TabNavigator animates selected TabView

The TabBar's selection is now represented by a TabBarSelection object which encapsulates both the previous and currently selected indices and the Performance used to animate the selection indicator.

Added a TabBarView class which displays a tab's contents. It uses a shared TabBarSelection to stay in sync with a TabBar. The TabBarView scrolls in sync with the TabBar when the selection changes. Eventually it will allow one to fling the selection forward or backwards.

Added a tabBar property to ToolBar. Typically the corresponding TabBarView will be the body of the toolbar's Scaffold.

Removed TabNavigatorView and TabNavigator.

Added a widget gallery tabs demo page. Removed the old tabs demo.

examples/material_gallery/flutter.yaml

@@ -3,3 +3,9 @@ material-design-icons:
   - name: navigation/arrow_drop_down
   - name: navigation/cancel
   - name: navigation/menu
+  - name: action/event
+  - name: action/home
+  - name: action/android
+  - name: action/alarm
+  - name: action/face
+  - name: action/language

examples/material_gallery/lib/demo/tabs_demo.dart

+// Copyright 2015 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/material.dart';
+
+import 'widget_demo.dart';
+
+final TabBarSelection _selection = new TabBarSelection();
+final List<String> _iconNames = <String>["event", "home", "android", "alarm", "face", "language"];
+
+Widget buildTabBar(_) {
+  return new TabBar(
+    selection: _selection,
+    isScrollable: true,
+    labels: _iconNames.map((String iconName) => new TabLabel(text: iconName, icon: "action/$iconName")).toList()
+  );
+}
+
+class TabsDemo extends StatefulComponent {
+  _TabsDemoState createState() => new _TabsDemoState();
+}
+
+class _TabsDemoState extends State<TabsDemo> {
+  double _viewWidth = 100.0;
+
+  void _handleSizeChanged(Size newSize) {
+    setState(() {
+      _viewWidth = newSize.width;
+    });
+  }
+
+  Widget build(_) {
+    return new SizeObserver(
+      onSizeChanged: _handleSizeChanged,
+      child: new TabBarView<String>(
+        selection: _selection,
+        items: _iconNames,
+        itemExtent: _viewWidth,
+        itemBuilder: (BuildContext context, String iconName, int index) {
+          return new Container(
+            key: new ValueKey<String>(iconName),
+            padding: const EdgeDims.all(12.0),
+            child: new Card(
+              child: new Center(child: new Icon(icon: "action/$iconName", size:IconSize.s48))
+            )
+          );
+        }
+      )
+    );
+  }
+}
+
+final WidgetDemo kTabsDemo = new WidgetDemo(
+  title: 'Tabs',
+  routeName: '/tabs',
+  tabBarBuilder: buildTabBar,
+  builder: (_) => new TabsDemo()
+);

examples/material_gallery/lib/demo/widget_demo.dart

@@ -5,9 +5,10 @@
 import 'package:flutter/material.dart';
 
 class WidgetDemo {
-  WidgetDemo({ this.title, this.routeName, this.builder });
+  WidgetDemo({ this.title, this.routeName, this.tabBarBuilder, this.builder });
 
   final String title;
   final String routeName;
+  final WidgetBuilder tabBarBuilder;
   final WidgetBuilder builder;
 }

examples/material_gallery/lib/gallery_page.dart

@@ -40,6 +40,11 @@ class GalleryPage extends StatelessComponent {
     );
   }
 
+  Widget _tabBar(BuildContext context) {
+    final WidgetBuilder builder = active?.tabBarBuilder;
+    return builder != null ? builder(context) : null;
+  }
+
   Widget build(BuildContext context) {
     return new Scaffold(
       toolBar: new ToolBar(
@@ -47,7 +52,8 @@ class GalleryPage extends StatelessComponent {
           icon: 'navigation/menu',
           onPressed: () { _showDrawer(context); }
         ),
-        center: new Text(active?.title ?? 'Material gallery')
+        center: new Text(active?.title ?? 'Material gallery'),
+        tabBar: _tabBar(context)
       ),
       body: _body(context)
     );

examples/material_gallery/lib/main.dart

@@ -9,6 +9,7 @@ import 'demo/date_picker_demo.dart';
 import 'demo/drop_down_demo.dart';
 import 'demo/selection_controls_demo.dart';
 import 'demo/slider_demo.dart';
+import 'demo/tabs_demo.dart';
 import 'demo/time_picker_demo.dart';
 import 'demo/widget_demo.dart';
 import 'gallery_page.dart';
@@ -18,6 +19,7 @@ final List<WidgetDemo> _kDemos = <WidgetDemo>[
   kSelectionControlsDemo,
   kSliderDemo,
   kDatePickerDemo,
+  kTabsDemo,
   kTimePickerDemo,
   kDropDownDemo,
 ];

examples/stocks/lib/stock_home.dart

@@ -6,6 +6,8 @@ part of stocks;
 
 typedef void ModeUpdater(StockMode mode);
 
+enum StockHomeTab { market, portfolio }
+
 class StockHome extends StatefulComponent {
   StockHome(this.stocks, this.symbols, this.stockMode, this.modeUpdater);
 
@@ -20,6 +22,7 @@ class StockHome extends StatefulComponent {
 class StockHomeState extends State<StockHome> {
 
   final GlobalKey scaffoldKey = new GlobalKey();
+  final TabBarSelection _tabBarSelection = new TabBarSelection();
   bool _isSearching = false;
   String _searchQuery;
 
@@ -160,7 +163,13 @@ class StockHomeState extends State<StockHome> {
           icon: "navigation/more_vert",
           onPressed: _handleMenuShow
         )
-      ]
+      ],
+      tabBar: new TabBar(
+        selection: _tabBarSelection,
+        labels: <TabLabel>[
+          const TabLabel(text: 'MARKET'),
+          const TabLabel(text: 'PORTFOLIO')]
+      )
     );
   }
 
@@ -208,25 +217,6 @@ class StockHomeState extends State<StockHome> {
 
   static const List<String> portfolioSymbols = const <String>["AAPL","FIZZ", "FIVE", "FLAT", "ZINC", "ZNGA"];
 
-  Widget buildTabNavigator() {
-    return new TabNavigator(
-      views: <TabNavigatorView>[
-        new TabNavigatorView(
-          label: const TabLabel(text: 'MARKET'),
-          builder: (BuildContext context) => buildStockList(context, _filterBySearchQuery(_getStockList(config.symbols)).toList())
-        ),
-        new TabNavigatorView(
-          label: const TabLabel(text: 'PORTFOLIO'),
-          builder: (BuildContext context) => buildStockList(context, _filterBySearchQuery(_getStockList(portfolioSymbols)).toList())
-        )
-      ],
-      selectedIndex: selectedTabIndex,
-      onChanged: (int tabIndex) {
-        setState(() { selectedTabIndex = tabIndex; } );
-      }
-    );
-  }
-
   static GlobalKey searchFieldKey = new GlobalKey();
   static GlobalKey companyNameKey = new GlobalKey();
 
@@ -270,12 +260,43 @@ class StockHomeState extends State<StockHome> {
     );
   }
 
+  Widget buildStockTab(BuildContext context, StockHomeTab tab, List<String> stockSymbols) {
+    return new Container(
+      key: new ValueKey<StockHomeTab>(tab),
+      child: buildStockList(context, _filterBySearchQuery(_getStockList(stockSymbols)).toList())
+    );
+  }
+
+  double _viewWidth = 100.0;
+  void _handleSizeChanged(Size newSize) {
+    setState(() {
+      _viewWidth = newSize.width;
+    });
+  }
+
   Widget build(BuildContext context) {
     return new Scaffold(
       key: scaffoldKey,
       toolBar: _isSearching ? buildSearchBar() : buildToolBar(),
-      body: buildTabNavigator(),
-      floatingActionButton: buildFloatingActionButton()
+      floatingActionButton: buildFloatingActionButton(),
+      body: new SizeObserver(
+        onSizeChanged: _handleSizeChanged,
+        child: new TabBarView<StockHomeTab>(
+          selection: _tabBarSelection,
+          items: [StockHomeTab.market, StockHomeTab.portfolio],
+          itemExtent: _viewWidth,
+          itemBuilder: (BuildContext context, StockHomeTab tab, _) {
+            switch (tab) {
+              case StockHomeTab.market:
+                return buildStockTab(context, tab, config.symbols);
+              case StockHomeTab.portfolio:
+                return buildStockTab(context, tab, portfolioSymbols);
+              default:
+                assert(false);
+            }
+          }
+        )
+      )
     );
   }
 }

examples/widgets/tabs.dart

-// Copyright 2015 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/material.dart';
-import 'package:flutter/painting.dart';
-
-class TabbedNavigatorApp extends StatefulComponent {
-  TabbedNavigatorAppState createState() => new TabbedNavigatorAppState();
-}
-
-class TabbedNavigatorAppState extends State<TabbedNavigatorApp> {
-  // The index of the selected tab for each of the TabNavigators constructed below.
-  List<int> selectedIndices = new List<int>.filled(5, 0);
-
-  TabNavigator _buildTabNavigator(int n, List<TabNavigatorView> views, Key key, {isScrollable: false}) {
-    return new TabNavigator(
-      key: key,
-      views: views,
-      selectedIndex: selectedIndices[n],
-      isScrollable: isScrollable,
-      onChanged: (int tabIndex) {
-        setState(() { selectedIndices[n] = tabIndex; } );
-      }
-    );
-  }
-
-  Widget _buildContent(String label) {
-    return new Center(
-      child: new Text(label, style: const TextStyle(fontSize: 48.0, fontWeight: FontWeight.w800))
-    );
-  }
-
-  TabNavigator _buildTextLabelsTabNavigator(int n) {
-    Iterable<TabNavigatorView> views = ["ONE", "TWO", "FREE", "FOUR"]
-      .map((text) {
-        return new TabNavigatorView(
-          label: new TabLabel(text: text),
-          builder: (BuildContext context) => _buildContent(text)
-        );
-      });
-    return _buildTabNavigator(n, views.toList(), const ValueKey<String>('textLabelsTabNavigator'));
-  }
-
-  TabNavigator _buildIconLabelsTabNavigator(int n) {
-    Iterable<TabNavigatorView> views = ["event", "home", "android", "alarm", "face", "language"]
-      .map((String iconName) {
-        return new TabNavigatorView(
-          label: new TabLabel(icon: "action/$iconName"),
-          builder: (BuildContext context) => _buildContent(iconName)
-        );
-      });
-    return _buildTabNavigator(n, views.toList(), const ValueKey<String>('iconLabelsTabNavigator'));
-  }
-
-  TabNavigator _buildTextAndIconLabelsTabNavigator(int n) {
-    List<TabNavigatorView> views = <TabNavigatorView>[
-      new TabNavigatorView(
-        label: const TabLabel(text: 'STOCKS', icon: 'action/list'),
-        builder: (BuildContext context) => _buildContent("Stocks")
-      ),
-      new TabNavigatorView(
-        label: const TabLabel(text: 'PORTFOLIO', icon: 'action/account_circle'),
-        builder: (BuildContext context) => _buildContent("Portfolio")
-      ),
-      new TabNavigatorView(
-        label: const TabLabel(text: 'SUMMARY', icon: 'action/assessment'),
-        builder: (BuildContext context) => _buildContent("Summary")
-      )
-    ];
-    return _buildTabNavigator(n, views, const ValueKey<String>('textAndIconLabelsTabNavigator'));
-  }
-
-  TabNavigator _buildScrollableTabNavigator(int n) {
-    Iterable<TabNavigatorView> views = [
-      "MIN WIDTH",
-      "THIS TAB LABEL IS SO WIDE THAT IT OCCUPIES TWO LINES",
-      "THIS TAB IS PRETTY WIDE TOO",
-      "MORE",
-      "TABS",
-      "TO",
-      "STRETCH",
-      "OUT",
-      "THE",
-      "TAB BAR"
-      ]
-      .map((text) {
-        return new TabNavigatorView(
-          label: new TabLabel(text: text),
-          builder: (BuildContext context) => _buildContent(text)
-        );
-      });
-    return _buildTabNavigator(n, views.toList(), const ValueKey<String>('scrollableTabNavigator'), isScrollable: true);
-  }
-
-
-  Container _buildCard(BuildContext context, TabNavigator tabNavigator) {
-    return new Container(
-      padding: const EdgeDims.all(12.0),
-      child: new Card(child: new Padding(child: tabNavigator, padding: const EdgeDims.all(8.0)))
-    );
-  }
-
-  Widget build(BuildContext context) {
-    List<TabNavigatorView> views = <TabNavigatorView>[
-      new TabNavigatorView(
-        label: const TabLabel(text: 'TEXT'),
-        builder: (BuildContext context) => _buildCard(context, _buildTextLabelsTabNavigator(0))
-      ),
-      new TabNavigatorView(
-        label: const TabLabel(text: 'ICONS'),
-        builder: (BuildContext context) => _buildCard(context, _buildIconLabelsTabNavigator(1))
-      ),
-      new TabNavigatorView(
-        label: const TabLabel(text: 'BOTH'),
-        builder: (BuildContext context) => _buildCard(context, _buildTextAndIconLabelsTabNavigator(2))
-      ),
-      new TabNavigatorView(
-        label: const TabLabel(text: 'SCROLL'),
-        builder: (BuildContext context) => _buildCard(context, _buildScrollableTabNavigator(3))
-      )
-    ];
-
-    TabNavigator tabNavigator = _buildTabNavigator(4, views, const ValueKey<String>('tabs'));
-    assert(selectedIndices.length == 5);
-
-    ToolBar toolbar = new ToolBar(
-      center: new Text('Tabbed Navigator', style: Typography.white.title),
-      elevation: 0
-    );
-
-    return new Scaffold(
-      toolBar: toolbar,
-      body: tabNavigator
-    );
-  }
-}
-
-void main() {
-  runApp(new MaterialApp(
-    title: 'Tabs',
-    routes: <String, RouteBuilder>{
-      '/': (RouteArguments args) => new TabbedNavigatorApp(),
-    }
-  ));
-}

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

@@ -292,14 +292,12 @@ class TabLabel {
   }
 }
 
-class Tab extends StatelessComponent {
-  Tab({
+class _Tab extends StatelessComponent {
+  _Tab({
     Key key,
     this.onSelected,
     this.label,
-    this.color,
-    this.selected: false,
-    this.selectedColor
+    this.color
   }) : super(key: key) {
     assert(label.text != null || label.icon != null);
   }
@@ -307,19 +305,16 @@ class Tab extends StatelessComponent {
   final VoidCallback onSelected;
   final TabLabel label;
   final Color color;
-  final bool selected;
-  final Color selectedColor;
 
   Widget _buildLabelText() {
     assert(label.text != null);
-    TextStyle style = new TextStyle(color: selected ? selectedColor : color);
+    TextStyle style = new TextStyle(color: color);
     return new Text(label.text, style: style);
   }
 
   Widget _buildLabelIcon() {
     assert(label.icon != null);
-    Color iconColor = selected ? selectedColor : color;
-    ColorFilter filter = new ColorFilter.mode(iconColor, TransferMode.srcATop);
+    ColorFilter filter = new ColorFilter.mode(color, TransferMode.srcATop);
     return new Icon(icon: label.icon, colorFilter: filter);
   }
 
@@ -381,18 +376,46 @@ class _TabsScrollBehavior extends BoundedBehavior {
   }
 }
 
+class TabBarSelection {
+  TabBarSelection({ int index: 0, this.onChanged }) : _index = index;
+
+  final VoidCallback onChanged;
+
+  PerformanceView get performance => _performance.view;
+  final _performance = new Performance(duration: _kTabBarScroll, progress: 1.0);
+
+  int get index => _index;
+  int _index;
+  void set index(int value) {
+    if (value == _index)
+      return;
+    _previousIndex = _index;
+    _index = value;
+    _performance
+      ..progress = 0.0
+      ..play().then((_) {
+      if (onChanged != null)
+        onChanged();
+      });
+  }
+
+  int get previousIndex => _previousIndex;
+  int _previousIndex = 0;
+}
+
 class TabBar extends Scrollable {
   TabBar({
     Key key,
     this.labels,
-    this.selectedIndex: 0,
-    this.onChanged,
+    this.selection,
     this.isScrollable: false
-  }) : super(key: key, scrollDirection: ScrollDirection.horizontal);
+  }) : super(key: key, scrollDirection: ScrollDirection.horizontal) {
+    assert(labels != null);
+    assert(selection != null);
+  }
 
   final Iterable<TabLabel> labels;
-  final int selectedIndex;
-  final TabSelectedIndexChanged onChanged;
+  final TabBarSelection selection;
   final bool isScrollable;
 
   _TabBarState createState() => new _TabBarState();
@@ -401,36 +424,53 @@ class TabBar extends Scrollable {
 class _TabBarState extends ScrollableState<TabBar> {
   void initState() {
     super.initState();
-    _indicatorAnimation = new ValuePerformance<Rect>()
-      ..duration = _kTabBarScroll
-      ..variable = new AnimatedRectValue(null, curve: Curves.ease);
     scrollBehavior.isScrollable = config.isScrollable;
+    config.selection._performance
+      ..addStatusListener(_handleStatusChange)
+      ..addListener(_handleProgressChange);
   }
 
-  Size _tabBarSize;
-  Size _viewportSize = Size.zero;
-  List<double> _tabWidths;
-  ValuePerformance<Rect> _indicatorAnimation;
-
-  void didUpdateConfig(TabBar oldConfig) {
-    super.didUpdateConfig(oldConfig);
-    if (!config.isScrollable)
-      scrollTo(0.0);
+  void dispose() {
+    config.selection._performance
+      ..removeStatusListener(_handleStatusChange)
+      ..removeListener(_handleProgressChange)
+      ..stop();
+    super.dispose();
   }
 
-  AnimatedRectValue get _indicatorRect => _indicatorAnimation.variable;
+  Performance get _performance => config.selection._performance;
+
+  bool _indicatorRectIsValid = false;
 
-  void _startIndicatorAnimation(int fromTabIndex, int toTabIndex) {
+  // The performance's status change is our indication that the selection index has
+  // changed. We don't start animating the _indicatorRect until after we've reset
+  // _indicatorRect here.
+  void _handleStatusChange(PerformanceStatus status) {
+    _indicatorRectIsValid = status == PerformanceStatus.forward;
+    if (status == PerformanceStatus.forward) {
+      if (config.isScrollable)
+        scrollTo(_centeredTabScrollOffset(config.selection.index), duration: _kTabBarScroll);
+      setState(() {
         _indicatorRect
-      ..begin = (_indicatorRect.value == null ? _tabIndicatorRect(fromTabIndex) : _indicatorRect.value)
-      ..end = _tabIndicatorRect(toTabIndex);
-    _indicatorAnimation
-      ..progress = 0.0
-      ..play();
+          ..begin = _indicatorRect.value ?? _tabIndicatorRect(config.selection.previousIndex)
+          ..end = _tabIndicatorRect(config.selection.index);
+      });
+    }
   }
 
-  ScrollBehavior createScrollBehavior() => new _TabsScrollBehavior();
-  _TabsScrollBehavior get scrollBehavior => super.scrollBehavior;
+  void _handleProgressChange() {
+    // Performance listeners are notified before statusListeners.
+    if (_indicatorRectIsValid && _performance.status == PerformanceStatus.forward) {
+      setState(() {
+        _indicatorRect.setProgress(_performance.progress, AnimationDirection.forward);
+      });
+    }
+  }
+
+  Size _viewportSize = Size.zero;
+  Size _tabBarSize;
+  List<double> _tabWidths;
+  AnimatedRectValue _indicatorRect = new AnimatedRectValue(null, curve: Curves.ease);
 
   Rect _tabRect(int tabIndex) {
     assert(_tabBarSize != null);
@@ -450,31 +490,40 @@ class _TabBarState extends ScrollableState<TabBar> {
     return new Rect.fromLTRB(r.left, r.bottom, r.right, r.bottom + _kTabIndicatorHeight);
   }
 
+  void didUpdateConfig(TabBar oldConfig) {
+    super.didUpdateConfig(oldConfig);
+    if (!config.isScrollable)
+      scrollTo(0.0);
+  }
+
+  ScrollBehavior createScrollBehavior() => new _TabsScrollBehavior();
+  _TabsScrollBehavior get scrollBehavior => super.scrollBehavior;
+
   double _centeredTabScrollOffset(int tabIndex) {
     double viewportWidth = scrollBehavior.containerExtent;
-    return (_tabRect(tabIndex).left + _tabWidths[tabIndex] / 2.0 - viewportWidth / 2.0)
+    Rect tabRect = _tabRect(tabIndex);
+    return (tabRect.left + tabRect.width / 2.0 - viewportWidth / 2.0)
       .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
   }
 
   void _handleTabSelected(int tabIndex) {
-    if (tabIndex != config.selectedIndex) {
-      if (_tabWidths != null) {
-        if (config.isScrollable)
-          scrollTo(_centeredTabScrollOffset(tabIndex), duration: _kTabBarScroll);
-        _startIndicatorAnimation(config.selectedIndex, tabIndex);
-      }
-      if (config.onChanged != null)
-        config.onChanged(tabIndex);
-    }
+    if (tabIndex != config.selection.index)
+      setState(() {
+        config.selection.index = tabIndex;
+      });
   }
 
   Widget _toTab(TabLabel label, int tabIndex, Color color, Color selectedColor) {
-    return new Tab(
-      onSelected: () => _handleTabSelected(tabIndex),
+    Color labelColor = color;
+    if (tabIndex == config.selection.index)
+      labelColor = Color.lerp(color, selectedColor, _performance.progress);
+    else if (tabIndex == config.selection.previousIndex)
+      labelColor = Color.lerp(selectedColor, color, _performance.progress);
+
+    return new _Tab(
+      onSelected: () { _handleTabSelected(tabIndex); },
       label: label,
-      color: color,
-      selected: tabIndex == config.selectedIndex,
-      selectedColor: selectedColor
+      color: labelColor
     );
   }
 
@@ -496,6 +545,8 @@ class _TabBarState extends ScrollableState<TabBar> {
   void _handleViewportSizeChanged(Size newSize) {
     _viewportSize = newSize;
     _updateScrollBehavior();
+    if (config.isScrollable)
+      scrollTo(_centeredTabScrollOffset(config.selection.index), duration: _kTabBarScroll);
   }
 
   Widget buildContent(BuildContext context) {
@@ -526,11 +577,11 @@ class _TabBarState extends ScrollableState<TabBar> {
         style: textStyle,
         child: new BuilderTransition(
           variables: <AnimatedValue<Rect>>[_indicatorRect],
-          performance: _indicatorAnimation.view,
+          performance: config.selection.performance,
           builder: (BuildContext context) {
             return new _TabBarWrapper(
               children: tabs,
-              selectedIndex: config.selectedIndex,
+              selectedIndex: config.selection.index,
               indicatorColor: indicatorColor,
               indicatorRect: _indicatorRect.value,
               textAndIcons: textAndIcons,
@@ -563,46 +614,111 @@ class _TabBarState extends ScrollableState<TabBar> {
   }
 }
 
-class TabNavigatorView {
-  TabNavigatorView({ this.label, this.builder }) {
-    assert(builder != null);
+class TabBarView<T> extends ScrollableList<T> {
+  TabBarView({
+    Key key,
+    this.selection,
+    List<T> items,
+    ItemBuilder<T> itemBuilder,
+    double itemExtent
+  }) : super(
+    key: key,
+    scrollDirection: ScrollDirection.horizontal,
+    items: items,
+    itemBuilder: itemBuilder,
+    itemExtent: itemExtent,
+    itemsWrap: false
+  ) {
+    assert(selection != null);
   }
 
-  // this uses a builder for the contents, rather than a raw Widget child,
-  // because there might be many, many tabs and some might be relatively
-  // expensive to create up front. This way, the view is only created lazily.
+  final TabBarSelection selection;
 
-  final TabLabel label;
-  final WidgetBuilder builder;
+  _TabBarViewState createState() => new _TabBarViewState<T>();
 }
 
-class TabNavigator extends StatelessComponent {
-  TabNavigator({
-    Key key,
-    this.views,
-    this.selectedIndex: 0,
-    this.onChanged,
-    this.isScrollable: false
-  }) : super(key: key);
+// TODO(hansmuller): horizontal scrolling should drive the TabSelection's performance.
+class _NotScrollable extends BoundedBehavior {
+  bool get isScrollable => false;
+}
 
-  final List<TabNavigatorView> views;
-  final int selectedIndex;
-  final TabSelectedIndexChanged onChanged;
-  final bool isScrollable;
+class _TabBarViewState<T> extends ScrollableListState<T, TabBarView<T>> {
 
-  Widget build(BuildContext context) {
-    assert(views != null && views.isNotEmpty);
-    assert(selectedIndex >= 0 && selectedIndex < views.length);
-    return new Column(<Widget>[
-      new TabBar(
-        labels: views.map((TabNavigatorView view) => view.label),
-        onChanged: onChanged,
-        selectedIndex: selectedIndex,
-        isScrollable: isScrollable
-      ),
-      new Flexible(child: views[selectedIndex].builder(context))
-    ],
-      alignItems: FlexAlignItems.stretch
-    );
+  ScrollBehavior createScrollBehavior() => new _NotScrollable();
+
+  List<int> _itemIndices = [0, 1];
+  AnimationDirection _scrollDirection = AnimationDirection.forward;
+
+  void _initItemIndicesAndScrollPosition() {
+    final int selectedIndex = config.selection.index;
+
+    if (selectedIndex == 0) {
+      _itemIndices = <int>[0, 1];
+      scrollTo(0.0);
+    } else if (selectedIndex == config.items.length - 1) {
+      _itemIndices = <int>[selectedIndex - 1, selectedIndex];
+      scrollTo(config.itemExtent);
+    } else {
+      _itemIndices = <int>[selectedIndex - 1, selectedIndex, selectedIndex + 1];
+      scrollTo(config.itemExtent);
+    }
+  }
+
+  Performance get _performance => config.selection._performance;
+
+  void initState() {
+    super.initState();
+    _initItemIndicesAndScrollPosition();
+    _performance
+      ..addStatusListener(_handleStatusChange)
+      ..addListener(_handleProgressChange);
+  }
+
+  void dispose() {
+    _performance
+      ..removeStatusListener(_handleStatusChange)
+      ..removeListener(_handleProgressChange)
+      ..stop();
+    super.dispose();
+  }
+
+  void didUpdateConfig(TabBarView oldConfig) {
+    super.didUpdateConfig(oldConfig);
+    if (oldConfig.itemExtent != config.itemExtent && !_performance.isAnimating)
+      _initItemIndicesAndScrollPosition();
+  }
+
+  void _handleStatusChange(PerformanceStatus status) {
+    final int selectedIndex = config.selection.index;
+    final int previousSelectedIndex = config.selection.previousIndex;
+
+    if (status == PerformanceStatus.forward) {
+      if (selectedIndex < previousSelectedIndex) {
+        _itemIndices = <int>[selectedIndex, previousSelectedIndex];
+        _scrollDirection = AnimationDirection.reverse;
+      } else {
+        _itemIndices = <int>[previousSelectedIndex, selectedIndex];
+        _scrollDirection = AnimationDirection.forward;
+      }
+    } else if (status == PerformanceStatus.completed) {
+      _initItemIndicesAndScrollPosition();
+    }
+  }
+
+  void _handleProgressChange() {
+    if (_scrollDirection == AnimationDirection.forward)
+      scrollTo(config.itemExtent * _performance.progress);
+    else
+      scrollTo(config.itemExtent * (1.0 - _performance.progress));
+  }
+
+  int get itemCount => _itemIndices.length;
+
+  List<Widget> buildItems(BuildContext context, int start, int count) {
+    return _itemIndices
+      .skip(start)
+      .take(count)
+      .map((int i) => config.itemBuilder(context, config.items[i], i))
+      .toList();
   }
 }

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

@@ -8,6 +8,7 @@ import 'constants.dart';
 import 'icon_theme.dart';
 import 'icon_theme_data.dart';
 import 'shadows.dart';
+import 'tabs.dart';
 import 'theme.dart';
 import 'typography.dart';
 
@@ -18,6 +19,7 @@ class ToolBar extends StatelessComponent {
     this.center,
     this.right,
     this.bottom,
+    this.tabBar,
     this.elevation: 4,
     this.backgroundColor,
     this.textTheme,
@@ -28,6 +30,7 @@ class ToolBar extends StatelessComponent {
   final Widget center;
   final List<Widget> right;
   final Widget bottom;
+  final TabBar tabBar;
   final int elevation;
   final Color backgroundColor;
   final TextTheme textTheme;
@@ -40,6 +43,7 @@ class ToolBar extends StatelessComponent {
       center: center,
       right: right,
       bottom: bottom,
+      tabBar: tabBar,
       elevation: elevation,
       backgroundColor: backgroundColor,
       textTheme: textTheme,
@@ -90,6 +94,9 @@ class ToolBar extends StatelessComponent {
         child: new Container(height: kExtendedToolBarHeight - kToolBarHeight, child: bottom)
       ));
 
+    if (tabBar != null)
+      columnChildren.add(tabBar);
+
     Widget content = new AnimatedContainer(
       duration: kThemeChangeDuration,
       padding: new EdgeDims.symmetric(horizontal: 8.0),

packages/unit/test/widget/tabs_test.dart

@@ -7,16 +7,13 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 import 'package:test/test.dart';
 
-int selectedIndex = 2;
+TabBarSelection selection;
 
 Widget buildFrame({ List<String> tabs, bool isScrollable: false }) {
   return new TabBar(
     labels: tabs.map((String tab) => new TabLabel(text: tab)).toList(),
-    selectedIndex: selectedIndex,
-    isScrollable: isScrollable,
-    onChanged: (int tabIndex) {
-      selectedIndex = tabIndex;
-    }
+    selection: selection,
+    isScrollable: isScrollable
   );
 }
 
@@ -24,56 +21,56 @@ void main() {
   test('TabBar tap selects tab', () {
     testWidgets((WidgetTester tester) {
       List<String> tabs = <String>['A', 'B', 'C'];
-      selectedIndex = 2;
+      selection = new TabBarSelection(index: 2);
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
       expect(tester.findText('A'), isNotNull);
       expect(tester.findText('B'), isNotNull);
       expect(tester.findText('C'), isNotNull);
-      expect(selectedIndex, equals(2));
+      expect(selection.index, equals(2));
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
       tester.tap(tester.findText('B'));
       tester.pump();
-      expect(selectedIndex, equals(1));
+      expect(selection.index, equals(1));
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
       tester.tap(tester.findText('C'));
       tester.pump();
-      expect(selectedIndex, equals(2));
+      expect(selection.index, equals(2));
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: false));
       tester.tap(tester.findText('A'));
       tester.pump();
-      expect(selectedIndex, equals(0));
+      expect(selection.index, equals(0));
     });
   });
 
   test('Scrollable TabBar tap selects tab', () {
     testWidgets((WidgetTester tester) {
       List<String> tabs = <String>['A', 'B', 'C'];
-      selectedIndex = 2;
+      selection = new TabBarSelection(index: 2);
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
       expect(tester.findText('A'), isNotNull);
       expect(tester.findText('B'), isNotNull);
       expect(tester.findText('C'), isNotNull);
-      expect(selectedIndex, equals(2));
+      expect(selection.index, equals(2));
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
       tester.tap(tester.findText('B'));
       tester.pump();
-      expect(selectedIndex, equals(1));
+      expect(selection.index, equals(1));
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
       tester.tap(tester.findText('C'));
       tester.pump();
-      expect(selectedIndex, equals(2));
+      expect(selection.index, equals(2));
 
       tester.pumpWidget(buildFrame(tabs: tabs, isScrollable: true));
       tester.tap(tester.findText('A'));
       tester.pump();
-      expect(selectedIndex, equals(0));
+      expect(selection.index, equals(0));
     });
   });
 }