),
+            maxLines: 1,
+            overflow: TextOverflow.ellipsis,
+            child: topBackLabel.child,
+          ),
+        ),
+      );
+    }
+
+    // The topBackLabel always comes from the large title first if available
+    // and expanded instead of middle.
+    if (bottomMiddle != null && topBackLabel != null) {
+      return new PositionedTransition(
+        rect: slideFromLeadingEdge(
+          fromKey: bottomComponents.middleKey,
+          fromNavBarBox: bottomNavBarBox,
+          toKey: topComponents.backLabelKey,
+          toNavBarBox: topNavBarBox,
+        ).animate(animation),
+        child: new FadeTransition(
+          opacity: midClickOpacity ?? fadeInFrom(0.3),
+          child: new DefaultTextStyleTransition(
+            style: TextStyleTween(
+              begin: _kMiddleTitleTextStyle,
+              end: topActionsStyle,
+            ).animate(animation),
+            child: topBackLabel.child,
+          ),
+        ),
+      );
+    }
+
+    return null;
+  }
+
+  Widget get topMiddle {
+    final KeyedSubtree topMiddle = topComponents.middleKey.currentWidget;
+
+    if (topMiddle == null) {
+      return null;
+    }
+
+    // The middle component is non-null when the nav bar is a large title
+    // nav bar but would be invisible when expanded, therefore don't show it here.
+    if (!topHasUserMiddle && topLargeExpanded) {
+      return null;
+    }
+
+    final RelativeRect to = positionInTransitionBox(topComponents.middleKey, from: topNavBarBox);
+
+    // Shift in from the trailing edge of the screen.
+    final RelativeRectTween positionTween = new RelativeRectTween(
+      begin: to.shift(new Offset(topNavBarBox.size.width / 2.0, 0.0)),
+      end: to,
+    );
+
+    return new PositionedTransition(
+      rect: positionTween.animate(animation),
+      child: new FadeTransition(
+        opacity: fadeInFrom(0.25),
+        child: new DefaultTextStyle(
+          style: _kMiddleTitleTextStyle,
+          child: topMiddle.child,
+        ),
+      ),
+    );
+  }
+
+  Widget get topTrailing {
+    final KeyedSubtree topTrailing = topComponents.trailingKey.currentWidget;
+
+    if (topTrailing == null) {
+      return null;
+    }
+
+    return new Positioned.fromRelativeRect(
+      rect: positionInTransitionBox(topComponents.trailingKey, from: topNavBarBox),
+      child: new FadeTransition(
+        opacity: fadeInFrom(0.4),
+        child: topTrailing.child,
+      ),
+    );
+  }
+
+  Widget get topLargeTitle {
+    final KeyedSubtree topLargeTitle = topComponents.largeTitleKey.currentWidget;
+
+    if (topLargeTitle == null || !topLargeExpanded) {
+      return null;
+    }
+
+    final RelativeRect to = positionInTransitionBox(topComponents.largeTitleKey, from: topNavBarBox);
+
+    // Shift in from the trailing edge of the screen.
+    final RelativeRectTween positionTween = new RelativeRectTween(
+      begin: to.shift(new Offset(topNavBarBox.size.width, 0.0)),
+      end: to,
+    );
+
+    return new PositionedTransition(
+      rect: positionTween.animate(animation),
+      child: new FadeTransition(
+        opacity: fadeInFrom(0.3),
+        child: new DefaultTextStyle(
+          style: _kLargeTitleTextStyle,
+          maxLines: 1,
+          overflow: TextOverflow.ellipsis,
+          child: topLargeTitle.child,
+        ),
+      ),
+    );
+  }
+}
+
+/// Navigation bars' hero rect tween that will move between the static bars
+/// but keep a constant size that's the bigger of both navigation bars.
+CreateRectTween _linearTranslateWithLargestRectSizeTween = (Rect begin, Rect end) {
+  final Size largestSize = new Size(
+    math.max(begin.size.width, end.size.width),
+    math.max(begin.size.height, end.size.height),
+  );
+  return new RectTween(
+    begin: begin.topLeft & largestSize,
+    end: end.topLeft & largestSize,
+  );
+};
+
+final TransitionBuilder _navBarHeroLaunchPadBuilder = (
+  BuildContext context,
+  Widget child,
+) {
+  assert(child is _TransitionableNavigationBar);
+  // Tree reshaping is fine here because the Heroes' child is always a
+  // _TransitionableNavigationBar which has a GlobalKey.
+
+  // Keeping the Hero subtree here is needed (instead of just swapping out the
+  // anchor nav bars for fixed size boxes during flights) because the nav bar
+  // and their specific component children may serve as anchor points again if
+  // another mid-transition flight diversion is triggered.
+
+  // This is ok performance-wise because static nav bars are generally cheap to
+  // build and layout but expensive to GPU render (due to clips and blurs) which
+  // we're skipping here.
+  return new Visibility(
+    maintainSize: true,
+    maintainAnimation: true,
+    maintainState: true,
+    visible: false,
+    child: child,
+  );
+};
+
+/// Navigation bars' hero flight shuttle builder.
+final HeroFlightShuttleBuilder _navBarHeroFlightShuttleBuilder = (
+  BuildContext flightContext,
+  Animation<double> animation,
+  HeroFlightDirection flightDirection,
+  BuildContext fromHeroContext,
+  BuildContext toHeroContext,
+) {
+  assert(animation != null);
+  assert(flightDirection != null);
+  assert(fromHeroContext != null);
+  assert(toHeroContext != null);
+  assert(fromHeroContext.widget is Hero);
+  assert(toHeroContext.widget is Hero);
+
+  final Hero fromHeroWidget = fromHeroContext.widget;
+  final Hero toHeroWidget = toHeroContext.widget;
+
+  assert(fromHeroWidget.child is _TransitionableNavigationBar);
+  assert(toHeroWidget.child is _TransitionableNavigationBar);
+
+  final _TransitionableNavigationBar fromNavBar = fromHeroWidget.child;
+  final _TransitionableNavigationBar toNavBar = toHeroWidget.child;
+
+  assert(fromNavBar.componentsKeys != null);
+  assert(toNavBar.componentsKeys != null);
+
+  assert(
+    fromNavBar.componentsKeys.navBarBoxKey.currentContext.owner != null,
+    'The from nav bar to Hero must have been mounted in the previous frame',
+  );
+  assert(
+    toNavBar.componentsKeys.navBarBoxKey.currentContext.owner != null,
+    'The to nav bar to Hero must have been mounted in the previous frame',
+  );
+
+  switch (flightDirection) {
+    case HeroFlightDirection.push:
+      return new _NavigationBarTransition(
+        animation: animation,
+        bottomNavBar: fromNavBar,
+        topNavBar: toNavBar,
+      );
+      break;
+    case HeroFlightDirection.pop:
+      return new _NavigationBarTransition(
+        animation: animation,
+        bottomNavBar: toNavBar,
+        topNavBar: fromNavBar,
+      );
+  }
+};

packages/flutter/lib/src/cupertino/tab_view.dart

@@ -37,7 +37,7 @@ import 'route.dart';
 ///  * [CupertinoTabScaffold], a typical host that supports switching between tabs.
 ///  * [CupertinoPageRoute], a typical modal page route pushed onto the
 ///    [CupertinoTabView]'s [Navigator].
-class CupertinoTabView extends StatelessWidget {
+class CupertinoTabView extends StatefulWidget {
   /// Creates the content area for a tab in a [CupertinoTabScaffold].
   const CupertinoTabView({
     Key key,
@@ -101,12 +101,41 @@ class CupertinoTabView extends StatelessWidget {
   /// This list of observers is not shared with ancestor or descendant [Navigator]s.
   final List<NavigatorObserver> navigatorObservers;
 
+  @override
+  _CupertinoTabViewState createState() {
+    return new _CupertinoTabViewState();
+  }
+}
+
+class _CupertinoTabViewState extends State<CupertinoTabView> {
+  HeroController _heroController;
+  List<NavigatorObserver> _navigatorObservers;
+
+  @override
+  void initState() {
+    super.initState();
+    _heroController = new HeroController(); // Linear tweening.
+    _updateObservers();
+  }
+
+  @override
+   void didUpdateWidget(CupertinoTabView oldWidget) {
+     super.didUpdateWidget(oldWidget);
+     _updateObservers();
+  }
+
+  void _updateObservers() {
+    _navigatorObservers =
+        new List<NavigatorObserver>.from(widget.navigatorObservers)
+          ..add(_heroController);
+  }
+
   @override
   Widget build(BuildContext context) {
     return new Navigator(
       onGenerateRoute: _onGenerateRoute,
       onUnknownRoute: _onUnknownRoute,
-      observers: navigatorObservers,
+      observers: _navigatorObservers,
     );
   }
 
@@ -114,12 +143,12 @@ class CupertinoTabView extends StatelessWidget {
     final String name = settings.name;
     WidgetBuilder routeBuilder;
     String title;
-    if (name == Navigator.defaultRouteName && builder != null) {
-      routeBuilder = builder;
-      title = defaultTitle;
+    if (name == Navigator.defaultRouteName && widget.builder != null) {
+      routeBuilder = widget.builder;
+      title = widget.defaultTitle;
     }
-    else if (routes != null)
-      routeBuilder = routes[name];
+    else if (widget.routes != null)
+      routeBuilder = widget.routes[name];
     if (routeBuilder != null) {
       return new CupertinoPageRoute<dynamic>(
         builder: routeBuilder,
@@ -127,14 +156,14 @@ class CupertinoTabView extends StatelessWidget {
         settings: settings,
       );
     }
-    if (onGenerateRoute != null)
-      return onGenerateRoute(settings);
+    if (widget.onGenerateRoute != null)
+      return widget.onGenerateRoute(settings);
     return null;
   }
 
   Route<dynamic> _onUnknownRoute(RouteSettings settings) {
     assert(() {
-      if (onUnknownRoute == null) {
+      if (widget.onUnknownRoute == null) {
         throw new FlutterError(
           'Could not find a generator for route $settings in the $runtimeType.\n'
           'Generators for routes are searched for in the following order:\n'
@@ -149,7 +178,7 @@ class CupertinoTabView extends StatelessWidget {
       }
       return true;
     }());
-    final Route<dynamic> result = onUnknownRoute(settings);
+    final Route<dynamic> result = widget.onUnknownRoute(settings);
     assert(() {
       if (result == null) {
         throw new FlutterError(

packages/flutter/lib/src/widgets/heroes.dart

@@ -20,11 +20,36 @@ import 'transitions.dart';
 /// [MaterialRectArcTween].
 typedef Tween<Rect> CreateRectTween(Rect begin, Rect end);
 
+/// A function that lets [Hero]s self supply a [Widget] that is shown during the
+/// hero's flight from one route to another instead of default (which is to
+/// show the destination route's instance of the Hero).
+typedef Widget HeroFlightShuttleBuilder(
+  BuildContext flightContext,
+  Animation<double> animation,
+  HeroFlightDirection flightDirection,
+  BuildContext fromHeroContext,
+  BuildContext toHeroContext,
+);
+
 typedef void _OnFlightEnded(_HeroFlight flight);
 
-enum _HeroFlightType {
-  push, // Fly the "to" hero and animate with the "to" route.
-  pop, // Fly the "to" hero and animate with the "from" route.
+/// Direction of the hero's flight based on the navigation operation.
+enum HeroFlightDirection {
+  /// A flight triggered by a route push.
+  ///
+  /// The animation goes from 0 to 1.
+  ///
+  /// If no custom [HeroFlightShuttleBuilder] is supplied, the top route's
+  /// [Hero] child is shown in flight.
+  push,
+
+  /// A flight triggered by a route pop.
+  ///
+  /// The animation goes from 1 to 0.
+  ///
+  /// If no custom [HeroFlightShuttleBuilder] is supplied, the bottom route's
+  /// [Hero] child is shown in flight.
+  pop,
 }
 
 // The bounding box for context in global coordinates.
@@ -42,8 +67,8 @@ Rect _globalBoundingBoxFor(BuildContext context) {
 /// be helpful for orienting the user for the feature to physically move from
 /// one page to the other during the routes' transition. Such an animation
 /// is called a *hero animation*. The hero widgets "fly" in the Navigator's
-/// overlay during the transition and while they're in-flight they're
-/// not shown in their original locations in the old and new routes.
+/// overlay during the transition and while they're in-flight they're, by
+/// default, not shown in their original locations in the old and new routes.
 ///
 /// To label a widget as such a feature, wrap it in a [Hero] widget. When
 /// navigation happens, the [Hero] widgets on each route are identified
@@ -52,6 +77,9 @@ Rect _globalBoundingBoxFor(BuildContext context) {
 ///
 /// If a [Hero] is already in flight when navigation occurs, its
 /// flight animation will be redirected to its new destination.
+
+/// The widget shown in-flight during the transition is, by default, the
+/// destination route's [Hero]'s child.
 ///
 /// Routes must not contain more than one [Hero] for each [tag].
 ///
@@ -67,12 +95,24 @@ Rect _globalBoundingBoxFor(BuildContext context) {
 ///
 /// To make the animations look good, it's critical that the widget tree for the
 /// hero in both locations be essentially identical. The widget of the *target*
-/// is used to do the transition: when going from route A to route B, route B's
-/// hero's widget is placed over route A's hero's widget, and route A's hero is
-/// hidden. Then the widget is animated to route B's hero's position, and then
-/// the widget is inserted into route B. When going back from B to A, route A's
-/// hero's widget is placed over where route B's hero's widget was, and then the
-/// animation goes the other way.
+/// is, by default, used to do the transition: when going from route A to route
+/// B, route B's hero's widget is placed over route A's hero's widget. If a
+/// [flightShuttleBuilder] is supplied, its output widget is shown during the
+/// flight transition instead.
+///
+/// By default, both route A and route B's heroes are hidden while the
+/// transitioning widget is animating in-flight above the 2 routes.
+/// [placeholderBuilder] can be used to show a custom widget in their place
+/// instead once the transition has taken flight.
+///
+/// During the transition, the transition widget is animated to route B's hero's
+/// position, and then the widget is inserted into route B. When going back from
+/// B to A, route A's hero's widget is, by default, placed over where route B's
+/// hero's widget was, and then the animation goes the other way.
+///
+/// ## Parts of a Hero Transition
+///
+/// ![Diagrams with parts of the Hero transition.](https://flutter.github.io/assets-for-api-docs/assets/interaction/heroes.png)
 class Hero extends StatefulWidget {
   /// Create a hero.
   ///
@@ -81,6 +121,8 @@ class Hero extends StatefulWidget {
     Key key,
     @required this.tag,
     this.createRectTween,
+    this.flightShuttleBuilder,
+    this.placeholderBuilder,
     @required this.child,
   }) : assert(tag != null),
        assert(child != null),
@@ -115,6 +157,25 @@ class Hero extends StatefulWidget {
   /// {@macro flutter.widgets.child}
   final Widget child;
 
+  /// Optional override to supply a widget that's shown during the hero's flight.
+  ///
+  /// This in-flight widget can depend on the route transition's animation as
+  /// well as the incoming and outgoing routes' [Hero] descendants' widgets and
+  /// layout.
+  ///
+  /// When both the source and destination [Hero]s provide a [flightShuttleBuilder],
+  /// the destination's [flightShuttleBuilder] takes precedence.
+  ///
+  /// If none is provided, the destination route's Hero child is shown in-flight
+  /// by default.
+  final HeroFlightShuttleBuilder flightShuttleBuilder;
+
+  /// Placeholder widget left in place as the Hero's child once the flight takes off.
+  ///
+  /// By default, an empty SizedBox keeping the Hero child's original size is
+  /// left in place once the Hero shuttle has taken flight.
+  final TransitionBuilder placeholderBuilder;
+
   // Returns a map of all of the heroes in context, indexed by hero tag.
   static Map<Object, _HeroState> _allHeroesFor(BuildContext context) {
     assert(context != null);
@@ -141,6 +202,10 @@ class Hero extends StatefulWidget {
         final _HeroState heroState = hero.state;
         result[tag] = heroState;
       }
+      // Don't perform transitions across different Navigators.
+      if (element.widget is Navigator) {
+        return;
+      }
       element.visitChildren(visitor);
     }
     context.visitChildElements(visitor);
@@ -181,10 +246,14 @@ class _HeroState extends State<Hero> {
   @override
   Widget build(BuildContext context) {
     if (_placeholderSize != null) {
-      return new SizedBox(
-        width: _placeholderSize.width,
-        height: _placeholderSize.height
-      );
+      if (widget.placeholderBuilder == null) {
+        return new SizedBox(
+          width: _placeholderSize.width,
+          height: _placeholderSize.height
+        );
+      } else {
+        return widget.placeholderBuilder(context, widget.child);
+      }
     }
     return new KeyedSubtree(
       key: _key,
@@ -204,9 +273,10 @@ class _HeroFlightManifest {
     @required this.fromHero,
     @required this.toHero,
     @required this.createRectTween,
+    @required this.shuttleBuilder,
   }) : assert(fromHero.widget.tag == toHero.widget.tag);
 
-  final _HeroFlightType type;
+  final HeroFlightDirection type;
   final OverlayState overlay;
   final Rect navigatorRect;
   final PageRoute<dynamic> fromRoute;
@@ -214,19 +284,21 @@ class _HeroFlightManifest {
   final _HeroState fromHero;
   final _HeroState toHero;
   final CreateRectTween createRectTween;
+  final HeroFlightShuttleBuilder shuttleBuilder;
 
   Object get tag => fromHero.widget.tag;
 
   Animation<double> get animation {
     return new CurvedAnimation(
-      parent: (type == _HeroFlightType.push) ? toRoute.animation : fromRoute.animation,
+      parent: (type == HeroFlightDirection.push) ? toRoute.animation : fromRoute.animation,
       curve: Curves.fastOutSlowIn,
     );
   }
 
   @override
   String toString() {
-    return '_HeroFlightManifest($type hero: $tag from: ${fromRoute.settings} to: ${toRoute.settings})';
+    return '_HeroFlightManifest($type tag: $tag from route: ${fromRoute.settings} '
+        'to route: ${toRoute.settings} with hero: $fromHero to $toHero)';
   }
 }
 
@@ -238,7 +310,9 @@ class _HeroFlight {
 
   final _OnFlightEnded onFlightEnded;
 
-  Tween<Rect> heroRect;
+  Tween<Rect> heroRectTween;
+  Widget shuttle;
+
   Animation<double> _heroOpacity = kAlwaysCompleteAnimation;
   ProxyAnimation _proxyAnimation;
   _HeroFlightManifest manifest;
@@ -255,9 +329,18 @@ class _HeroFlight {
   // The OverlayEntry WidgetBuilder callback for the hero's overlay.
   Widget _buildOverlay(BuildContext context) {
     assert(manifest != null);
+    shuttle ??= manifest.shuttleBuilder(
+      context,
+      manifest.animation,
+      manifest.type,
+      manifest.fromHero.context,
+      manifest.toHero.context,
+    );
+    assert(shuttle != null);
+
     return new AnimatedBuilder(
       animation: _proxyAnimation,
-      child: manifest.toHero.widget,
+      child: shuttle,
       builder: (BuildContext context, Widget child) {
         final RenderBox toHeroBox = manifest.toHero.context?.findRenderObject();
         if (_aborted || toHeroBox == null || !toHeroBox.attached) {
@@ -273,13 +356,13 @@ class _HeroFlight {
           // supposed to end up then recreate the heroRect tween.
           final RenderBox finalRouteBox = manifest.toRoute.subtreeContext?.findRenderObject();
           final Offset toHeroOrigin = toHeroBox.localToGlobal(Offset.zero, ancestor: finalRouteBox);
-          if (toHeroOrigin != heroRect.end.topLeft) {
-            final Rect heroRectEnd = toHeroOrigin & heroRect.end.size;
-            heroRect = _doCreateRectTween(heroRect.begin, heroRectEnd);
+          if (toHeroOrigin != heroRectTween.end.topLeft) {
+            final Rect heroRectEnd = toHeroOrigin & heroRectTween.end.size;
+            heroRectTween = _doCreateRectTween(heroRectTween.begin, heroRectEnd);
           }
         }
 
-        final Rect rect = heroRect.evaluate(_proxyAnimation);
+        final Rect rect = heroRectTween.evaluate(_proxyAnimation);
         final Size size = manifest.navigatorRect.size;
         final RelativeRect offsets = new RelativeRect.fromSize(rect, size);
 
@@ -291,7 +374,6 @@ class _HeroFlight {
           child: new IgnorePointer(
             child: new RepaintBoundary(
               child: new Opacity(
-                key: manifest.toHero._key,
                 opacity: _heroOpacity.value,
                 child: child,
               ),
@@ -322,12 +404,12 @@ class _HeroFlight {
     assert(() {
       final Animation<double> initial = initialManifest.animation;
       assert(initial != null);
-      final _HeroFlightType type = initialManifest.type;
+      final HeroFlightDirection type = initialManifest.type;
       assert(type != null);
       switch (type) {
-        case _HeroFlightType.pop:
+        case HeroFlightDirection.pop:
           return initial.value == 1.0 && initial.status == AnimationStatus.reverse;
-        case _HeroFlightType.push:
+        case HeroFlightDirection.push:
           return initial.value == 0.0 && initial.status == AnimationStatus.forward;
       }
       return null;
@@ -335,7 +417,7 @@ class _HeroFlight {
 
     manifest = initialManifest;
 
-    if (manifest.type == _HeroFlightType.pop)
+    if (manifest.type == HeroFlightDirection.pop)
       _proxyAnimation.parent = new ReverseAnimation(manifest.animation);
     else
       _proxyAnimation.parent = manifest.animation;
@@ -343,7 +425,7 @@ class _HeroFlight {
     manifest.fromHero.startFlight();
     manifest.toHero.startFlight();
 
-    heroRect = _doCreateRectTween(
+    heroRectTween = _doCreateRectTween(
       _globalBoundingBoxFor(manifest.fromHero.context),
       _globalBoundingBoxFor(manifest.toHero.context),
     );
@@ -357,7 +439,7 @@ class _HeroFlight {
   void divert(_HeroFlightManifest newManifest) {
     assert(manifest.tag == newManifest.tag);
 
-    if (manifest.type == _HeroFlightType.push && newManifest.type == _HeroFlightType.pop) {
+    if (manifest.type == HeroFlightDirection.push && newManifest.type == HeroFlightDirection.pop) {
       // A push flight was interrupted by a pop.
       assert(newManifest.animation.status == AnimationStatus.reverse);
       assert(manifest.fromHero == newManifest.toHero);
@@ -371,8 +453,8 @@ class _HeroFlight {
       // path for swapped begin and end parameters. We want the pop flight
       // path to be the same (in reverse) as the push flight path.
       _proxyAnimation.parent = new ReverseAnimation(newManifest.animation);
-      heroRect = new ReverseTween<Rect>(heroRect);
-    } else if (manifest.type == _HeroFlightType.pop && newManifest.type == _HeroFlightType.push) {
+      heroRectTween = new ReverseTween<Rect>(heroRectTween);
+    } else if (manifest.type == HeroFlightDirection.pop && newManifest.type == HeroFlightDirection.push) {
       // A pop flight was interrupted by a push.
       assert(newManifest.animation.status == AnimationStatus.forward);
       assert(manifest.toHero == newManifest.fromHero);
@@ -386,10 +468,10 @@ class _HeroFlight {
       if (manifest.fromHero != newManifest.toHero) {
         manifest.fromHero.endFlight();
         newManifest.toHero.startFlight();
-        heroRect = _doCreateRectTween(heroRect.end, _globalBoundingBoxFor(newManifest.toHero.context));
+        heroRectTween = _doCreateRectTween(heroRectTween.end, _globalBoundingBoxFor(newManifest.toHero.context));
       } else {
         // TODO(hansmuller): Use ReverseTween here per github.com/flutter/flutter/pull/12203.
-        heroRect = _doCreateRectTween(heroRect.end, heroRect.begin);
+        heroRectTween = _doCreateRectTween(heroRectTween.end, heroRectTween.begin);
       }
     } else {
       // A push or a pop flight is heading to a new route, i.e.
@@ -398,17 +480,24 @@ class _HeroFlight {
       assert(manifest.fromHero != newManifest.fromHero);
       assert(manifest.toHero != newManifest.toHero);
 
-      heroRect = _doCreateRectTween(heroRect.evaluate(_proxyAnimation), _globalBoundingBoxFor(newManifest.toHero.context));
+      heroRectTween = _doCreateRectTween(heroRectTween.evaluate(_proxyAnimation), _globalBoundingBoxFor(newManifest.toHero.context));
+      shuttle = null;
 
-      if (newManifest.type == _HeroFlightType.pop)
+      if (newManifest.type == HeroFlightDirection.pop)
         _proxyAnimation.parent = new ReverseAnimation(newManifest.animation);
       else
         _proxyAnimation.parent = newManifest.animation;
 
       manifest.fromHero.endFlight();
       manifest.toHero.endFlight();
+
+      // Let the heroes in each of the routes rebuild with their placeholders.
       newManifest.fromHero.startFlight();
       newManifest.toHero.startFlight();
+
+      // Let the transition overlay on top of the routes also rebuild since
+      // we cleared the old shuttle.
+      overlayEntry.markNeedsBuild();
     }
 
     _aborted = false;
@@ -455,14 +544,14 @@ class HeroController extends NavigatorObserver {
   void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
     assert(navigator != null);
     assert(route != null);
-    _maybeStartHeroTransition(previousRoute, route, _HeroFlightType.push);
+    _maybeStartHeroTransition(previousRoute, route, HeroFlightDirection.push);
   }
 
   @override
   void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
     assert(navigator != null);
     assert(route != null);
-    _maybeStartHeroTransition(route, previousRoute, _HeroFlightType.pop);
+    _maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop);
   }
 
   @override
@@ -477,14 +566,14 @@ class HeroController extends NavigatorObserver {
 
   // If we're transitioning between different page routes, start a hero transition
   // after the toRoute has been laid out with its animation's value at 1.0.
-  void _maybeStartHeroTransition(Route<dynamic> fromRoute, Route<dynamic> toRoute, _HeroFlightType flightType) {
+  void _maybeStartHeroTransition(Route<dynamic> fromRoute, Route<dynamic> toRoute, HeroFlightDirection flightType) {
     if (_questsEnabled && toRoute != fromRoute && toRoute is PageRoute<dynamic> && fromRoute is PageRoute<dynamic>) {
       final PageRoute<dynamic> from = fromRoute;
       final PageRoute<dynamic> to = toRoute;
-      final Animation<double> animation = (flightType == _HeroFlightType.push) ? to.animation : from.animation;
+      final Animation<double> animation = (flightType == HeroFlightDirection.push) ? to.animation : from.animation;
 
       // A user gesture may have already completed the pop.
-      if (flightType == _HeroFlightType.pop && animation.status == AnimationStatus.dismissed)
+      if (flightType == HeroFlightDirection.pop && animation.status == AnimationStatus.dismissed)
         return;
 
       // Putting a route offstage changes its animation value to 1.0. Once this
@@ -493,14 +582,19 @@ class HeroController extends NavigatorObserver {
       to.offstage = to.animation.value == 0.0;
 
       WidgetsBinding.instance.addPostFrameCallback((Duration value) {
-        _startHeroTransition(from, to, flightType);
+        _startHeroTransition(from, to, animation, flightType);
       });
     }
   }
 
   // Find the matching pairs of heros in from and to and either start or a new
   // hero flight, or divert an existing one.
-  void _startHeroTransition(PageRoute<dynamic> from, PageRoute<dynamic> to, _HeroFlightType flightType) {
+  void _startHeroTransition(
+    PageRoute<dynamic> from,
+    PageRoute<dynamic> to,
+    Animation<double> animation,
+    HeroFlightDirection flightType,
+  ) {
     // If the navigator or one of the routes subtrees was removed before this
     // end-of-frame callback was called, then don't actually start a transition.
     if (navigator == null || from.subtreeContext == null || to.subtreeContext == null) {
@@ -520,6 +614,9 @@ class HeroController extends NavigatorObserver {
 
     for (Object tag in fromHeroes.keys) {
       if (toHeroes[tag] != null) {
+        final HeroFlightShuttleBuilder fromShuttleBuilder = fromHeroes[tag].widget.flightShuttleBuilder;
+        final HeroFlightShuttleBuilder toShuttleBuilder = toHeroes[tag].widget.flightShuttleBuilder;
+
         final _HeroFlightManifest manifest = new _HeroFlightManifest(
           type: flightType,
           overlay: navigator.overlay,
@@ -529,7 +626,10 @@ class HeroController extends NavigatorObserver {
           fromHero: fromHeroes[tag],
           toHero: toHeroes[tag],
           createRectTween: createRectTween,
+          shuttleBuilder:
+              toShuttleBuilder ?? fromShuttleBuilder ?? _defaultHeroFlightShuttleBuilder,
         );
+
         if (_flights[tag] != null)
           _flights[tag].divert(manifest);
         else
@@ -543,4 +643,15 @@ class HeroController extends NavigatorObserver {
   void _handleFlightEnded(_HeroFlight flight) {
     _flights.remove(flight.manifest.tag);
   }
+
+  static final HeroFlightShuttleBuilder _defaultHeroFlightShuttleBuilder = (
+    BuildContext flightContext,
+    Animation<double> animation,
+    HeroFlightDirection flightDirection,
+    BuildContext fromHeroContext,
+    BuildContext toHeroContext,
+  ) {
+    final Hero toHero = toHeroContext.widget;
+    return toHero.child;
+  };
 }

packages/flutter/lib/src/widgets/implicit_animations.dart

@@ -127,6 +127,24 @@ class BorderRadiusTween extends Tween<BorderRadius> {
   BorderRadius lerp(double t) => BorderRadius.lerp(begin, end, t);
 }
 
+/// An interpolation between two [Border]s.
+///
+/// This class specializes the interpolation of [Tween<Border>] to use
+/// [Border.lerp].
+///
+/// See [Tween] for a discussion on how to use interpolation objects.
+class BorderTween extends Tween<Border> {
+  /// Creates a [Border] tween.
+  ///
+  /// The [begin] and [end] properties may be null; the null value
+  /// is treated as having no border.
+  BorderTween({ Border begin, Border end }) : super(begin: begin, end: end);
+
+  /// Returns the value this variable has at the given animation clock value.
+  @override
+  Border lerp(double t) => Border.lerp(begin, end, t);
+}
+
 /// An interpolation between two [Matrix4]s.
 ///
 /// This class specializes the interpolation of [Tween<Matrix4>] to be

packages/flutter/lib/src/widgets/sliver_persistent_header.dart

@@ -188,6 +188,7 @@ class _SliverPersistentHeaderElement extends RenderObjectElement {
 
   @override
   void performRebuild() {
+    super.performRebuild();
     renderObject.triggerRebuild();
   }
 

packages/flutter/lib/src/widgets/transitions.dart

@@ -10,6 +10,7 @@ import 'package:vector_math/vector_math_64.dart' show Matrix4;
 import 'basic.dart';
 import 'container.dart';
 import 'framework.dart';
+import 'text.dart';
 
 export 'package:flutter/rendering.dart' show RelativeRect;
 
@@ -671,6 +672,64 @@ class AlignTransition extends AnimatedWidget {
   }
 }
 
+/// Animated version of a [DefaultTextStyle] that animates the different properties
+/// of its [TextStyle].
+///
+/// See also:
+///
+/// * [DefaultTextStyle], which also defines a [TextStyle] for its descendants
+///   but is not animated.
+class DefaultTextStyleTransition extends AnimatedWidget {
+  /// Creates an animated [DefaultTextStyle] whose [TextStyle] animation updates
+  /// the widget.
+  const DefaultTextStyleTransition({
+    Key key,
+    @required Animation<TextStyle> style,
+    @required this.child,
+    this.textAlign,
+    this.softWrap = true,
+    this.overflow = TextOverflow.clip,
+    this.maxLines,
+  }) : super(key: key, listenable: style);
+
+  /// The animation that controls the descendants' text style.
+  Animation<TextStyle> get style => listenable;
+
+  /// How the text should be aligned horizontally.
+  final TextAlign textAlign;
+
+  /// Whether the text should break at soft line breaks.
+  ///
+  /// See [DefaultTextStyle.softWrap] for more details.
+  final bool softWrap;
+
+  /// How visual overflow should be handled.
+  ///
+  final TextOverflow overflow;
+
+  /// An optional maximum number of lines for the text to span, wrapping if necessary.
+  ///
+  /// See [DefaultTextStyle.maxLines] for more details.
+  final int maxLines;
+
+  /// The widget below this widget in the tree.
+  ///
+  /// {@macro flutter.widgets.child}
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    return new DefaultTextStyle(
+      style: style.value,
+      textAlign: textAlign,
+      softWrap: softWrap,
+      overflow: overflow,
+      maxLines: maxLines,
+      child: child,
+    );
+  }
+}
+
 /// A general-purpose widget for building animations.
 ///
 /// AnimatedBuilder is useful for more complex widgets that wish to include

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -306,6 +306,65 @@ void main() {
     expect(tester.getSize(find.widgetWithText(OverflowBox, 'Title')).height, 0.0);
   });
 
+  testWidgets('User specified middle is always visible in sliver', (WidgetTester tester) async {
+    final ScrollController scrollController = new ScrollController();
+    final Key segmentedControlsKey = new UniqueKey();
+    await tester.pumpWidget(
+      new CupertinoApp(
+        home: new CupertinoPageScaffold(
+          child: new CustomScrollView(
+            controller: scrollController,
+            slivers: <Widget>[
+              new CupertinoSliverNavigationBar(
+                middle: new ConstrainedBox(
+                  constraints: const BoxConstraints(maxWidth: 200.0),
+                  child: new CupertinoSegmentedControl<int>(
+                    key: segmentedControlsKey,
+                    children: const <int, Widget>{
+                      0: Text('Option A'),
+                      1: Text('Option B'),
+                    },
+                    onValueChanged: (int selected) { },
+                    groupValue: 0,
+                  ),
+                ),
+                largeTitle: const Text('Title'),
+              ),
+              new SliverToBoxAdapter(
+                child: new Container(
+                  height: 1200.0,
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(scrollController.offset, 0.0);
+    expect(tester.getTopLeft(find.byType(NavigationToolbar)).dy, 0.0);
+    expect(tester.getSize(find.byType(NavigationToolbar)).height, 44.0);
+
+    expect(find.text('Title'), findsOneWidget);
+    expect(tester.getCenter(find.byKey(segmentedControlsKey)).dx, 400.0);
+
+    expect(tester.getTopLeft(find.widgetWithText(OverflowBox, 'Title')).dy, 44.0);
+    expect(tester.getSize(find.widgetWithText(OverflowBox, 'Title')).height, 52.0);
+
+    scrollController.jumpTo(600.0);
+    await tester.pump(); // Once to trigger the opacity animation.
+    await tester.pump(const Duration(milliseconds: 300));
+
+    expect(tester.getCenter(find.byKey(segmentedControlsKey)).dx, 400.0);
+    // The large title is invisible now.
+    expect(
+      tester.renderObject<RenderAnimatedOpacity>(
+        find.widgetWithText(AnimatedOpacity, 'Title')
+      ).opacity.value,
+      0.0,
+    );
+  });
+
   testWidgets('Small title can be overridden', (WidgetTester tester) async {
     final ScrollController scrollController = new ScrollController();
     await tester.pumpWidget(
@@ -390,7 +449,7 @@ void main() {
     ));
 
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 500));
 
     expect(find.byType(CupertinoButton), findsOneWidget);
     expect(find.text(new String.fromCharCode(CupertinoIcons.back.codePoint)), findsOneWidget);
@@ -405,23 +464,22 @@ void main() {
     ));
 
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 500));
 
-    expect(find.byType(CupertinoButton), findsNWidgets(2));
-    expect(find.text('Close'), findsOneWidget);
+    expect(find.widgetWithText(CupertinoButton, 'Close'), findsOneWidget);
 
     // Test popping goes back correctly.
     await tester.tap(find.text('Close'));
 
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 500));
 
     expect(find.text('Page 2'), findsOneWidget);
 
     await tester.tap(find.text(new String.fromCharCode(CupertinoIcons.back.codePoint)));
 
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 500));
 
     expect(find.text('Home page'), findsOneWidget);
   });
@@ -438,7 +496,7 @@ void main() {
         builder: (BuildContext context) {
           return const CupertinoPageScaffold(
             navigationBar: CupertinoNavigationBar(
-              previousPageTitle: '0123456789',
+              previousPageTitle: '012345678901',
             ),
             child: Placeholder(),
           );
@@ -449,14 +507,14 @@ void main() {
     await tester.pump();
     await tester.pump(const Duration(milliseconds: 500));
 
-    expect(find.widgetWithText(CupertinoButton, '0123456789'), findsOneWidget);
+    expect(find.widgetWithText(CupertinoButton, '012345678901'), findsOneWidget);
 
     tester.state<NavigatorState>(find.byType(Navigator)).push(
       new CupertinoPageRoute<void>(
         builder: (BuildContext context) {
           return const CupertinoPageScaffold(
             navigationBar: CupertinoNavigationBar(
-              previousPageTitle: '01234567890',
+              previousPageTitle: '0123456789012',
             ),
             child: Placeholder(),
           );

packages/flutter/test/cupertino/nav_bar_transition_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+Future<void> startTransitionBetween(
+  WidgetTester tester, {
+  Widget from,
+  Widget to,
+  String fromTitle,
+  String toTitle,
+}) async {
+  await tester.pumpWidget(
+    new CupertinoApp(
+      home: const Placeholder(),
+    ),
+  );
+
+  tester
+      .state<NavigatorState>(find.byType(Navigator))
+      .push(new CupertinoPageRoute<void>(
+        title: fromTitle,
+        builder: (BuildContext context) => scaffoldForNavBar(from),
+      ));
+
+  await tester.pump();
+  await tester.pump(const Duration(milliseconds: 500));
+
+  tester
+      .state<NavigatorState>(find.byType(Navigator))
+      .push(new CupertinoPageRoute<void>(
+        title: toTitle,
+        builder: (BuildContext context) => scaffoldForNavBar(to),
+      ));
+
+  await tester.pump();
+}
+
+CupertinoPageScaffold scaffoldForNavBar(Widget navBar) {
+  if (navBar is CupertinoNavigationBar || navBar == null) {
+    return new CupertinoPageScaffold(
+      navigationBar: navBar ?? const CupertinoNavigationBar(),
+      child: const Placeholder(),
+    );
+  } else if (navBar is CupertinoSliverNavigationBar) {
+    return new CupertinoPageScaffold(
+      child: new CustomScrollView(
+        slivers: <Widget>[
+          navBar,
+          // Add filler so it's scrollable.
+          const SliverToBoxAdapter(
+            child: Placeholder(fallbackHeight: 1000.0),
+          ),
+        ],
+      ),
+    );
+  }
+  assert(false, 'Unexpected nav bar type ${navBar.runtimeType}');
+  return null;
+}
+
+Finder flying(WidgetTester tester, Finder finder) {
+  final RenderObjectWithChildMixin<RenderStack> theater =
+      tester.renderObject(find.byType(Overlay));
+  final RenderStack theaterStack = theater.child;
+  final Finder lastOverlayFinder = find.byElementPredicate((Element element) {
+    return element is RenderObjectElement &&
+        element.renderObject == theaterStack.lastChild;
+  });
+
+  assert(
+      find
+              .descendant(
+                of: lastOverlayFinder,
+                matching: find.byWidgetPredicate(
+                  (Widget widget) =>
+                      widget.runtimeType.toString() ==
+                      '_NavigationBarTransition',
+                ),
+              )
+              .evaluate()
+              .length ==
+          1,
+      'The last overlay in the navigator was not a flying hero',);
+
+  return find.descendant(
+    of: lastOverlayFinder,
+    matching: finder,
+  );
+}
+
+void checkBackgroundBoxHeight(WidgetTester tester, double height) {
+  final Widget transitionBackgroundBox =
+      tester.widget<Stack>(flying(tester, find.byType(Stack))).children[0];
+  expect(
+    tester
+        .widget<SizedBox>(
+          find.descendant(
+            of: find.byWidget(transitionBackgroundBox),
+            matching: find.byType(SizedBox),
+          ),
+        )
+        .height,
+    height,
+  );
+}
+
+void checkOpacity(WidgetTester tester, Finder finder, double opacity) {
+  expect(
+    tester
+        .renderObject<RenderAnimatedOpacity>(find.ancestor(
+          of: finder,
+          matching: find.byType(FadeTransition),
+        ))
+        .opacity
+        .value,
+    opacity,
+  );
+}
+
+void main() {
+  testWidgets('Bottom middle moves between middle and back label',
+      (WidgetTester tester) async {
+    await startTransitionBetween(tester, fromTitle: 'Page 1');
+
+    // Be mid-transition.
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // There's 2 of them. One from the top route's back label and one from the
+    // bottom route's middle widget.
+    expect(flying(tester, find.text('Page 1')), findsNWidgets(2));
+
+    // Since they have the same text, they should be more or less at the same
+    // place.
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1')).first),
+      const Offset(331.0724935531616, 13.5),
+    );
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1')).last),
+      const Offset(331.0724935531616, 13.5),
+    );
+  });
+
+  testWidgets('Bottom middle and top back label transitions their font',
+      (WidgetTester tester) async {
+    await startTransitionBetween(tester, fromTitle: 'Page 1');
+
+    // Be mid-transition.
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // The transition's stack is ordered. The bottom middle is inserted first.
+    final RenderParagraph bottomMiddle =
+        tester.renderObject(flying(tester, find.text('Page 1')).first);
+    expect(bottomMiddle.text.style.color, const Color(0xFF00070F));
+    expect(bottomMiddle.text.style.fontWeight, FontWeight.w600);
+    expect(bottomMiddle.text.style.fontFamily, '.SF UI Text');
+    expect(bottomMiddle.text.style.letterSpacing, -0.08952957153320312);
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 1')).first, 0.8609542846679688);
+
+    // The top back label is styled exactly the same way. But the opacity tweens
+    // are flipped.
+    final RenderParagraph topBackLabel =
+        tester.renderObject(flying(tester, find.text('Page 1')).last);
+    expect(topBackLabel.text.style.color, const Color(0xFF00070F));
+    expect(topBackLabel.text.style.fontWeight, FontWeight.w600);
+    expect(topBackLabel.text.style.fontFamily, '.SF UI Text');
+    expect(topBackLabel.text.style.letterSpacing, -0.08952957153320312);
+
+    checkOpacity(tester, flying(tester, find.text('Page 1')).last, 0.0);
+
+    // Move animation further a bit.
+    await tester.pump(const Duration(milliseconds: 200));
+    expect(bottomMiddle.text.style.color, const Color(0xFF0073F0));
+    expect(bottomMiddle.text.style.fontWeight, FontWeight.w400);
+    expect(bottomMiddle.text.style.fontFamily, '.SF UI Text');
+    expect(bottomMiddle.text.style.letterSpacing, -0.231169798374176);
+
+    checkOpacity(tester, flying(tester, find.text('Page 1')).first, 0.0);
+
+    expect(topBackLabel.text.style.color, const Color(0xFF0073F0));
+    expect(topBackLabel.text.style.fontWeight, FontWeight.w400);
+    expect(topBackLabel.text.style.fontFamily, '.SF UI Text');
+    expect(topBackLabel.text.style.letterSpacing, -0.231169798374176);
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 1')).last, 0.8733493089675903);
+  });
+
+  testWidgets('Fullscreen dialogs do not create heroes',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new CupertinoApp(
+        home: const Placeholder(),
+      ),
+    );
+
+    tester
+        .state<NavigatorState>(find.byType(Navigator))
+        .push(new CupertinoPageRoute<void>(
+          title: 'Page 1',
+          builder: (BuildContext context) => scaffoldForNavBar(null),
+        ));
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    tester
+        .state<NavigatorState>(find.byType(Navigator))
+        .push(new CupertinoPageRoute<void>(
+          title: 'Page 2',
+          fullscreenDialog: true,
+          builder: (BuildContext context) => scaffoldForNavBar(null),
+        ));
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 100));
+
+    // Only the first (non-fullscreen-dialog) page has a Hero.
+    expect(find.byType(Hero), findsOneWidget);
+    // No Hero transition happened.
+    expect(() => flying(tester, find.text('Page 2')), throwsAssertionError);
+  });
+
+  testWidgets('Turning off transition works', (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoNavigationBar(
+        transitionBetweenRoutes: false,
+        middle: Text('Page 1'),
+      ),
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // Only the second page that doesn't have the transitionBetweenRoutes
+    // override off has a Hero.
+    expect(find.byType(Hero), findsOneWidget);
+    expect(
+      find.descendant(of: find.byType(Hero), matching: find.text('Page 2')),
+      findsOneWidget,
+    );
+
+    // No Hero transition happened.
+    expect(() => flying(tester, find.text('Page 2')), throwsAssertionError);
+  });
+
+  testWidgets('Popping mid-transition is symmetrical',
+      (WidgetTester tester) async {
+    await startTransitionBetween(tester, fromTitle: 'Page 1');
+
+    // Be mid-transition.
+    await tester.pump(const Duration(milliseconds: 50));
+
+    void checkColorAndPositionAt50ms() {
+      // The transition's stack is ordered. The bottom middle is inserted first.
+      final RenderParagraph bottomMiddle =
+          tester.renderObject(flying(tester, find.text('Page 1')).first);
+      expect(bottomMiddle.text.style.color, const Color(0xFF00070F));
+      expect(
+        tester.getTopLeft(flying(tester, find.text('Page 1')).first),
+        const Offset(331.0724935531616, 13.5),
+      );
+
+      // The top back label is styled exactly the same way. But the opacity tweens
+      // are flipped.
+      final RenderParagraph topBackLabel =
+          tester.renderObject(flying(tester, find.text('Page 1')).last);
+      expect(topBackLabel.text.style.color, const Color(0xFF00070F));
+      expect(
+        tester.getTopLeft(flying(tester, find.text('Page 1')).last),
+        const Offset(331.0724935531616, 13.5),
+      );
+    }
+
+    checkColorAndPositionAt50ms();
+
+    // Advance more.
+    await tester.pump(const Duration(milliseconds: 100));
+
+    // Pop and reverse the same amount of time.
+    tester.state<NavigatorState>(find.byType(Navigator)).pop();
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 100));
+
+    // Check that everything's the same as on the way in.
+    checkColorAndPositionAt50ms();
+  });
+
+  testWidgets('There should be no global keys in the hero flight',
+      (WidgetTester tester) async {
+    await startTransitionBetween(tester, fromTitle: 'Page 1');
+
+    // Be mid-transition.
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(
+      flying(
+        tester,
+        find.byWidgetPredicate((Widget widget) => widget.key != null),
+      ),
+      findsNothing,
+    );
+  });
+
+  testWidgets('Transition box grows to large title size',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+      to: const CupertinoSliverNavigationBar(),
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 47.097110748291016);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 61.0267448425293);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 78.68475294113159);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 88.32722091674805);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 93.13018447160721);
+  });
+
+  testWidgets('Large transition box shrinks to standard nav bar size',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(),
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 92.90288925170898);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 78.9732551574707);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 61.31524705886841);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 51.67277908325195);
+
+    await tester.pump(const Duration(milliseconds: 50));
+    checkBackgroundBoxHeight(tester, 46.86981552839279);
+  });
+
+  testWidgets('Hero flight removed at the end of page transition',
+      (WidgetTester tester) async {
+    await startTransitionBetween(tester, fromTitle: 'Page 1');
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // There's 2 of them. One from the top route's back label and one from the
+    // bottom route's middle widget.
+    expect(flying(tester, find.text('Page 1')), findsNWidgets(2));
+
+    // End the transition.
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(() => flying(tester, find.text('Page 1')), throwsAssertionError);
+  });
+
+  testWidgets('Exact widget is reused to build inside the transition',
+      (WidgetTester tester) async {
+    const Widget userMiddle = Placeholder();
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(
+        middle: userMiddle,
+      ),
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(flying(tester, find.byWidget(userMiddle)), findsOneWidget);
+  });
+
+  testWidgets('First appearance of back chevron fades in from the right',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new CupertinoApp(
+        home: scaffoldForNavBar(null),
+      ),
+    );
+
+    tester
+        .state<NavigatorState>(find.byType(Navigator))
+        .push(new CupertinoPageRoute<void>(
+          title: 'Page 1',
+          builder: (BuildContext context) => scaffoldForNavBar(null),
+        ));
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 50));
+
+    final Finder backChevron = flying(tester,
+        find.text(new String.fromCharCode(CupertinoIcons.back.codePoint)));
+
+    expect(
+      backChevron,
+      // Only one exists from the top page. The bottom page has no back chevron.
+      findsOneWidget,
+    );
+
+    // Come in from the right and fade in.
+    checkOpacity(tester, backChevron, 0.0);
+    expect(
+        tester.getTopLeft(backChevron), const Offset(71.94993209838867, 5.0));
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, backChevron, 0.32467134296894073);
+    expect(
+        tester.getTopLeft(backChevron), const Offset(18.033634185791016, 5.0));
+  });
+
+  testWidgets('Back chevron fades out and in when both pages have it',
+      (WidgetTester tester) async {
+    await startTransitionBetween(tester, fromTitle: 'Page 1');
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    final Finder backChevrons = flying(tester,
+        find.text(new String.fromCharCode(CupertinoIcons.back.codePoint)));
+
+    expect(
+      backChevrons,
+      findsNWidgets(2),
+    );
+
+    checkOpacity(tester, backChevrons.first, 0.8393326997756958);
+    checkOpacity(tester, backChevrons.last, 0.0);
+    // Both overlap at the same place.
+    expect(tester.getTopLeft(backChevrons.first), const Offset(8.0, 5.0));
+    expect(tester.getTopLeft(backChevrons.last), const Offset(8.0, 5.0));
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, backChevrons.first, 0.0);
+    checkOpacity(tester, backChevrons.last, 0.6276369094848633);
+    // Still in the same place.
+    expect(tester.getTopLeft(backChevrons.first), const Offset(8.0, 5.0));
+    expect(tester.getTopLeft(backChevrons.last), const Offset(8.0, 5.0));
+  });
+
+  testWidgets('Bottom middle just fades if top page has a custom leading',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+      to: const CupertinoSliverNavigationBar(
+        leading: Text('custom'),
+      ),
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // There's just 1 in flight because there's no back label on the top page.
+    expect(flying(tester, find.text('Page 1')), findsOneWidget);
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 1')), 0.8609542846679688);
+
+    // The middle widget doesn't move.
+    expect(
+      tester.getCenter(flying(tester, find.text('Page 1'))),
+      const Offset(400.0, 22.0),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, flying(tester, find.text('Page 1')), 0.0);
+    expect(
+      tester.getCenter(flying(tester, find.text('Page 1'))),
+      const Offset(400.0, 22.0),
+    );
+  });
+
+  testWidgets('Bottom leading fades in place', (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(leading: Text('custom')),
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(flying(tester, find.text('custom')), findsOneWidget);
+
+    checkOpacity(
+        tester, flying(tester, find.text('custom')), 0.7655444294214249);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('custom'))),
+      const Offset(16.0, 0.0),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, flying(tester, find.text('custom')), 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('custom'))),
+      const Offset(16.0, 0.0),
+    );
+  });
+
+  testWidgets('Bottom trailing fades in place', (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(trailing: Text('custom')),
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(flying(tester, find.text('custom')), findsOneWidget);
+
+    checkOpacity(
+        tester, flying(tester, find.text('custom')), 0.8393326997756958);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('custom'))),
+      const Offset(683.0, 13.5),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, flying(tester, find.text('custom')), 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('custom'))),
+      const Offset(683.0, 13.5),
+    );
+  });
+
+  testWidgets('Bottom back label fades and slides to the left',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 500));
+    tester
+        .state<NavigatorState>(find.byType(Navigator))
+        .push(new CupertinoPageRoute<void>(
+          title: 'Page 3',
+          builder: (BuildContext context) => scaffoldForNavBar(null),
+        ));
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // 'Page 1' appears once on Page 2 as the back label.
+    expect(flying(tester, find.text('Page 1')), findsOneWidget);
+
+    // Back label fades out faster.
+    checkOpacity(
+        tester, flying(tester, find.text('Page 1')), 0.5584745407104492);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1'))),
+      const Offset(24.176071166992188, 13.5),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, flying(tester, find.text('Page 1')), 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1'))),
+      const Offset(-292.97862243652344, 13.5),
+    );
+  });
+
+  testWidgets('Bottom large title moves to top back label',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(),
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // There's 2, one from the bottom large title fading out and one from the
+    // bottom back label fading in.
+    expect(flying(tester, find.text('Page 1')), findsNWidgets(2));
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 1')).first, 0.8393326997756958);
+    checkOpacity(tester, flying(tester, find.text('Page 1')).last, 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1')).first),
+      const Offset(17.905914306640625, 51.58156871795654),
+    );
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1')).last),
+      const Offset(17.905914306640625, 51.58156871795654),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(tester, flying(tester, find.text('Page 1')).first, 0.0);
+    checkOpacity(
+        tester, flying(tester, find.text('Page 1')).last, 0.6276369094848633);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1')).first),
+      const Offset(43.278289794921875, 19.23011875152588),
+    );
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 1')).last),
+      const Offset(43.278289794921875, 19.23011875152588),
+    );
+  });
+
+  testWidgets('Long title turns into the word back mid transition',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(),
+      fromTitle: 'A title too long to fit',
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(
+        flying(tester, find.text('A title too long to fit')), findsOneWidget);
+    // Automatically changed to the word 'Back' in the back label.
+    expect(flying(tester, find.text('Back')), findsOneWidget);
+
+    checkOpacity(tester, flying(tester, find.text('A title too long to fit')),
+        0.8393326997756958);
+    checkOpacity(tester, flying(tester, find.text('Back')), 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('A title too long to fit'))),
+      const Offset(17.905914306640625, 51.58156871795654),
+    );
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Back'))),
+      const Offset(17.905914306640625, 51.58156871795654),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+    checkOpacity(
+        tester, flying(tester, find.text('A title too long to fit')), 0.0);
+    checkOpacity(tester, flying(tester, find.text('Back')), 0.6276369094848633);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('A title too long to fit'))),
+      const Offset(43.278289794921875, 19.23011875152588),
+    );
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Back'))),
+      const Offset(43.278289794921875, 19.23011875152588),
+    );
+  });
+
+  testWidgets('Bottom large title and top back label transitions their font',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      from: const CupertinoSliverNavigationBar(),
+      fromTitle: 'Page 1',
+    );
+
+    // Be mid-transition.
+    await tester.pump(const Duration(milliseconds: 50));
+
+    // The transition's stack is ordered. The bottom large title is inserted first.
+    final RenderParagraph bottomLargeTitle =
+        tester.renderObject(flying(tester, find.text('Page 1')).first);
+    expect(bottomLargeTitle.text.style.color, const Color(0xFF00070F));
+    expect(bottomLargeTitle.text.style.fontWeight, FontWeight.w700);
+    expect(bottomLargeTitle.text.style.fontFamily, '.SF Pro Display');
+    expect(bottomLargeTitle.text.style.letterSpacing, 0.21141128540039061);
+
+    // The top back label is styled exactly the same way.
+    final RenderParagraph topBackLabel =
+        tester.renderObject(flying(tester, find.text('Page 1')).last);
+    expect(topBackLabel.text.style.color, const Color(0xFF00070F));
+    expect(topBackLabel.text.style.fontWeight, FontWeight.w700);
+    expect(topBackLabel.text.style.fontFamily, '.SF Pro Display');
+    expect(topBackLabel.text.style.letterSpacing, 0.21141128540039061);
+
+    // Move animation further a bit.
+    await tester.pump(const Duration(milliseconds: 200));
+    expect(bottomLargeTitle.text.style.color, const Color(0xFF0073F0));
+    expect(bottomLargeTitle.text.style.fontWeight, FontWeight.w400);
+    expect(bottomLargeTitle.text.style.fontFamily, '.SF UI Text');
+    expect(bottomLargeTitle.text.style.letterSpacing, -0.2135093951225281);
+
+    expect(topBackLabel.text.style.color, const Color(0xFF0073F0));
+    expect(topBackLabel.text.style.fontWeight, FontWeight.w400);
+    expect(topBackLabel.text.style.fontFamily, '.SF UI Text');
+    expect(topBackLabel.text.style.letterSpacing, -0.2135093951225281);
+  });
+
+  testWidgets('Top middle fades in and slides in from the right',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(flying(tester, find.text('Page 2')), findsOneWidget);
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 2')), 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(725.1760711669922, 13.5),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 2')), 0.6972532719373703);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(408.02137756347656, 13.5),
+    );
+  });
+
+  testWidgets('Top large title fades in and slides in from the right',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      to: const CupertinoSliverNavigationBar(),
+      toTitle: 'Page 2',
+    );
+
+    await tester.pump(const Duration(milliseconds: 50));
+
+    expect(flying(tester, find.text('Page 2')), findsOneWidget);
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 2')), 0.0);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(768.3521423339844, 54.0),
+    );
+
+    await tester.pump(const Duration(milliseconds: 150));
+
+    checkOpacity(
+        tester, flying(tester, find.text('Page 2')), 0.6753286570310593);
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(134.04275512695312, 54.0),
+    );
+  });
+
+  testWidgets('Components are not unnecessarily rebuilt during transitions',
+          (WidgetTester tester) async {
+    int bottomBuildTimes = 0;
+    int topBuildTimes = 0;
+    await startTransitionBetween(
+      tester,
+      from: new CupertinoNavigationBar(
+        middle: new Builder(builder: (BuildContext context) {
+          bottomBuildTimes++;
+          return const Text('Page 1');
+        }),
+      ),
+      to: new CupertinoSliverNavigationBar(
+        largeTitle: new Builder(builder: (BuildContext context) {
+          topBuildTimes++;
+          return const Text('Page 2');
+        }),
+      ),
+    );
+
+    expect(bottomBuildTimes, 1);
+    // RenderSliverPersistentHeader.layoutChild causes 2 builds.
+    expect(topBuildTimes, 2);
+
+    await tester.pump();
+
+    // The shuttle builder builds the component widgets one more time.
+    expect(bottomBuildTimes, 2);
+    expect(topBuildTimes, 3);
+
+    // Subsequent animation needs to use reprojection of children.
+    await tester.pump();
+    expect(bottomBuildTimes, 2);
+    expect(topBuildTimes, 3);
+
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(bottomBuildTimes, 2);
+    expect(topBuildTimes, 3);
+
+    // Finish animations.
+    await tester.pump(const Duration(milliseconds: 400));
+
+    expect(bottomBuildTimes, 2);
+    expect(topBuildTimes, 3);
+  });
+}

packages/flutter/test/cupertino/route_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -36,6 +37,71 @@ void main() {
     expect(tester.getCenter(find.text('An iPod')).dx, 400.0);
   });
 
+  testWidgets('Large title auto-populates with title', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new CupertinoApp(
+        home: const Placeholder(),
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).push(
+      new CupertinoPageRoute<void>(
+        title: 'An iPod',
+        builder: (BuildContext context) {
+          return new CupertinoPageScaffold(
+            child: new CustomScrollView(
+              slivers: const <Widget>[
+                CupertinoSliverNavigationBar(),
+              ],
+            ),
+          );
+        }
+      )
+    );
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // There should be 2 Text widget with the title in the nav bar. One in the
+    // large title position and one in the middle position (though the middle
+    // position Text is initially invisible while the sliver is expanded).
+    expect(
+      find.widgetWithText(CupertinoSliverNavigationBar, 'An iPod'),
+      findsNWidgets(2),
+    );
+
+    final List<Element> titles = tester.elementList(find.text('An iPod'))
+        .toList()
+        ..sort((Element a, Element b) {
+          final RenderParagraph aParagraph = a.renderObject;
+          final RenderParagraph bParagraph = b.renderObject;
+          return aParagraph.text.style.fontSize.compareTo(
+            bParagraph.text.style.fontSize
+          );
+        });
+
+    final Iterable<double> opacities = titles.map((Element element) {
+      final RenderAnimatedOpacity renderOpacity =
+          element.ancestorRenderObjectOfType(const TypeMatcher<RenderAnimatedOpacity>());
+      return renderOpacity.opacity.value;
+    });
+
+    expect(opacities, <double> [
+        0.0, // Initially the smaller font title is invisible.
+        1.0, // The larger font title is visible.
+    ]);
+
+    // Check that the large font title is at the right spot.
+    expect(
+      tester.getTopLeft(find.byWidget(titles[1].widget)),
+      const Offset(16.0, 54.0),
+    );
+
+    // The smaller, initially invisible title, should still be positioned in the
+    // center.
+    expect(tester.getCenter(find.byWidget(titles[0].widget)).dx, 400.0);
+  });
+
   testWidgets('Leading auto-populates with back button with previous title', (WidgetTester tester) async {
     await tester.pumpWidget(
       new CupertinoApp(

packages/flutter/test/cupertino/scaffold_test.dart

@@ -239,7 +239,7 @@ void main() {
     // Navigate in tab 2.
     await tester.tap(find.text('Next'));
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 300));
+    await tester.pump(const Duration(milliseconds: 500));
 
     expect(find.text('Page 2 of tab 2'), isOnstage);
     expect(find.text('Page 1 of tab 1', skipOffstage: false), isOffstage);
@@ -254,7 +254,7 @@ void main() {
     // Navigate in tab 1.
     await tester.tap(find.text('Next'));
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 300));
+    await tester.pump(const Duration(milliseconds: 500));
 
     expect(find.text('Page 2 of tab 1'), isOnstage);
     expect(find.text('Page 2 of tab 2', skipOffstage: false), isOffstage);
@@ -268,7 +268,7 @@ void main() {
     // Pop in tab 2
     await tester.tap(find.text('Back'));
     await tester.pump();
-    await tester.pump(const Duration(milliseconds: 300));
+    await tester.pump(const Duration(milliseconds: 500));
 
     expect(find.text('Page 1 of tab 2'), isOnstage);
     expect(find.text('Page 2 of tab 1', skipOffstage: false), isOffstage);

packages/flutter/test/widgets/heroes_test.dart

@@ -1244,4 +1244,95 @@ void main() {
     await tester.pump(duration * 0.1);
     expect(tester.getTopLeft(find.byKey(firstKey)).dx, x0);
   });
+
+  testWidgets('Can override flight shuttle', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: new Material(
+        child: new ListView(
+          children: <Widget>[
+            const Hero(tag: 'a', child: Text('foo')),
+            new Builder(builder: (BuildContext context) {
+              return new FlatButton(
+                child: const Text('two'),
+                onPressed: () => Navigator.push<void>(context, new MaterialPageRoute<void>(
+                  builder: (BuildContext context) {
+                    return new Material(
+                      child: new Hero(
+                        tag: 'a',
+                        child: const Text('bar'),
+                        flightShuttleBuilder: (
+                          BuildContext flightContext,
+                          Animation<double> animation,
+                          HeroFlightDirection flightDirection,
+                          BuildContext fromHeroContext,
+                          BuildContext toHeroContext,
+                        ) {
+                          return const Text('baz');
+                        },
+                      ),
+                    );
+                  },
+                )),
+              );
+            }),
+          ],
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text('two'));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 10));
+
+    expect(find.text('foo'), findsNothing);
+    expect(find.text('bar'), findsNothing);
+    expect(find.text('baz'), findsOneWidget);
+  });
+
+  testWidgets('Can override flight launch pads', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: new Material(
+        child: new ListView(
+          children: <Widget>[
+            new Hero(
+              tag: 'a',
+              child: const Text('Batman'),
+              placeholderBuilder: (BuildContext context, Widget child) {
+                return const Text('Venom');
+              },
+            ),
+            new Builder(builder: (BuildContext context) {
+              return new FlatButton(
+                child: const Text('two'),
+                onPressed: () => Navigator.push<void>(context, new MaterialPageRoute<void>(
+                  builder: (BuildContext context) {
+                    return new Material(
+                      child: new Hero(
+                        tag: 'a',
+                        child: const Text('Wolverine'),
+                        placeholderBuilder: (BuildContext context, Widget child) {
+                          return const Text('Joker');
+                        },
+                      ),
+                    );
+                  },
+                )),
+              );
+            }),
+          ],
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text('two'));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 10));
+
+    expect(find.text('Batman'), findsNothing);
+    // This shows up once but in the Hero because by default, the destination
+    // Hero child is the widget in flight.
+    expect(find.text('Wolverine'), findsOneWidget);
+    expect(find.text('Venom'), findsOneWidget);
+    expect(find.text('Joker'), findsOneWidget);
+  });
 }