Add iOS 11 style large titles to cupertino nav bar (#12002)

* Things lay out but the effects not right yet

* Remaining functionalities and tests

* one line large title only

* Add more docs

* review

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

@@ -9,12 +9,32 @@ import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
 
-// Standard iOS 10 nav bar height without the status bar.
-const double _kNavBarHeight = 44.0;
+/// Standard iOS nav bar height without the status bar.
+const double _kNavBarPersistentHeight = 44.0;
+
+/// Size increase from expanding the nav bar into an iOS 11 style large title
+/// form in a [CustomScrollView].
+const double _kNavBarLargeTitleHeightExtension = 56.0;
+
+/// Number of logical pixels scrolled down before the title text is transferred
+/// from the normal nav bar to a big title below the nav bar.
+const double _kNavBarShowLargeTitleThreshold = 10.0;
+
+const double _kNavBarEdgePadding = 16.0;
+
+/// Title text transfer fade.
+const Duration _kNavBarTitleFadeDuration = const Duration(milliseconds: 150);
 
 const Color _kDefaultNavBarBackgroundColor = const Color(0xCCF8F8F8);
 const Color _kDefaultNavBarBorderColor = const Color(0x4C000000);
 
+const TextStyle _kLargeTitleTextStyle = const TextStyle(
+  fontSize: 34.0,
+  fontWeight: FontWeight.bold,
+  letterSpacing: 0.41,
+  color: CupertinoColors.black,
+);
+
 /// An iOS-styled navigation bar.
 ///
 /// The navigation bar is a toolbar that minimally consists of a widget, normally
@@ -28,6 +48,10 @@ const Color _kDefaultNavBarBorderColor = const Color(0x4C000000);
 ///
 /// 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.
+///
+/// Enabling [largeTitle] will create a scrollable second row showing the title
+/// in a larger font introduced in iOS 11. The [middle] widget must be a text
+/// and the [CupertinoNavigationBar] must be placed in a sliver group in this case.
 //
 // TODO(xster): document automatic addition of a CupertinoBackButton.
 // TODO(xster): add sample code using icons.
@@ -41,6 +65,7 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
     this.trailing,
     this.backgroundColor: _kDefaultNavBarBackgroundColor,
     this.actionsForegroundColor: CupertinoColors.activeBlue,
+    this.largeTitle: false,
   }) : assert(middle != null, 'There must be a middle widget, usually a title.'),
        super(key: key);
 
@@ -73,8 +98,110 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
   /// True if the nav bar's background color has no transparency.
   bool get opaque => backgroundColor.alpha == 0xFF;
 
+  /// Use iOS 11 style large title navigation bars.
+  ///
+  /// When true, the navigation bar will split into 2 sections. The static
+  /// top 44px section will be wrapped in a SliverPersistentHeader and a
+  /// second scrollable section behind it will show and replace the `middle`
+  /// text in a larger font when scrolled down.
+  ///
+  /// Navigation bars with large titles must be used in a sliver group such
+  /// as [CustomScrollView].
+  final bool largeTitle;
+
+  @override
+  Size get preferredSize => const Size.fromHeight(_kNavBarPersistentHeight);
+
+  @override
+  Widget build(BuildContext context) {
+    assert(
+      !largeTitle || middle is Text,
+      "largeTitle mode is only possible when 'middle' is a Text widget",
+    );
+
+    if (!largeTitle) {
+      return _wrapWithBackground(
+        backgroundColor: backgroundColor,
+        child: new _CupertinoPersistentNavigationBar(
+          leading: leading,
+          middle: middle,
+          trailing: trailing,
+          actionsForegroundColor: actionsForegroundColor,
+        ),
+      );
+    } else {
+      return new SliverPersistentHeader(
+        pinned: true, // iOS navigation bars are always pinned.
+        delegate: new _CupertinoLargeTitleNavigationBarSliverDelegate(
+          persistentHeight: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
+          leading: leading,
+          middle: middle,
+          trailing: trailing,
+          backgroundColor: backgroundColor,
+          actionsForegroundColor: actionsForegroundColor,
+        ),
+      );
+    }
+  }
+}
+
+/// Returns `child` wrapped with background and a bottom border if background color
+/// is opaque. Otherwise, also blur with [BackdropFilter].
+Widget _wrapWithBackground({Color backgroundColor, Widget child}) {
+  final DecoratedBox childWithBackground = new DecoratedBox(
+    decoration: new BoxDecoration(
+      border: const Border(
+        bottom: const BorderSide(
+          color: _kDefaultNavBarBorderColor,
+          width: 0.0, // One physical pixel.
+          style: BorderStyle.solid,
+        ),
+      ),
+      color: backgroundColor,
+    ),
+    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,
+    ),
+  );
+}
+
+/// The top part of the nav bar that's never scrolled away.
+///
+/// Consists of the entire nav 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({
+    Key key,
+    this.leading,
+    @required this.middle,
+    this.trailing,
+    this.actionsForegroundColor,
+    this.middleVisible,
+  }) : super(key: key);
+
+  final Widget leading;
+
+  final Widget middle;
+
+  final Widget trailing;
+
+  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(_kNavBarHeight);
+  Size get preferredSize => const Size.fromHeight(_kNavBarPersistentHeight);
 
   @override
   Widget build(BuildContext context) {
@@ -101,21 +228,18 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
       child: middle,
     );
 
+    final Widget animatedStyledMiddle = middleVisible == null
+      ? styledMiddle
+      : new AnimatedOpacity(
+        opacity: middleVisible ? 1.0 : 0.0,
+        duration: _kNavBarTitleFadeDuration,
+        child: styledMiddle,
+      );
+
     // TODO(xster): automatically build a CupertinoBackButton.
 
-    Widget result = new DecoratedBox(
-      decoration: new BoxDecoration(
-        border: const Border(
-          bottom: const BorderSide(
-            color: _kDefaultNavBarBorderColor,
-            width: 0.0, // One physical pixel.
-            style: BorderStyle.solid,
-          ),
-        ),
-        color: backgroundColor,
-      ),
-      child: new SizedBox(
-        height: _kNavBarHeight + MediaQuery.of(context).padding.top,
+    return new SizedBox(
+      height: _kNavBarPersistentHeight + MediaQuery.of(context).padding.top,
       child: IconTheme.merge(
         data: new IconThemeData(
           color: actionsForegroundColor,
@@ -126,30 +250,117 @@ class CupertinoNavigationBar extends StatelessWidget implements PreferredSizeWid
             top: MediaQuery.of(context).padding.top,
             // TODO(xster): dynamically reduce padding when an automatic
             // CupertinoBackButton is present.
-              left: 16.0,
-              right: 16.0,
+            left: _kNavBarEdgePadding,
+            right: _kNavBarEdgePadding,
           ),
           child: new NavigationToolbar(
             leading: styledLeading,
-              middle: styledMiddle,
+            middle: animatedStyledMiddle,
             trailing: styledTrailing,
             centerMiddle: true,
           ),
         ),
       ),
-      ),
     );
+  }
+}
 
-    if (!opaque) {
-      // For non-opaque backgrounds, apply a blur effect.
-      result = new ClipRect(
-        child: new BackdropFilter(
-          filter: new ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),
-          child: result,
+class _CupertinoLargeTitleNavigationBarSliverDelegate extends SliverPersistentHeaderDelegate {
+  const _CupertinoLargeTitleNavigationBarSliverDelegate({
+    @required this.persistentHeight,
+    this.leading,
+    @required this.middle,
+    this.trailing,
+    this.backgroundColor,
+    this.actionsForegroundColor,
+  }) : assert(persistentHeight != null);
+
+  final double persistentHeight;
+
+  final Widget leading;
+
+  final Text middle;
+
+  final Widget trailing;
+
+  final Color backgroundColor;
+
+  final Color actionsForegroundColor;
+
+  @override
+  double get minExtent => persistentHeight;
+
+  @override
+  double get maxExtent => persistentHeight + _kNavBarLargeTitleHeightExtension;
+
+  @override
+  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
+    final bool showLargeTitle = shrinkOffset < maxExtent - minExtent - _kNavBarShowLargeTitleThreshold;
+
+    final _CupertinoPersistentNavigationBar persistentNavigationBar =
+        new _CupertinoPersistentNavigationBar(
+      leading: leading,
+      middle: middle,
+      trailing: trailing,
+      middleVisible: !showLargeTitle,
+      actionsForegroundColor: actionsForegroundColor,
+    );
+
+    return _wrapWithBackground(
+      backgroundColor: backgroundColor,
+      child: new Stack(
+        fit: StackFit.expand,
+        children: <Widget>[
+          new Positioned(
+            top: persistentHeight,
+            left: 0.0,
+            right: 0.0,
+            bottom: 0.0,
+            child: new ClipRect(
+              // The large title starts at the persistent bar.
+              // It's aligned with the bottom of the sliver and expands clipped
+              // and behind the persistent bar.
+              child: new OverflowBox(
+                minHeight: 0.0,
+                maxHeight: double.INFINITY,
+                alignment: FractionalOffsetDirectional.bottomStart,
+                child: new Padding(
+                  padding: const EdgeInsetsDirectional.only(
+                    start: _kNavBarEdgePadding,
+                    bottom: 8.0, // Bottom has a different padding.
+                  ),
+                  child: new DefaultTextStyle(
+                    style: _kLargeTitleTextStyle,
+                    maxLines: 1,
+                    overflow: TextOverflow.ellipsis,
+                    child: new AnimatedOpacity(
+                      opacity: showLargeTitle ? 1.0 : 0.0,
+                      duration: _kNavBarTitleFadeDuration,
+                      child: middle,
+                    )
+                  ),
+                ),
+              ),
+            ),
+          ),
+          new Positioned(
+            left: 0.0,
+            right: 0.0,
+            top: 0.0,
+            child: persistentNavigationBar,
+          ),
+        ],
       ),
     );
   }
 
-    return result;
+  @override
+  bool shouldRebuild(_CupertinoLargeTitleNavigationBarSliverDelegate oldDelegate) {
+    return persistentHeight != oldDelegate.persistentHeight ||
+        leading != oldDelegate.leading ||
+        middle != oldDelegate.middle ||
+        trailing != oldDelegate.trailing ||
+        backgroundColor != oldDelegate.backgroundColor ||
+        actionsForegroundColor != oldDelegate.actionsForegroundColor;
   }
 }

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

@@ -113,9 +113,10 @@ class _CupertinoScaffoldState extends State<CupertinoScaffold> {
   /// Pad the given middle widget with or without top and bottom offsets depending
   /// on whether the middle widget should slide behind translucent bars.
   Widget _padMiddle(Widget middle) {
-    double topPadding = MediaQuery.of(context).padding.top;
+    double topPadding = 0.0;
     if (widget.navigationBar is CupertinoNavigationBar) {
       final CupertinoNavigationBar top = widget.navigationBar;
+      topPadding += MediaQuery.of(context).padding.top;
       if (top.opaque)
         topPadding += top.preferredSize.height;
     }

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -4,7 +4,7 @@
 
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/rendering.dart';
-import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_test/flutter_test.dart' hide TypeMatcher;
 
 int count = 0;
 
@@ -14,9 +14,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return const CupertinoNavigationBar(
                 leading: const CupertinoButton(child: const Text('Something'), onPressed: null,),
                 middle: const Text('Title'),
@@ -36,9 +36,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return const CupertinoNavigationBar(
                 middle: const Text('Title'),
                 backgroundColor: const Color(0xFFE5E5E5),
@@ -56,9 +56,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return const CupertinoNavigationBar(
                 middle: const Text('Title'),
               );
@@ -76,9 +76,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return const CupertinoNavigationBar(
                 leading: const _ExpectStyles(color: const Color(0xFF001122), index: 0x000001),
                 middle: const _ExpectStyles(color: const Color(0xFF000000), index: 0x000100),
@@ -92,6 +92,118 @@ void main() {
     );
     expect(count, 0x010101);
   });
+
+  testWidgets('No slivers with no large titles', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          return new CupertinoPageRoute<Null>(
+            settings: settings,
+            builder: (BuildContext context) {
+              return const CupertinoScaffold(
+                navigationBar: const CupertinoNavigationBar(
+                  middle: const Text('Title'),
+                ),
+                child: const Center(),
+              );
+            },
+          );
+        },
+      ),
+    );
+
+    expect(find.byType(SliverPersistentHeader), findsNothing);
+  });
+
+  testWidgets('Large title nav bar scrolls', (WidgetTester tester) async {
+    final ScrollController scrollController = new ScrollController();
+    await tester.pumpWidget(
+      new WidgetsApp(
+        color: const Color(0xFFFFFFFF),
+        onGenerateRoute: (RouteSettings settings) {
+          return new CupertinoPageRoute<Null>(
+            settings: settings,
+            builder: (BuildContext context) {
+              return new CupertinoScaffold(
+                child: new CustomScrollView(
+                  controller: scrollController,
+                  slivers: <Widget>[
+                    const CupertinoNavigationBar(
+                      middle: const Text('Title'),
+                      largeTitle: true,
+                    ),
+                    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'), findsNWidgets(2)); // Though only one is visible.
+
+    List<Element> titles = tester.elementList(find.text('Title'))
+        .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);
+        });
+
+    Iterable<double> opacities = titles.map((Element element) {
+      final RenderOpacity renderOpacity = element.ancestorRenderObjectOfType(const TypeMatcher<RenderOpacity>());
+      return renderOpacity.opacity;
+    });
+
+    expect(opacities, <double> [
+        0.0, // Initially the smaller font title is invisible.
+        1.0, // The larger font title is visible.
+    ]);
+
+    expect(tester.getTopLeft(find.widgetWithText(OverflowBox, 'Title')).dy, 44.0);
+    expect(tester.getSize(find.widgetWithText(OverflowBox, 'Title')).height, 56.0);
+
+    scrollController.jumpTo(600.0);
+    await tester.pump(); // Once to trigger the opacity animation.
+    await tester.pump(const Duration(milliseconds: 300));
+
+    titles = tester.elementList(find.text('Title'))
+        .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);
+        });
+
+    opacities = titles.map((Element element) {
+      final RenderOpacity renderOpacity = element.ancestorRenderObjectOfType(const TypeMatcher<RenderOpacity>());
+      return renderOpacity.opacity;
+    });
+
+    expect(opacities, <double> [
+        1.0, // Smaller font title now visiblee
+        0.0, // Larger font title invisible.
+    ]);
+
+    // The persistent toolbar doesn't move or change size.
+    expect(tester.getTopLeft(find.byType(NavigationToolbar)).dy, 0.0);
+    expect(tester.getSize(find.byType(NavigationToolbar)).height, 44.0);
+
+    expect(tester.getTopLeft(find.widgetWithText(OverflowBox, 'Title')).dy, 44.0);
+    // The OverflowBox is squished with the text in it.
+    expect(tester.getSize(find.widgetWithText(OverflowBox, 'Title')).height, 0.0);
+  });
 }
 
 class _ExpectStyles extends StatelessWidget {

packages/flutter/test/cupertino/scaffold_test.dart

@@ -20,10 +20,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          // TODO(xster): change to a CupertinoPageRoute.
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return const CupertinoScaffold(
                 // Default nav bar is translucent.
                 navigationBar: const CupertinoNavigationBar(
@@ -47,10 +46,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          // TODO(xster): change to a CupertinoPageRoute.
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return new CupertinoScaffold.tabbed(
                 navigationBar: const CupertinoNavigationBar(
                   backgroundColor: CupertinoColors.white,
@@ -77,10 +75,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          // TODO(xster): change to a CupertinoPageRoute.
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return new CupertinoScaffold.tabbed(
                 navigationBar: const CupertinoNavigationBar(
                   backgroundColor: CupertinoColors.white,
@@ -142,10 +139,9 @@ void main() {
       new WidgetsApp(
         color: const Color(0xFFFFFFFF),
         onGenerateRoute: (RouteSettings settings) {
-          // TODO(xster): change to a CupertinoPageRoute.
-          return new PageRouteBuilder<Null>(
+          return new CupertinoPageRoute<Null>(
             settings: settings,
-            pageBuilder: (BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+            builder: (BuildContext context) {
               return new CupertinoScaffold.tabbed(
                   navigationBar: const CupertinoNavigationBar(
                     backgroundColor: CupertinoColors.white,