AppBar bottom widget, not necessarily a TabBar (#4631)

examples/flutter_gallery/lib/demo/colors_demo.dart

@@ -119,7 +119,7 @@ class ColorsDemo extends StatelessWidget {
         appBar: new AppBar(
           elevation: 0,
           title: new Text('Colors'),
-          tabBar: new TabBar<ColorSwatch>(
+          bottom: new TabBar<ColorSwatch>(
             isScrollable: true,
             labels: new Map<ColorSwatch, TabLabel>.fromIterable(colorSwatches, value: (ColorSwatch swatch) {
               return new TabLabel(text: swatch.name);

examples/flutter_gallery/lib/demo/scrollable_tabs_demo.dart

@@ -71,7 +71,7 @@ class ScrollableTabsDemoState extends State<ScrollableTabsDemo> {
               ]
             )
           ],
-          tabBar: new TabBar<IconData>(
+          bottom: new TabBar<IconData>(
             isScrollable: true,
             labels: new Map<IconData, TabLabel>.fromIterable(
               icons,

examples/flutter_gallery/lib/demo/tabs_demo.dart

@@ -50,7 +50,7 @@ class TabsDemoState extends State<TabsDemo> {
         appBarBehavior: AppBarBehavior.under,
         appBar: new AppBar(
           title: new Text('Tabs and scrolling'),
-          tabBar: new TabBar<_Page>(
+          bottom: new TabBar<_Page>(
             labels: new Map<_Page, TabLabel>.fromIterable(_pages, value: (_Page page) {
               return new TabLabel(text: page.label);
             })

examples/flutter_gallery/lib/demo/tabs_fab_demo.dart

@@ -100,7 +100,7 @@ class _TabsFabDemoState extends State<TabsFabDemo> {
         key: scaffoldKey,
         appBar: new AppBar(
           title: new Text('FAB per tab'),
-          tabBar: new TabBar<_Page>(
+          bottom: new TabBar<_Page>(
             labels: new Map<_Page, TabLabel>.fromIterable(pages, value: (_Page page) => page.tabLabel)
           )
         ),

examples/flutter_gallery/lib/demo/two_level_list_demo.dart

@@ -16,15 +16,16 @@ class TwoLevelListDemo extends StatelessWidget {
         children: <Widget>[
           new TwoLevelListItem(title: new Text('Top')),
           new TwoLevelSublist(
-            title: new Text('Sublist'),
-            children: <Widget>[
-              new TwoLevelListItem(title: new Text('One')),
-              new TwoLevelListItem(title: new Text('Two')),
-              new TwoLevelListItem(title: new Text('Free')),
-              new TwoLevelListItem(title: new Text('Four'))
-            ]
-          ),
-          new TwoLevelListItem(title: new Text('Bottom'))
+             title: new Text('Sublist'),
+              children: <Widget>[
+                new TwoLevelListItem(title: new Text('One')),
+                new TwoLevelListItem(title: new Text('Two')),
+                // https://en.wikipedia.org/wiki/Free_Four
+                new TwoLevelListItem(title: new Text('Free')),
+                new TwoLevelListItem(title: new Text('Four'))
+              ]
+           ),
+           new TwoLevelListItem(title: new Text('Bottom'))
         ]
       )
     );

examples/flutter_gallery/lib/gallery/demo.dart

@@ -73,7 +73,7 @@ class TabbedComponentDemoScaffold extends StatelessWidget {
       child: new Scaffold(
         appBar: new AppBar(
           title: new Text(title),
-          tabBar: new TabBar<ComponentDemoTabData>(
+          bottom: new TabBar<ComponentDemoTabData>(
             isScrollable: true,
             labels: ComponentDemoTabData.buildTabLabels(demos)
           )

examples/stocks/lib/stock_home.dart

@@ -241,7 +241,7 @@ class StockHomeState extends State<StockHome> {
           ]
         )
       ],
-      tabBar: new TabBar<StockHomeTab>(
+      bottom: new TabBar<StockHomeTab>(
         labels: <StockHomeTab, TabLabel>{
           StockHomeTab.market: new TabLabel(text: StockStrings.of(context).market()),
           StockHomeTab.portfolio: new TabLabel(text: StockStrings.of(context).portfolio())

packages/flutter/lib/src/material/app_bar.dart

@@ -14,6 +14,14 @@ import 'tabs.dart';
 import 'theme.dart';
 import 'typography.dart';
 
+/// A widget that can appear at the bottom of an [AppBar]. The [Scaffold] uses
+/// the bottom widget's [bottomHeight] to handle layout for
+/// [AppBarBehavior.scroll] and [AppBarBehavior.under].
+abstract class AppBarBottomWidget extends Widget {
+  /// Defines the height of the app bar's optional bottom widget.
+  double get bottomHeight;
+}
+
 // TODO(eseidel) Toolbar needs to change size based on orientation:
 // http://www.google.com/design/spec/layout/structure.html#structure-app-bar
 // Mobile Landscape: 48dp
@@ -30,10 +38,13 @@ import 'typography.dart';
 /// App bars are most commonly used in the [Scaffold.appBar] property, which
 /// places the app bar at the top of the app.
 ///
-/// The AppBar displays the toolbar widgets, [leading], [title],
-/// and [actions], above the [tabBar] (if any). If a [flexibleSpace] widget is
-/// specified then it is stacked behind the toolbar and tabbar. The [Scaffold]
-/// typically creates the appbar with an initial height equal to [expandedHeight].
+/// The AppBar displays the toolbar widgets, [leading], [title], and
+/// [actions], above the [bottom] (if any). If a [flexibleSpace] widget is
+/// specified then it is stacked behind the toolbar and the bottom widget.
+/// The [Scaffold] typically creates the appbar with an initial height equal to
+/// [expandedHeight]. If the [Scaffold.appBarBehavior] is set then the
+/// AppBar's [collapsedHeight] and [bottomHeight] define how small the app bar
+/// will become when the application is scrolled.
 ///
 /// See also:
 ///
@@ -53,18 +64,16 @@ class AppBar extends StatelessWidget {
     this.title,
     this.actions,
     this.flexibleSpace,
-    this.tabBar,
+    this.bottom,
     this.elevation: 4,
     this.backgroundColor,
     this.brightness,
     this.textTheme,
     this.padding: EdgeInsets.zero,
     double expandedHeight,
-    double collapsedHeight,
-    double minimumHeight
+    double collapsedHeight
   }) : _expandedHeight = expandedHeight,
        _collapsedHeight = collapsedHeight,
-       _minimumHeight = minimumHeight,
        super(key: key);
 
   /// A widget to display before the [title].
@@ -77,7 +86,7 @@ class AppBar extends StatelessWidget {
   /// field with an [IconButton] that calls [Navigator.pop].
   final Widget leading;
 
-  /// The primary widget displayed in the app bar.
+  /// The primary widget displayed in the appbar.
   ///
   /// Typically a [Text] widget containing a description of the current contents
   /// of the app.
@@ -97,8 +106,10 @@ class AppBar extends StatelessWidget {
   /// Typically a [FlexibleSpaceBar]. See [FlexibleSpaceBar] for details.
   final Widget flexibleSpace;
 
-  /// A horizontal bar of tabs to display at the bottom of the app bar.
-  final TabBar<dynamic> tabBar;
+  /// This widget appears across the bottom of the appbar.
+  ///
+  /// Typically a [TabBar].
+  final AppBarBottomWidget bottom;
 
   /// The z-coordinate at which to place this app bar.
   ///
@@ -129,7 +140,6 @@ class AppBar extends StatelessWidget {
 
   final double _expandedHeight;
   final double _collapsedHeight;
-  final double _minimumHeight;
 
   /// Creates a copy of this app bar but with the given fields replaced with the new values.
   AppBar copyWith({
@@ -138,6 +148,7 @@ class AppBar extends StatelessWidget {
     Widget title,
     List<Widget> actions,
     Widget flexibleSpace,
+    AppBarBottomWidget bottom,
     int elevation,
     Color backgroundColor,
     Brightness brightness,
@@ -152,7 +163,7 @@ class AppBar extends StatelessWidget {
       title: title ?? this.title,
       actions: actions ?? this.actions,
       flexibleSpace: flexibleSpace ?? this.flexibleSpace,
-      tabBar: tabBar ?? this.tabBar,
+      bottom: bottom ?? this.bottom,
       elevation: elevation ?? this.elevation,
       backgroundColor: backgroundColor ?? this.backgroundColor,
       brightness: brightness ?? this.brightness,
@@ -163,31 +174,31 @@ class AppBar extends StatelessWidget {
     );
   }
 
-  double get _tabBarHeight => tabBar == null ? null : tabBar.minimumHeight;
-
   double get _toolBarHeight => kToolBarHeight;
 
-  /// By default, the height of the toolbar and the tabbar (if any).
-  /// The [Scaffold] gives its appbar this height initially. If a
+  /// The height of the bottom widget. The [Scaffold] uses this value to control
+  /// the size of the app bar when its appBarBehavior is [AppBarBehavior.scroll]
+  /// or [AppBarBehavior.under].
+  double get bottomHeight => bottom?.bottomHeight ?? 0.0;
+
+  /// By default, the total height of the toolbar and the bottom widget (if any).
+  /// The [Scaffold] gives its app bar this height initially. If a
   /// [flexibleSpace] widget is specified this height should be big
   /// enough to accommodate whatever that widget contains.
-  double get expandedHeight => _expandedHeight ?? (_toolBarHeight + (_tabBarHeight ?? 0.0));
+  double get expandedHeight => _expandedHeight ?? (_toolBarHeight + bottomHeight);
 
-  /// By default, the height of the toolbar and the tabbar (if any).
+  /// By default, the height of the toolbar and the bottom widget (if any).
   /// If the height of the app bar is constrained to be less than this value
-  /// the toolbar and tabbar are scrolled upwards, out of view.
-  double get collapsedHeight => _collapsedHeight ?? (_toolBarHeight + (_tabBarHeight ?? 0.0));
-
-  double get minimumHeight => _minimumHeight ?? _tabBarHeight ?? _toolBarHeight;
+  /// then the toolbar and bottom widget are scrolled upwards, out of view.
+  double get collapsedHeight => _collapsedHeight ?? (_toolBarHeight + bottomHeight);
 
   // Defines the opacity of the toolbar's text and icons.
   double _toolBarOpacity(double appBarHeight, double statusBarHeight) {
-    return ((appBarHeight - (_tabBarHeight ?? 0.0) - statusBarHeight) / _toolBarHeight).clamp(0.0, 1.0);
+    return ((appBarHeight - bottomHeight - statusBarHeight) / _toolBarHeight).clamp(0.0, 1.0);
   }
 
-  double _tabBarOpacity(double appBarHeight, double statusBarHeight) {
-    final double tabBarHeight = _tabBarHeight ?? 0.0;
-    return ((appBarHeight - statusBarHeight) / tabBarHeight).clamp(0.0, 1.0);
+  double _bottomOpacity(double appBarHeight, double statusBarHeight) {
+    return ((appBarHeight - statusBarHeight) / bottomHeight).clamp(0.0, 1.0);
   }
 
   Widget _buildForSize(BuildContext context, BoxConstraints constraints) {
@@ -254,15 +265,15 @@ class AppBar extends StatelessWidget {
       )
     );
 
-    final double tabBarOpacity = _tabBarOpacity(size.height, statusBarHeight);
-    if (tabBar != null) {
+    final double bottomOpacity = _bottomOpacity(size.height, statusBarHeight);
+    if (bottom != null) {
       appBar = new Column(
         mainAxisAlignment: MainAxisAlignment.spaceBetween,
         children: <Widget>[
           appBar,
-          tabBarOpacity == 1.0 ? tabBar : new Opacity(
-            child: tabBar,
-            opacity: const Interval(0.25, 1.0, curve: Curves.ease).transform(tabBarOpacity)
+          bottomOpacity == 1.0 ? bottom : new Opacity(
+            child: bottom,
+            opacity: const Interval(0.25, 1.0, curve: Curves.ease).transform(bottomOpacity)
           )
         ]
       );

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

@@ -20,24 +20,27 @@ const double _kFloatingActionButtonMargin = 16.0; // TODO(hmuller): should be de
 const Duration _kFloatingActionButtonSegue = const Duration(milliseconds: 200);
 final Tween<double> _kFloatingActionButtonTurnTween = new Tween<double>(begin: -0.125, end: 0.0);
 
-/// The Scaffold's appbar is the toolbar, tabbar, and the "flexible space" that's
-/// stacked behind them. The Scaffold's appBarBehavior defines how the appbar
-/// responds to scrolling the application.
+/// The Scaffold's appbar is the toolbar, bottom, and the "flexible space"
+/// that's stacked behind them. The Scaffold's appBarBehavior defines how
+/// its layout responds to scrolling the application's body.
 enum AppBarBehavior {
-  /// The tool bar's layout does not respond to scrolling.
+  /// The app bar's layout does not respond to scrolling.
   anchor,
 
-  /// The tool bar's appearance and layout depend on the scrollOffset of the
+  /// The app bar's appearance and layout depend on the scrollOffset of the
   /// Scrollable identified by the Scaffold's scrollableKey. With the scrollOffset
   /// at 0.0, scrolling downwards causes the toolbar's flexible space to shrink,
-  /// and then the entire toolbar fade outs and scrolls off the top of the screen.
-  /// Scrolling upwards always causes the toolbar to reappear.
+  /// and then the app bar fades out and scrolls off the top of the screen.
+  /// Scrolling upwards always causes the app bar's bottom widget to reappear
+  /// if the bottom widget isn't null, otherwise the app bar's toolbar reappears.
   scroll,
 
-  /// The tool bar's appearance and layout depend on the scrollOffset of the
+  /// The app bar's appearance and layout depend on the scrollOffset of the
   /// Scrollable identified by the Scaffold's scrollableKey. With the scrollOffset
   /// at 0.0, Scrolling downwards causes the toolbar's flexible space to shrink.
-  /// Other than that, the toolbar remains anchored at the top.
+  /// If the bottom widget isn't null the app bar shrinks to the bottom widget's
+  /// [AppBarBottomWidget.bottomHeight], otherwise the app bar shrinks to its
+  /// [AppBar.collapsedHeight].
   under,
 }
 
@@ -612,13 +615,14 @@ class ScaffoldState extends State<Scaffold> {
   Widget _buildScrollableAppBar(BuildContext context, EdgeInsets padding) {
     final double expandedHeight = (config.appBar?.expandedHeight ?? 0.0) + padding.top;
     final double collapsedHeight = (config.appBar?.collapsedHeight ?? 0.0) + padding.top;
-    final double minimumHeight = (config.appBar?.minimumHeight ?? 0.0) + padding.top;
+    final double bottomHeight = config.appBar?.bottomHeight + padding.top;
+    final double underHeight = config.appBar.bottom != null ? bottomHeight : collapsedHeight;
     Widget appBar;
 
-    if (_scrollOffset <= expandedHeight && _scrollOffset >= expandedHeight - minimumHeight) {
+    if (_scrollOffset <= expandedHeight && _scrollOffset >= expandedHeight - underHeight) {
       // scrolled to the top, flexible space collapsed, only the toolbar and tabbar are (partially) visible.
       if (config.appBarBehavior == AppBarBehavior.under) {
-        appBar = _buildAnchoredAppBar(expandedHeight, minimumHeight, padding);
+        appBar = _buildAnchoredAppBar(expandedHeight, underHeight, padding);
       } else {
         final double height = math.max(_floatingAppBarHeight, expandedHeight - _scrollOffset);
         _appBarController.value = (expandedHeight - height) / expandedHeight;
@@ -630,7 +634,7 @@ class ScaffoldState extends State<Scaffold> {
     } else if (_scrollOffset > expandedHeight) {
       // scrolled past the entire app bar, maybe show the "floating" toolbar.
       if (config.appBarBehavior == AppBarBehavior.under) {
-        appBar = _buildAnchoredAppBar(expandedHeight, minimumHeight, padding);
+        appBar = _buildAnchoredAppBar(expandedHeight, underHeight, padding);
       } else {
         _floatingAppBarHeight = (_floatingAppBarHeight + _scrollOffsetDelta).clamp(0.0, collapsedHeight);
         _appBarController.value = (expandedHeight - _floatingAppBarHeight) / expandedHeight;

packages/flutter/lib/src/material/tabs.dart

@@ -10,6 +10,7 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:meta/meta.dart';
 
+import 'app_bar.dart';
 import 'colors.dart';
 import 'debug.dart';
 import 'icon.dart';
@@ -643,7 +644,7 @@ class TabBarSelectionState<T> extends State<TabBarSelection<T>> {
 ///  * [TabBarView]
 ///  * [AppBar.tabBar]
 ///  * <https://www.google.com/design/spec/components/tabs.html>
-class TabBar<T> extends Scrollable {
+class TabBar<T> extends Scrollable implements AppBarBottomWidget {
   TabBar({
     Key key,
     this.labels,
@@ -671,7 +672,9 @@ class TabBar<T> extends Scrollable {
   /// the color of the theme's body2 text color is used.
   final Color labelColor;
 
-  double get minimumHeight {
+  /// The height of the tab labels and indicator.
+  @override
+  double get bottomHeight {
     for (TabLabel label in labels.values) {
       if (label.text != null && (label.icon != null || label.iconBuilder != null))
         return _kTextAndIconTabHeight + _kTabIndicatorHeight;