Convert Persistent Bottom Sheets to a Scaffold API

- `Scaffold.of(context).showBottomSheet(widget);`
- Returns an object with .closed Future and .close() method.
- Uses a StateRoute to handle back button.
- Take the Navigator logic out of the BottomSheet widget.
- Support showing a sheet while an old one is going away.
- Add Navigator.remove().

examples/stocks/lib/stock_home.dart

@@ -20,7 +20,6 @@ class StockHome extends StatefulComponent {
 class StockHomeState extends State<StockHome> {
 
   final GlobalKey scaffoldKey = new GlobalKey();
-  final GlobalKey<PlaceholderState> _bottomSheetPlaceholderKey = new GlobalKey<PlaceholderState>();
   bool _isSearching = false;
   String _searchQuery;
 
@@ -202,11 +201,7 @@ class StockHomeState extends State<StockHome> {
         Navigator.of(context).pushNamed('/stock/${stock.symbol}', mostValuableKeys: mostValuableKeys);
       },
       onShow: (Stock stock, Key arrowKey) {
-        showBottomSheet(
-          placeholderKey: _bottomSheetPlaceholderKey,
-          context: context,
-          child: new StockSymbolBottomSheet(stock: stock)
-        );
+        scaffoldKey.currentState.showBottomSheet((BuildContext context) => new StockSymbolBottomSheet(stock: stock));
       }
     );
   }
@@ -256,12 +251,14 @@ class StockHomeState extends State<StockHome> {
     showModalBottomSheet(
       // TODO(ianh): Fill this out.
       context: context,
-      child: new Column([
-        new Input(
-          key: companyNameKey,
-          placeholder: 'Company Name'
-        ),
-      ])
+      builder: (BuildContext context) {
+        return new Column([
+          new Input(
+            key: companyNameKey,
+            placeholder: 'Company Name'
+          ),
+        ]);
+      }
     );
   }
 
@@ -278,7 +275,6 @@ class StockHomeState extends State<StockHome> {
       key: scaffoldKey,
       toolBar: _isSearching ? buildSearchBar() : buildToolBar(),
       body: buildTabNavigator(),
-      bottomSheet: new Placeholder(key: _bottomSheetPlaceholderKey),
       floatingActionButton: buildFloatingActionButton()
     );
   }

examples/stocks/lib/stock_symbol_viewer.dart

@@ -80,11 +80,11 @@ class StockSymbolBottomSheet extends StatelessComponent {
 
   Widget build(BuildContext context) {
     return new Container(
-      child: new StockSymbolView(stock: stock),
       padding: new EdgeDims.all(10.0),
       decoration: new BoxDecoration(
         border: new Border(top: new BorderSide(color: Colors.black26, width: 1.0))
-      )
+      ),
+      child: new StockSymbolView(stock: stock)
    );
   }
 }

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

@@ -18,17 +18,31 @@ const double _kCloseProgressThreshold = 0.5;
 const Color _kTransparent = const Color(0x00000000);
 const Color _kBarrierColor = Colors.black54;
 
-class _BottomSheetDragController extends StatelessComponent {
-  _BottomSheetDragController({
+class BottomSheet extends StatelessComponent {
+  BottomSheet({
     Key key,
     this.performance,
-    this.child,
-    this.childHeight
-  }) : super(key: key);
+    this.onClosing,
+    this.childHeight,
+    this.builder
+  }) : super(key: key) {
+    assert(onClosing != null);
+  }
 
+  /// The performance that controls the bottom sheet's position. The BottomSheet
+  /// widget will manipulate the position of this performance, it is not just a
+  /// passive observer.
   final Performance performance;
-  final Widget child;
+  final VoidCallback onClosing;
   final double childHeight;
+  final WidgetBuilder builder;
+
+  static Performance createPerformance() {
+    return new Performance(
+      duration: _kBottomSheetDuration,
+      debugLabel: 'BottomSheet'
+    );
+  }
 
   bool get _dismissUnderway => performance.direction == AnimationDirection.reverse;
 
@@ -42,13 +56,11 @@ class _BottomSheetDragController extends StatelessComponent {
     if (_dismissUnderway)
       return;
     if (velocity.dy > _kMinFlingVelocity) {
-      performance.fling(velocity: -velocity.dy / childHeight).then((_) {
-        Navigator.of(context).pop();
-      });
+      performance.fling(velocity: -velocity.dy / childHeight);
+      onClosing();
     } else if (performance.progress < _kCloseProgressThreshold) {
-      performance.fling(velocity: -1.0).then((_) {
-        Navigator.of(context).pop();
-      });
+      performance.fling(velocity: -1.0);
+      onClosing();
     } else {
       performance.forward();
     }
@@ -58,46 +70,19 @@ class _BottomSheetDragController extends StatelessComponent {
     return new GestureDetector(
       onVerticalDragUpdate: _handleDragUpdate,
       onVerticalDragEnd: (Offset velocity) { _handleDragEnd(velocity, context); },
-      child: child
+      child: new Material(
+        child: builder(context)
+      )
     );
   }
 }
 
-class _BottomSheetRoute extends OverlayRoute {
-  _BottomSheetRoute({ this.completer, this.child });
+// PERSISTENT BOTTOM SHEETS
 
-  final Completer completer;
-  final Widget child;
-  Performance performance;
+// See scaffold.dart
 
-  void didPush(OverlayState overlay, OverlayEntry insertionPoint) {
-    performance = new Performance(duration: _kBottomSheetDuration, debugLabel: debugLabel)
-      ..forward();
-    super.didPush(overlay, insertionPoint);
-  }
 
-  void didPop(dynamic result) {
-    void finish() {
-      super.didPop(result); // clear the overlay entries
-      completer.complete(result);
-    }
-    if (performance.isDismissed)
-      finish();
-    else
-      performance.reverse().then((_) { finish(); });
-  }
-
-  String get debugLabel => '$runtimeType';
-  String toString() => '$runtimeType(performance: $performance)';
-}
-
-class _ModalBottomSheet extends StatefulComponent {
-  _ModalBottomSheet({ Key key, this.route }) : super(key: key);
-
-  final _ModalBottomSheetRoute route;
-
-  _ModalBottomSheetState createState() => new _ModalBottomSheetState();
-}
+// MODAL BOTTOM SHEETS
 
 class _ModalBottomSheetLayout extends OneChildLayoutDelegate {
   // The distance from the bottom of the parent to the top of the BottomSheet child.
@@ -118,6 +103,14 @@ class _ModalBottomSheetLayout extends OneChildLayoutDelegate {
   }
 }
 
+class _ModalBottomSheet extends StatefulComponent {
+  _ModalBottomSheet({ Key key, this.route }) : super(key: key);
+
+  final _ModalBottomSheetRoute route;
+
+  _ModalBottomSheetState createState() => new _ModalBottomSheetState();
+}
+
 class _ModalBottomSheetState extends State<_ModalBottomSheet> {
 
   final _ModalBottomSheetLayout _layout = new _ModalBottomSheetLayout();
@@ -133,10 +126,11 @@ class _ModalBottomSheetState extends State<_ModalBottomSheet> {
             child: new CustomOneChildLayout(
               delegate: _layout,
               token: _layout.childTop.value,
-              child: new _BottomSheetDragController(
+              child: new BottomSheet(
                 performance: config.route.performance,
-                child: new Material(child: config.route.child),
-                childHeight: _layout.childTop.end
+                onClosing: () { Navigator.of(context).pop(); },
+                childHeight: _layout.childTop.end,
+                builder: config.route.builder
               )
             )
           );
@@ -146,9 +140,30 @@ class _ModalBottomSheetState extends State<_ModalBottomSheet> {
   }
 }
 
-class _ModalBottomSheetRoute extends _BottomSheetRoute {
-  _ModalBottomSheetRoute({ Completer completer, Widget child })
-    : super(completer: completer, child: child);
+class _ModalBottomSheetRoute extends OverlayRoute {
+  _ModalBottomSheetRoute({ this.completer, this.builder });
+
+  final Completer completer;
+  final WidgetBuilder builder;
+  Performance performance;
+
+  void didPush(OverlayState overlay, OverlayEntry insertionPoint) {
+    performance = BottomSheet.createPerformance()
+      ..forward();
+    super.didPush(overlay, insertionPoint);
+  }
+
+  void _finish(dynamic result) {
+    super.didPop(result); // clear the overlay entries
+    completer.complete(result);
+  }
+
+  void didPop(dynamic result) {
+    if (performance.isDismissed)
+      _finish(result);
+    else
+      performance.reverse().then((_) { _finish(result); });
+  }
 
   Widget _buildModalBarrier(BuildContext context) {
     return new AnimatedModalBarrier(
@@ -168,61 +183,18 @@ class _ModalBottomSheetRoute extends _BottomSheetRoute {
     _buildModalBarrier,
     _buildBottomSheet,
   ];
+
+  String get debugLabel => '$runtimeType';
+  String toString() => '$runtimeType(performance: $performance)';
 }
 
-Future showModalBottomSheet({ BuildContext context, Widget child }) {
-  assert(child != null);
+Future showModalBottomSheet({ BuildContext context, WidgetBuilder builder }) {
+  assert(context != null);
+  assert(builder != null);
   final Completer completer = new Completer();
   Navigator.of(context).pushEphemeral(new _ModalBottomSheetRoute(
     completer: completer,
-    child: child
+    builder: builder
   ));
   return completer.future;
 }
-
-class _PersistentBottomSheet extends StatefulComponent {
-  _PersistentBottomSheet({ Key key, this.route }) : super(key: key);
-
-  final _BottomSheetRoute route;
-
-  _PersistentBottomSheetState createState() => new _PersistentBottomSheetState();
-}
-
-class _PersistentBottomSheetState extends State<_PersistentBottomSheet> {
-
-  double _childHeight;
-  void _updateChildHeight(Size newSize) {
-    setState(() {
-      _childHeight = newSize.height;
-    });
-  }
-
-  Widget build(BuildContext context) {
-    return new AlignTransition(
-      performance: config.route.performance,
-      alignment: new AnimatedValue<FractionalOffset>(const FractionalOffset(0.0, 0.0)),
-      heightFactor: new AnimatedValue<double>(0.0, end: 1.0),
-      child: new _BottomSheetDragController(
-        performance: config.route.performance,
-        childHeight: _childHeight,
-        child: new Material(
-          child: new SizeObserver(child: config.route.child, onSizeChanged: _updateChildHeight)
-        )
-      )
-    );
-  }
-}
-
-Future showBottomSheet({ BuildContext context, GlobalKey<PlaceholderState> placeholderKey, Widget child }) {
-  assert(child != null);
-  assert(placeholderKey != null);
-  final Completer completer = new Completer();
-  _BottomSheetRoute route = new _BottomSheetRoute(child: child, completer: completer);
-  placeholderKey.currentState.child = new _PersistentBottomSheet(route: route);
-  Navigator.of(context).pushEphemeral(route);
-  return completer.future.then((_) {
-    // If our overlay has been obscured by an opaque OverlayEntry then currentState
-    // will have been cleared already.
-    placeholderKey.currentState?.child = null;
-  });
-}

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

@@ -11,9 +11,10 @@ import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
+import 'bottom_sheet.dart';
 import 'material.dart';
-import 'tool_bar.dart';
 import 'snack_bar.dart';
+import 'tool_bar.dart';
 
 const double _kFloatingActionButtonMargin = 16.0; // TODO(hmuller): should be device dependent
 
@@ -57,7 +58,7 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
 
     if (isChild(_Child.bottomSheet)) {
       bottomSheetSize = layoutChild(_Child.bottomSheet, fullWidthConstraints);
-      positionChild(_Child.bottomSheet, new Point(0.0, size.height - bottomSheetSize.height));
+      positionChild(_Child.bottomSheet, new Point((size.width - bottomSheetSize.width) / 2.0, size.height - bottomSheetSize.height));
     }
 
     if (isChild(_Child.snackBar)) {
@@ -85,13 +86,11 @@ class Scaffold extends StatefulComponent {
     Key key,
     this.toolBar,
     this.body,
-    this.bottomSheet,
     this.floatingActionButton
   }) : super(key: key);
 
   final ToolBar toolBar;
   final Widget body;
-  final Widget bottomSheet; // this is for non-modal bottom sheets
   final Widget floatingActionButton;
 
   static ScaffoldState of(BuildContext context) => context.ancestorStateOfType(ScaffoldState);
@@ -101,6 +100,8 @@ class Scaffold extends StatefulComponent {
 
 class ScaffoldState extends State<Scaffold> {
 
+  // SNACKBAR API
+
   Queue<SnackBar> _snackBars = new Queue<SnackBar>();
   Performance _snackBarPerformance;
   Timer _snackBarTimer;
@@ -108,6 +109,10 @@ class ScaffoldState extends State<Scaffold> {
   void showSnackBar(SnackBar snackbar) {
     _snackBarPerformance ??= SnackBar.createPerformance()
       ..addStatusListener(_handleSnackBarStatusChange);
+    if (_snackBars.isEmpty) {
+      assert(_snackBarPerformance.isDismissed);
+      _snackBarPerformance.forward();
+    }
     setState(() {
       _snackBars.addLast(snackbar.withPerformance(_snackBarPerformance));
     });
@@ -120,6 +125,8 @@ class ScaffoldState extends State<Scaffold> {
         setState(() {
           _snackBars.removeFirst();
         });
+        if (_snackBars.isNotEmpty)
+          _snackBarPerformance.forward();
         break;
       case PerformanceStatus.completed:
         setState(() {
@@ -138,6 +145,63 @@ class ScaffoldState extends State<Scaffold> {
     _snackBarTimer = null;
   }
 
+
+  // PERSISTENT BOTTOM SHEET API
+
+  List<Widget> _dismissedBottomSheets;
+  BottomSheetController _currentBottomSheet;
+
+  BottomSheetController showBottomSheet(WidgetBuilder builder) {
+    if (_currentBottomSheet != null) {
+      _currentBottomSheet.close();
+      assert(_currentBottomSheet == null);
+    }
+    Completer completer = new Completer();
+    GlobalKey<_PersistentBottomSheetState> bottomSheetKey = new GlobalKey<_PersistentBottomSheetState>();
+    Performance performance = BottomSheet.createPerformance()
+      ..forward();
+    _PersistentBottomSheet bottomSheet;
+    Route route = new StateRoute(
+      onPop: () {
+        assert(_currentBottomSheet._widget == bottomSheet);
+        assert(bottomSheetKey.currentState != null);
+        bottomSheetKey.currentState.close();
+        _dismissedBottomSheets ??= <Widget>[];
+        _dismissedBottomSheets.add(bottomSheet);
+        _currentBottomSheet = null;
+        completer.complete();
+      }
+    );
+    bottomSheet = new _PersistentBottomSheet(
+      key: bottomSheetKey,
+      performance: performance,
+      onClosing: () {
+        assert(_currentBottomSheet._widget == bottomSheet);
+        Navigator.of(context).remove(route);
+      },
+      onDismissed: () {
+        assert(_dismissedBottomSheets != null);
+        setState(() {
+          _dismissedBottomSheets.remove(bottomSheet);
+        });
+      },
+      builder: builder
+    );
+    Navigator.of(context).push(route);
+    setState(() {
+      _currentBottomSheet = new BottomSheetController._(
+        bottomSheet,
+        completer.future,
+        () => Navigator.of(context).remove(route),
+        setState
+      );
+    });
+    return _currentBottomSheet;
+  }
+
+
+  // INTERNALS
+
   void dispose() {
     _snackBarPerformance?.stop();
     _snackBarPerformance = null;
@@ -156,8 +220,6 @@ class ScaffoldState extends State<Scaffold> {
     final Widget materialBody = config.body != null ? new Material(child: config.body) : null;
 
     if (_snackBars.length > 0) {
-      if (_snackBarPerformance.isDismissed)
-        _snackBarPerformance.forward();
       ModalRoute route = ModalRoute.of(context);
       if (route == null || route.isCurrent) {
         if (_snackBarPerformance.isCompleted && _snackBarTimer == null)
@@ -171,12 +233,105 @@ class ScaffoldState extends State<Scaffold> {
     final List<LayoutId>children = new List<LayoutId>();
     _addIfNonNull(children, materialBody, _Child.body);
     _addIfNonNull(children, paddedToolBar, _Child.toolBar);
-    _addIfNonNull(children, config.bottomSheet, _Child.bottomSheet);
+
+    if (_currentBottomSheet != null ||
+        (_dismissedBottomSheets != null && _dismissedBottomSheets.isNotEmpty)) {
+      List<Widget> bottomSheets = <Widget>[];
+      if (_dismissedBottomSheets != null && _dismissedBottomSheets.isNotEmpty)
+        bottomSheets.addAll(_dismissedBottomSheets);
+      if (_currentBottomSheet != null)
+        bottomSheets.add(_currentBottomSheet._widget);
+      Widget stack = new Stack(
+        bottomSheets,
+        alignment: const FractionalOffset(0.5, 1.0) // bottom-aligned, centered
+      );
+      _addIfNonNull(children, stack, _Child.bottomSheet);
+    }
+
     if (_snackBars.isNotEmpty)
       _addIfNonNull(children, _snackBars.first, _Child.snackBar);
+
     _addIfNonNull(children, config.floatingActionButton, _Child.floatingActionButton);
 
     return new CustomMultiChildLayout(children, delegate: _scaffoldLayout);
   }
 
 }
+
+class BottomSheetController {
+  const BottomSheetController._(this._widget, this.closed, this.close, this.setState);
+  final Widget _widget;
+  final Future closed;
+  final VoidCallback close; // call this to close the bottom sheet
+  final StateSetter setState;
+}
+
+class _PersistentBottomSheet extends StatefulComponent {
+  _PersistentBottomSheet({
+    Key key,
+    this.performance,
+    this.onClosing,
+    this.onDismissed,
+    this.builder
+  }) : super(key: key);
+
+  final Performance performance;
+  final VoidCallback onClosing;
+  final VoidCallback onDismissed;
+  final WidgetBuilder builder;
+
+  _PersistentBottomSheetState createState() => new _PersistentBottomSheetState();
+}
+
+class _PersistentBottomSheetState extends State<_PersistentBottomSheet> {
+
+  // We take ownership of the performance given in the first configuration.
+  // We also share control of that performance with out BottomSheet widget.
+
+  void initState() {
+    super.initState();
+    assert(config.performance.status == PerformanceStatus.forward);
+    config.performance.addStatusListener(_handleStatusChange);
+  }
+
+  void didUpdateConfig(_PersistentBottomSheet oldConfig) {
+    super.didUpdateConfig(oldConfig);
+    assert(config.performance == oldConfig.performance);
+  }
+
+  void dispose() {
+    config.performance.stop();
+    super.dispose();
+  }
+
+  void close() {
+    config.performance.reverse();
+  }
+
+  void _handleStatusChange(PerformanceStatus status) {
+    if (status == PerformanceStatus.dismissed && config.onDismissed != null)
+      config.onDismissed();
+  }
+
+  double _childHeight;
+  void _updateChildHeight(Size newSize) {
+    setState(() {
+      _childHeight = newSize.height;
+    });
+  }
+
+  Widget build(BuildContext context) {
+    return new AlignTransition(
+      performance: config.performance,
+      alignment: new AnimatedValue<FractionalOffset>(const FractionalOffset(0.0, 0.0)),
+      heightFactor: new AnimatedValue<double>(0.0, end: 1.0),
+      child: new BottomSheet(
+        performance: config.performance,
+        onClosing: config.onClosing,
+        childHeight: _childHeight,
+        builder: (BuildContext context) => new SizeObserver(child: config.builder(context), onSizeChanged: _updateChildHeight)
+      )
+    );
+  }
+
+}

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

@@ -21,10 +21,10 @@ const Color _kSnackBackground = const Color(0xFF323232);
 
 // TODO(ianh): Implement the Tablet version of snackbar if we're "on a tablet".
 
-const Duration kSnackBarTransitionDuration = const Duration(milliseconds: 250);
+const Duration _kSnackBarTransitionDuration = const Duration(milliseconds: 250);
 const Duration kSnackBarShortDisplayDuration = const Duration(milliseconds: 1500);
 const Duration kSnackBarMediumDisplayDuration = const Duration(milliseconds: 2750);
-const Curve snackBarFadeCurve = const Interval(0.72, 1.0, curve: Curves.fastOutSlowIn);
+const Curve _snackBarFadeCurve = const Interval(0.72, 1.0, curve: Curves.fastOutSlowIn);
 
 class SnackBarAction extends StatelessComponent {
   SnackBarAction({Key key, this.label, this.onPressed }) : super(key: key) {
@@ -91,7 +91,7 @@ class SnackBar extends StatelessComponent {
               style: new TextStyle(color: Theme.of(context).accentColor),
               child: new FadeTransition(
                 performance: performance,
-                opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: snackBarFadeCurve),
+                opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: _snackBarFadeCurve),
                 child: new Row(children)
               )
             )
@@ -105,7 +105,7 @@ class SnackBar extends StatelessComponent {
 
   static Performance createPerformance() {
     return new Performance(
-      duration: kSnackBarTransitionDuration,
+      duration: _kSnackBarTransitionDuration,
       debugLabel: 'SnackBar'
     );
   }

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

@@ -332,6 +332,9 @@ enum _StateLifecycle {
   defunct,
 }
 
+/// The signature of setState() methods.
+typedef void StateSetter(VoidCallback fn);
+
 /// The logic and internal state for a StatefulComponent.
 abstract class State<T extends StatefulComponent> {
   /// The current configuration (an instance of the corresponding
@@ -377,7 +380,7 @@ abstract class State<T extends StatefulComponent> {
   /// If you just change the state directly without calling setState(), then the
   /// component will not be scheduled for rebuilding, meaning that its rendering
   /// will not be updated.
-  void setState(void fn()) {
+  void setState(VoidCallback fn) {
     assert(_debugLifecycleState != _StateLifecycle.defunct);
     fn();
     _element.markNeedsBuild();

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

@@ -133,6 +133,28 @@ class NavigatorState extends State<Navigator> {
     assert(_ephemeral.isEmpty);
   }
 
+  /// Pops the given route, if it's the current route. If it's not the current
+  /// route, removes it from the list of active routes without notifying any
+  /// observers or adjacent routes.
+  ///
+  /// Do not use this for ModalRoutes, or indeed anything other than
+  /// StateRoutes. Doing so would cause very odd results, e.g. ModalRoutes would
+  /// get confused about who is current.
+  void remove(Route route, [dynamic result]) {
+    assert(_modal.contains(route));
+    assert(route.overlayEntries.isEmpty);
+    if (_modal.last == route) {
+      pop(result);
+    } else {
+      setState(() {
+        _modal.remove(route);
+        route.didPop(result);
+      });
+    }
+  }
+
+  /// Removes the current route, notifying the observer (if any), and the
+  /// previous routes (using [Route.didPopNext]).
   void pop([dynamic result]) {
     setState(() {
       // We use setState to guarantee that we'll rebuild, since the routes can't

packages/unit/test/widget/bottom_sheet_test.dart

@@ -26,7 +26,10 @@ void main() {
       tester.pump();
       expect(tester.findText('BottomSheet'), isNull);
 
-      showModalBottomSheet(context: context, child: new Text('BottomSheet')).then((_) {
+      showModalBottomSheet(
+        context: context,
+        builder: (BuildContext context) => new Text('BottomSheet')
+      ).then((_) {
         showBottomSheetThenCalled = true;
       });
 
@@ -42,7 +45,7 @@ void main() {
       expect(showBottomSheetThenCalled, isTrue);
       expect(tester.findText('BottomSheet'), isNull);
 
-      showModalBottomSheet(context: context, child: new Text('BottomSheet'));
+      showModalBottomSheet(context: context, builder: (BuildContext context) => new Text('BottomSheet'));
       tester.pump(); // bottom sheet show animation starts
       tester.pump(new Duration(seconds: 1)); // animation done
       expect(tester.findText('BottomSheet'), isNotNull);
@@ -58,45 +61,60 @@ void main() {
 
   test('Verify that a downwards fling dismisses a persistent BottomSheet', () {
     testWidgets((WidgetTester tester) {
-      GlobalKey<PlaceholderState> _bottomSheetPlaceholderKey = new GlobalKey<PlaceholderState>();
-      BuildContext context;
+      GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();
       bool showBottomSheetThenCalled = false;
 
       tester.pumpWidget(new MaterialApp(
           routes: <String, RouteBuilder>{
             '/': (RouteArguments args) {
-              context = args.context;
               return new Scaffold(
-                bottomSheet: new Placeholder(key: _bottomSheetPlaceholderKey),
+                key: scaffoldKey,
                 body: new Center(child: new Text('body'))
               );
             }
           }
       ));
 
-      tester.pump();
+      expect(showBottomSheetThenCalled, isFalse);
       expect(tester.findText('BottomSheet'), isNull);
 
-      showBottomSheet(
-        context: context,
-        child: new Container(child: new Text('BottomSheet'), margin: new EdgeDims.all(40.0)),
-        placeholderKey: _bottomSheetPlaceholderKey
-      ).then((_) {
+      scaffoldKey.currentState.showBottomSheet((BuildContext context) { 
+        return new Container(
+          margin: new EdgeDims.all(40.0),
+          child: new Text('BottomSheet')
+        );
+      }).closed.then((_) {
         showBottomSheetThenCalled = true;
       });
 
-      expect(_bottomSheetPlaceholderKey.currentState.child, isNotNull);
+      expect(showBottomSheetThenCalled, isFalse);
+      expect(tester.findText('BottomSheet'), isNull);
+
       tester.pump(); // bottom sheet show animation starts
+
+      expect(showBottomSheetThenCalled, isFalse);
+      expect(tester.findText('BottomSheet'), isNotNull);
+
       tester.pump(new Duration(seconds: 1)); // animation done
+
+      expect(showBottomSheetThenCalled, isFalse);
       expect(tester.findText('BottomSheet'), isNotNull);
 
       tester.fling(tester.findText('BottomSheet'), const Offset(0.0, 20.0), 1000.0);
+      tester.pump(); // drain the microtask queue (Future completion callback)
+
+      expect(showBottomSheetThenCalled, isTrue);
+      expect(tester.findText('BottomSheet'), isNotNull);
+
       tester.pump(); // bottom sheet dismiss animation starts
+
+      expect(showBottomSheetThenCalled, isTrue);
+      expect(tester.findText('BottomSheet'), isNotNull);
+
       tester.pump(new Duration(seconds: 1)); // animation done
-      tester.pump(new Duration(seconds: 1)); // rebuild frame without the bottom sheet
+
       expect(showBottomSheetThenCalled, isTrue);
       expect(tester.findText('BottomSheet'), isNull);
-      expect(_bottomSheetPlaceholderKey.currentState.child, isNull);
     });
   });
 

packages/unit/test/widget/widget_tester.dart

@@ -162,7 +162,7 @@ class WidgetTester {
     assert(velocity != 0.0);   // velocity is pixels/second
     final TestPointer p = new TestPointer(pointer);
     final HitTestResult result = _hitTest(startLocation);
-    final kMoveCount = 50; // Needs to be >= kHistorySize, see _LeastSquaresVelocityTrackerStrategy
+    const int kMoveCount = 50; // Needs to be >= kHistorySize, see _LeastSquaresVelocityTrackerStrategy
     final double timeStampDelta = 1000.0 * offset.distance / (kMoveCount * velocity);
     double timeStamp = 0.0;
     _dispatchEvent(p.down(startLocation, timeStamp: new Duration(milliseconds: timeStamp.round())), result);