Re-land ScaffoldMessenger (#65416)

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

@@ -17,6 +17,7 @@ import 'floating_action_button.dart';
 import 'icons.dart';
 import 'material_localizations.dart';
 import 'page.dart';
+import 'scaffold.dart';
 import 'theme.dart';
 
 /// [MaterialApp] uses this [TextStyle] as its [DefaultTextStyle] to encourage
@@ -168,6 +169,7 @@ class MaterialApp extends StatefulWidget {
   const MaterialApp({
     Key key,
     this.navigatorKey,
+    this.scaffoldMessengerKey,
     this.home,
     this.routes = const <String, WidgetBuilder>{},
     this.initialRoute,
@@ -215,6 +217,7 @@ class MaterialApp extends StatefulWidget {
   /// Creates a [MaterialApp] that uses the [Router] instead of a [Navigator].
   const MaterialApp.router({
     Key key,
+    this.scaffoldMessengerKey,
     this.routeInformationProvider,
     @required this.routeInformationParser,
     @required this.routerDelegate,
@@ -263,6 +266,14 @@ class MaterialApp extends StatefulWidget {
   /// {@macro flutter.widgets.widgetsApp.navigatorKey}
   final GlobalKey<NavigatorState> navigatorKey;
 
+  /// A key to use when building the [ScaffoldMessenger].
+  ///
+  /// If a [scaffoldMessengerKey] is specified, the [ScaffoldMessenger] can be
+  /// directly manipulated without first obtaining it from a [BuildContext] via
+  /// [ScaffoldMessenger.of]: from the [scaffoldMessengerKey], use the
+  /// [GlobalKey.currentState] getter.
+  final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey;
+
   /// {@macro flutter.widgets.widgetsApp.home}
   final Widget home;
 
@@ -722,7 +733,9 @@ class _MaterialAppState extends State<MaterialApp> {
     }
     theme ??= widget.theme ?? ThemeData.light();
 
-    return AnimatedTheme(
+    return ScaffoldMessenger(
+      key: widget.scaffoldMessengerKey,
+      child: AnimatedTheme(
         data: theme,
         isMaterialAppTheme: true,
         child: widget.builder != null
@@ -743,6 +756,7 @@ class _MaterialAppState extends State<MaterialApp> {
               },
             )
           : child,
+      )
     );
   }
 

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

@@ -137,7 +137,7 @@ class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
 ///           icon: const Icon(Icons.add_alert),
 ///           tooltip: 'Show Snackbar',
 ///           onPressed: () {
-///             scaffoldKey.currentState.showSnackBar(snackBar);
+///             ScaffoldMessenger.of(context).showSnackBar(snackBar);
 ///           },
 ///         ),
 ///         IconButton(

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

@@ -9,7 +9,7 @@ import 'package:flutter/widgets.dart';
 
 import 'material.dart';
 import 'material_localizations.dart';
-import 'scaffold.dart' show Scaffold;
+import 'scaffold.dart' show Scaffold, ScaffoldMessenger;
 
 /// Asserts that the given context has a [Material] ancestor.
 ///
@@ -125,3 +125,34 @@ bool debugCheckHasScaffold(BuildContext context) {
   }());
   return true;
 }
+
+/// Asserts that the given context has a [ScaffoldMessenger] ancestor.
+///
+/// Used by various widgets to make sure that they are only used in an
+/// appropriate context.
+///
+/// To invoke this function, use the following pattern, typically in the
+/// relevant Widget's build method:
+///
+/// ```dart
+/// assert(debugCheckHasScaffoldMessenger(context));
+/// ```
+///
+/// Does nothing if asserts are disabled. Always returns true.
+bool debugCheckHasScaffoldMessenger(BuildContext context) {
+  assert(() {
+    if (context.widget is! ScaffoldMessenger && context.findAncestorWidgetOfExactType<ScaffoldMessenger>() == null) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No ScaffoldMessenger widget found.'),
+        ErrorDescription('${context.widget.runtimeType} widgets require a ScaffoldMessenger widget ancestor.'),
+        ...context.describeMissingAncestor(expectedAncestorType: ScaffoldMessenger),
+        ErrorHint(
+          'Typically, the ScaffoldMessenger widget is introduced by the MaterialApp '
+          'at the top of your application widget tree.'
+        )
+      ]);
+    }
+    return true;
+  }());
+  return true;
+}

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

@@ -19,6 +19,7 @@ import 'bottom_sheet.dart';
 import 'button_bar.dart';
 import 'colors.dart';
 import 'curves.dart';
+import 'debug.dart';
 import 'divider.dart';
 import 'drawer.dart';
 import 'flexible_space_bar.dart';
@@ -61,6 +62,372 @@ enum _ScaffoldSlot {
   statusBar,
 }
 
+/// Manages [SnackBar]s for descendant [Scaffold]s.
+///
+/// This class provides APIs for showing snack bars.
+///
+/// To display a snack bar, obtain the [ScaffoldMessengerState] for the current
+/// [BuildContext] via [ScaffoldMessenger.of] and use the
+/// [ScaffoldMessengerState.showSnackBar] function.
+///
+/// See also:
+///
+///  * [SnackBar], which is a temporary notification typically shown near the
+///    bottom of the app using the [ScaffoldMessengerState.showSnackBar] method.
+///  * Cookbook: [Display a snackbar](https://flutter.dev/docs/cookbook/design/snackbars)
+class ScaffoldMessenger extends StatefulWidget {
+  /// Creates a widget that manages [SnackBar]s for [Scaffold] descendants.
+  const ScaffoldMessenger({
+    Key key,
+    @required this.child,
+  }) : assert(child != null),
+       super(key: key);
+
+  /// The widget below this widget in the tree.
+  ///
+  /// {@macro flutter.widgets.child}
+  final Widget child;
+
+  /// The state from the closest instance of this class that encloses the given
+  /// context.
+  ///
+  /// {@tool dartpad --template=stateless_widget_scaffold_center}
+  /// Typical usage of the [ScaffoldMessenger.of] function is to call it in
+  /// response to a user gesture or an application state change.
+  ///
+  /// ```dart
+  /// Widget build(BuildContext context) {
+  ///   return ElevatedButton(
+  ///     child: const Text('SHOW A SNACKBAR'),
+  ///     onPressed: () {
+  ///       ScaffoldMessenger.of(context).showSnackBar(
+  ///         const SnackBar(
+  ///           content: Text('Have a snack!'),
+  ///         ),
+  ///       );
+  ///     },
+  ///   );
+  /// }
+  /// ```
+  /// {@end-tool}
+  ///
+  /// A less elegant but more expedient solution is assign a [GlobalKey] to the
+  /// [ScaffoldMessenger], then use the `key.currentState` property to obtain the
+  /// [ScaffoldMessengerState] rather than using the [ScaffoldMessenger.of]
+  /// function. The [MaterialApp.scaffoldMessengerKey] refers to the root
+  /// ScaffoldMessenger that is provided by default.
+  ///
+  /// {@tool dartpad --template=freeform}
+  /// Sometimes [SnackBar]s are produced by code that doesn't have ready access
+  /// to a valid [BuildContext]. One such example of this is when you may want
+  /// to show a SnackBar from a method outside of the `build` function. In these
+  /// cases, you can assign a [GlobalKey] to the [ScaffoldMessenger]. This
+  /// example shows a key being used to obtain the [ScaffoldMessengerState]
+  /// provided by the [MaterialApp].
+  ///
+  /// ```dart imports
+  /// import 'package:flutter/material.dart';
+  /// ```
+  /// ```dart
+  /// void main() => runApp(MyApp());
+  ///
+  /// class MyApp extends StatefulWidget {
+  ///   @override
+  ///  _MyAppState createState() => _MyAppState();
+  /// }
+  ///
+  /// class _MyAppState extends State<MyApp> {
+  ///   final GlobalKey<ScaffoldMessengerState> _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
+  ///   int _counter = 0;
+  ///
+  ///   void _incrementCounter() {
+  ///     setState(() {
+  ///       _counter++;
+  ///     });
+  ///     if (_counter % 10 == 0) {
+  ///       _scaffoldMessengerKey.currentState.showSnackBar(const SnackBar(
+  ///         content: Text('A multiple of ten!'),
+  ///       ));
+  ///     }
+  ///   }
+  ///
+  ///   @override
+  ///   Widget build(BuildContext context) {
+  ///     return MaterialApp(
+  ///       scaffoldMessengerKey: _scaffoldMessengerKey,
+  ///       home: Scaffold(
+  ///         appBar: AppBar(title: Text('ScaffoldMessenger Demo')),
+  ///         body: Center(
+  ///           child: Column(
+  ///             mainAxisAlignment: MainAxisAlignment.center,
+  ///             children: <Widget>[
+  ///               Text(
+  ///                 'You have pushed the button this many times:',
+  ///               ),
+  ///               Text(
+  ///                 '$_counter',
+  ///                 style: Theme.of(context).textTheme.headline4,
+  ///               ),
+  ///             ],
+  ///           ),
+  ///         ),
+  ///         floatingActionButton: FloatingActionButton(
+  ///           onPressed: _incrementCounter,
+  ///           tooltip: 'Increment',
+  ///           child: Icon(Icons.add),
+  ///         ),
+  ///       ),
+  ///     );
+  ///   }
+  /// }
+  ///
+  /// ```
+  /// {@end-tool}
+  ///
+  /// If there is no [ScaffoldMessenger] in scope, then this will throw an
+  /// exception.
+  static ScaffoldMessengerState of(BuildContext context) {
+    assert(context != null);
+    final _ScaffoldMessengerScope scope = context.dependOnInheritedWidgetOfExactType<_ScaffoldMessengerScope>();
+    return scope?._scaffoldMessengerState;
+  }
+
+  @override
+  ScaffoldMessengerState createState() => ScaffoldMessengerState();
+}
+
+/// State for a [ScaffoldMessenger].
+///
+/// A [ScaffoldMessengerState] object can be used to [showSnackBar] for every
+/// registered [Scaffold] that is a descendant of the associated
+/// [ScaffoldMessenger]. Scaffolds will register to receive [SnackBar]s from
+/// their closest ScaffoldMessenger ancestor.
+///
+/// Typically obtained via [ScaffoldMessenger.of].
+class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProviderStateMixin {
+  final LinkedHashSet<ScaffoldState> _scaffolds = LinkedHashSet<ScaffoldState>();
+  final Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>> _snackBars = Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>>();
+  AnimationController _snackBarController;
+  Timer _snackBarTimer;
+  bool _accessibleNavigation;
+
+  @override
+  void didChangeDependencies() {
+    final MediaQueryData mediaQuery = MediaQuery.of(context);
+    // If we transition from accessible navigation to non-accessible navigation
+    // and there is a SnackBar that would have timed out that has already
+    // completed its timer, dismiss that SnackBar. If the timer hasn't finished
+    // yet, let it timeout as normal.
+    if (_accessibleNavigation == true
+        && !mediaQuery.accessibleNavigation
+        && _snackBarTimer != null
+        && !_snackBarTimer.isActive) {
+      hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
+    }
+    _accessibleNavigation = mediaQuery.accessibleNavigation;
+    super.didChangeDependencies();
+  }
+
+  void _register(ScaffoldState scaffold) {
+    _scaffolds.add(scaffold);
+    if (_snackBars.isNotEmpty) {
+      scaffold._updateSnackBar();
+    }
+  }
+
+  void _unregister(ScaffoldState scaffold) {
+    final bool removed = _scaffolds.remove(scaffold);
+    // ScaffoldStates should only be removed once.
+    assert(removed);
+  }
+
+  /// Shows  a [SnackBar] across all registered [Scaffold]s.
+  ///
+  /// A scaffold can show at most one snack bar at a time. If this function is
+  /// called while another snack bar is already visible, the given snack bar
+  /// will be added to a queue and displayed after the earlier snack bars have
+  /// closed.
+  ///
+  /// To control how long a [SnackBar] remains visible, use [SnackBar.duration].
+  ///
+  /// To remove the [SnackBar] with an exit animation, use [hideCurrentSnackBar]
+  /// or call [ScaffoldFeatureController.close] on the returned
+  /// [ScaffoldFeatureController]. To remove a [SnackBar] suddenly (without an
+  /// animation), use [removeCurrentSnackBar].
+  ///
+  /// See [ScaffoldMessenger.of] for information about how to obtain the
+  /// [ScaffoldMessengerState].
+  ///
+  /// {@tool dartpad --template=stateless_widget_scaffold_center}
+  ///
+  /// Here is an example of showing a [SnackBar] when the user presses a button.
+  ///
+  /// ```dart
+  ///   Widget build(BuildContext context) {
+  ///     return OutlinedButton(
+  ///       onPressed: () {
+  ///         ScaffoldMessenger.of(context).showSnackBar(
+  ///           const SnackBar(
+  ///             content: Text('A SnackBar has been shown.'),
+  ///           ),
+  ///         );
+  ///       },
+  ///       child: const Text('Show SnackBar'),
+  ///     );
+  ///   }
+  /// ```
+  /// {@end-tool}
+  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackBar) {
+    _snackBarController ??= SnackBar.createAnimationController(vsync: this)
+      ..addStatusListener(_handleStatusChanged);
+    if (_snackBars.isEmpty) {
+      assert(_snackBarController.isDismissed);
+      _snackBarController.forward();
+    }
+    ScaffoldFeatureController<SnackBar, SnackBarClosedReason> controller;
+    controller = ScaffoldFeatureController<SnackBar, SnackBarClosedReason>._(
+      // We provide a fallback key so that if back-to-back snackbars happen to
+      // match in structure, material ink splashes and highlights don't survive
+      // from one to the next.
+      snackBar.withAnimation(_snackBarController, fallbackKey: UniqueKey()),
+      Completer<SnackBarClosedReason>(),
+        () {
+        assert(_snackBars.first == controller);
+        hideCurrentSnackBar(reason: SnackBarClosedReason.hide);
+      },
+      null, // SnackBar doesn't use a builder function so setState() wouldn't rebuild it
+    );
+    setState(() {
+      _snackBars.addLast(controller);
+    });
+    _updateScaffolds();
+    return controller;
+  }
+
+  void _handleStatusChanged(AnimationStatus status) {
+    switch (status) {
+      case AnimationStatus.dismissed:
+        assert(_snackBars.isNotEmpty);
+        setState(() {
+          _snackBars.removeFirst();
+        });
+        _updateScaffolds();
+        if (_snackBars.isNotEmpty) {
+          _snackBarController.forward();
+        }
+        break;
+      case AnimationStatus.completed:
+        setState(() {
+          assert(_snackBarTimer == null);
+          // build will create a new timer if necessary to dismiss the snackBar.
+        });
+        _updateScaffolds();
+        break;
+      case AnimationStatus.forward:
+        break;
+      case AnimationStatus.reverse:
+        break;
+    }
+  }
+
+  void _updateScaffolds() {
+    for (final ScaffoldState scaffold in _scaffolds) {
+      scaffold._updateSnackBar();
+    }
+  }
+
+  /// Removes the current [SnackBar] (if any) immediately from registered
+  /// [Scaffold]s.
+  ///
+  /// The removed snack bar does not run its normal exit animation. If there are
+  /// any queued snack bars, they begin their entrance animation immediately.
+  void removeCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.remove }) {
+    assert(reason != null);
+    if (_snackBars.isEmpty)
+      return;
+    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
+    if (!completer.isCompleted)
+      completer.complete(reason);
+    _snackBarTimer?.cancel();
+    _snackBarTimer = null;
+    // This will trigger the animation's status callback.
+    _snackBarController.value = 0.0;
+  }
+
+  /// Removes the current [SnackBar] by running its normal exit animation.
+  ///
+  /// The closed completer is called after the animation is complete.
+  void hideCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.hide }) {
+    assert(reason != null);
+    if (_snackBars.isEmpty || _snackBarController.status == AnimationStatus.dismissed)
+      return;
+    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
+    if (_accessibleNavigation) {
+      _snackBarController.value = 0.0;
+      completer.complete(reason);
+    } else {
+      _snackBarController.reverse().then<void>((void value) {
+        assert(mounted);
+        if (!completer.isCompleted)
+          completer.complete(reason);
+      });
+    }
+    _snackBarTimer?.cancel();
+    _snackBarTimer = null;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    assert(debugCheckHasMediaQuery(context));
+    final MediaQueryData mediaQuery = MediaQuery.of(context);
+    _accessibleNavigation = mediaQuery.accessibleNavigation;
+
+    if (_snackBars.isNotEmpty) {
+      final ModalRoute<dynamic> route = ModalRoute.of(context);
+      if (route == null || route.isCurrent) {
+        if (_snackBarController.isCompleted && _snackBarTimer == null) {
+          final SnackBar snackBar = _snackBars.first._widget;
+          _snackBarTimer = Timer(snackBar.duration, () {
+            assert(_snackBarController.status == AnimationStatus.forward || _snackBarController.status == AnimationStatus.completed);
+            // Look up MediaQuery again in case the setting changed.
+            final MediaQueryData mediaQuery = MediaQuery.of(context);
+            if (mediaQuery.accessibleNavigation && snackBar.action != null)
+              return;
+            hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
+          });
+        }
+      }
+    }
+
+    return _ScaffoldMessengerScope(
+      scaffoldMessengerState: this,
+      child: widget.child,
+    );
+  }
+
+  @override
+  void dispose() {
+    _snackBarController?.dispose();
+    _snackBarTimer?.cancel();
+    _snackBarTimer = null;
+    super.dispose();
+  }
+}
+
+class _ScaffoldMessengerScope extends InheritedWidget {
+  const _ScaffoldMessengerScope({
+    Key key,
+    Widget child,
+    ScaffoldMessengerState scaffoldMessengerState,
+  }) : _scaffoldMessengerState = scaffoldMessengerState,
+      super(key: key, child: child);
+
+  final ScaffoldMessengerState _scaffoldMessengerState;
+
+  @override
+  bool updateShouldNotify(_ScaffoldMessengerScope old) => _scaffoldMessengerState != old._scaffoldMessengerState;
+}
+
 /// The geometry of the [Scaffold] after all its contents have been laid out
 /// except the [FloatingActionButton].
 ///
@@ -837,11 +1204,11 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 
 /// Implements the basic material design visual layout structure.
 ///
-/// This class provides APIs for showing drawers, snack bars, and bottom sheets.
+/// This class provides APIs for showing drawers and bottom sheets.
 ///
-/// To display a snackbar or a persistent bottom sheet, obtain the
+/// To display a persistent bottom sheet, obtain the
 /// [ScaffoldState] for the current [BuildContext] via [Scaffold.of] and use the
-/// [ScaffoldState.showSnackBar] and [ScaffoldState.showBottomSheet] functions.
+/// [ScaffoldState.showBottomSheet] function.
 ///
 /// {@tool dartpad --template=stateful_widget_material}
 /// This example shows a [Scaffold] with a [body] and [FloatingActionButton].
@@ -918,7 +1285,7 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// Widget build(BuildContext context) {
 ///   return Scaffold(
 ///     appBar: AppBar(
-///       title: Text('Sample Code'),
+///       title: const Text('Sample Code'),
 ///     ),
 ///     body: Center(
 ///       child: Text('You have pressed the button $_count times.'),
@@ -1006,8 +1373,6 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 ///  * [BottomNavigationBar], which is a horizontal array of buttons typically
 ///    shown along the bottom of the app using the [bottomNavigationBar]
 ///    property.
-///  * [SnackBar], which is a temporary notification typically shown near the
-///    bottom of the app using the [ScaffoldState.showSnackBar] method.
 ///  * [BottomSheet], which is an overlay typically shown near the bottom of the
 ///    app. A bottom sheet can either be persistent, in which case it is shown
 ///    using the [ScaffoldState.showBottomSheet] method, or modal, in which case
@@ -1015,7 +1380,6 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 ///  * [ScaffoldState], which is the state associated with this widget.
 ///  * <https://material.io/design/layout/responsive-layout-grid.html>
 ///  * Cookbook: [Add a Drawer to a screen](https://flutter.dev/docs/cookbook/design/drawer)
-///  * Cookbook: [Display a snackbar](https://flutter.dev/docs/cookbook/design/snackbars)
 ///  * See our
 ///    [Scaffold Sample Apps](https://flutter.dev/docs/catalog/samples/Scaffold).
 class Scaffold extends StatefulWidget {
@@ -1387,7 +1751,7 @@ class Scaffold extends StatefulWidget {
   ///       ),
   ///       home: Scaffold(
   ///         body: MyScaffoldBody(),
-  ///         appBar: AppBar(title: Text('Scaffold.of Example')),
+  ///         appBar: AppBar(title: const Text('Scaffold.of Example')),
   ///       ),
   ///       color: Colors.white,
   ///     );
@@ -1401,14 +1765,32 @@ class Scaffold extends StatefulWidget {
   ///   Widget build(BuildContext context) {
   ///     return Center(
   ///       child: ElevatedButton(
-  ///         child: Text('SHOW A SNACKBAR'),
+  ///         child: const Text('SHOW BOTTOM SHEET'),
   ///         onPressed: () {
-  ///           Scaffold.of(context).showSnackBar(
-  ///             SnackBar(
-  ///               content: Text('Have a snack!'),
+  ///           Scaffold.of(context).showBottomSheet<void>(
+  ///             (BuildContext context) {
+  ///               return Container(
+  ///                 alignment: Alignment.center,
+  ///                 height: 200,
+  ///                 color: Colors.amber,
+  ///                 child: Center(
+  ///                   child: Column(
+  ///                     mainAxisSize: MainAxisSize.min,
+  ///                     children: <Widget>[
+  ///                       const Text('BottomSheet'),
+  ///                       ElevatedButton(
+  ///                         child: const Text('Close BottomSheet'),
+  ///                         onPressed: () {
+  ///                           Navigator.pop(context);
+  ///                         },
+  ///                       )
+  ///                     ],
+  ///                   ),
   ///                 ),
   ///               );
   ///             },
+  ///           );
+  ///         },
   ///       ),
   ///     );
   ///   }
@@ -1427,20 +1809,38 @@ class Scaffold extends StatefulWidget {
   /// ```dart
   /// Widget build(BuildContext context) {
   ///   return Scaffold(
-  ///     appBar: AppBar(
-  ///       title: Text('Demo')
-  ///     ),
+  ///     appBar: AppBar(title: const Text('Demo')),
   ///     body: Builder(
   ///       // Create an inner BuildContext so that the onPressed methods
   ///       // can refer to the Scaffold with Scaffold.of().
   ///       builder: (BuildContext context) {
   ///         return Center(
   ///           child: ElevatedButton(
-  ///             child: Text('SHOW A SNACKBAR'),
+  ///             child: const Text('SHOW BOTTOM SHEET'),
   ///             onPressed: () {
-  ///               Scaffold.of(context).showSnackBar(SnackBar(
-  ///                 content: Text('Have a snack!'),
-  ///               ));
+  ///               Scaffold.of(context).showBottomSheet<void>(
+  ///                 (BuildContext context) {
+  ///                   return Container(
+  ///                     alignment: Alignment.center,
+  ///                     height: 200,
+  ///                     color: Colors.amber,
+  ///                     child: Center(
+  ///                       child: Column(
+  ///                         mainAxisSize: MainAxisSize.min,
+  ///                         children: <Widget>[
+  ///                           const Text('BottomSheet'),
+  ///                           ElevatedButton(
+  ///                             child: const Text('Close BottomSheet'),
+  ///                             onPressed: () {
+  ///                               Navigator.pop(context);
+  ///                             },
+  ///                           )
+  ///                         ],
+  ///                       ),
+  ///                     ),
+  ///                   );
+  ///                 },
+  ///               );
   ///             },
   ///           ),
   ///         );
@@ -1557,7 +1957,7 @@ class Scaffold extends StatefulWidget {
   /// See also:
   ///
   ///  * [Scaffold.of], which provides access to the [ScaffoldState] object as a
-  ///    whole, from which you can show snackbars, bottom sheets, and so forth.
+  ///    whole, from which you can show bottom sheets, and so forth.
   static bool hasDrawer(BuildContext context, { bool registerForUpdates = true }) {
     assert(registerForUpdates != null);
     assert(context != null);
@@ -1576,8 +1976,8 @@ class Scaffold extends StatefulWidget {
 
 /// State for a [Scaffold].
 ///
-/// Can display [SnackBar]s and [BottomSheet]s. Retrieve a [ScaffoldState] from
-/// the current [BuildContext] using [Scaffold.of].
+/// Can display [BottomSheet]s. Retrieve a [ScaffoldState] from the current
+/// [BuildContext] using [Scaffold.of].
 class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
   // DRAWER API
@@ -1668,12 +2068,10 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
   // SNACKBAR API
 
-  final Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>> _snackBars = Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>>();
-  AnimationController _snackBarController;
-  Timer _snackBarTimer;
-  bool _accessibleNavigation;
+  ScaffoldMessengerState _scaffoldMessenger;
 
-  /// Shows a [SnackBar] at the bottom of the scaffold.
+  /// [ScaffoldMessengerState.showSnackBar] shows a [SnackBar] at the bottom of
+  /// the scaffold. This method should not be used.
   ///
   /// A scaffold can show at most one snack bar at a time. If this function is
   /// called while another snack bar is already visible, the given snack bar
@@ -1682,12 +2080,14 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   ///
   /// To control how long a [SnackBar] remains visible, use [SnackBar.duration].
   ///
-  /// To remove the [SnackBar] with an exit animation, use [hideCurrentSnackBar]
-  /// or call [ScaffoldFeatureController.close] on the returned
-  /// [ScaffoldFeatureController]. To remove a [SnackBar] suddenly (without an
-  /// animation), use [removeCurrentSnackBar].
+  /// To remove the [SnackBar] with an exit animation, use
+  /// [ScaffoldMessengerState.hideCurrentSnackBar] or call
+  /// [ScaffoldFeatureController.close] on the returned [ScaffoldFeatureController].
+  /// To remove a [SnackBar] suddenly (without an animation), use
+  /// [ScaffoldMessengerState.removeCurrentSnackBar].
   ///
-  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
+  /// See [ScaffoldMessenger.of] for information about how to obtain the
+  /// [ScaffoldMessengerState].
   ///
   /// {@tool dartpad --template=stateless_widget_scaffold_center}
   ///
@@ -1697,104 +2097,66 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   ///   Widget build(BuildContext context) {
   ///     return OutlinedButton(
   ///       onPressed: () {
-  ///         Scaffold.of(context).showSnackBar(
+  ///         ScaffoldMessenger.of(context).showSnackBar(
   ///           SnackBar(
-  ///             content: Text('A SnackBar has been shown.'),
+  ///             content: const Text('A SnackBar has been shown.'),
   ///           ),
   ///         );
   ///       },
-  ///       child: Text('Show SnackBar'),
+  ///       child: const Text('Show SnackBar'),
   ///     );
   ///   }
   /// ```
   /// {@end-tool}
+  ///
+  /// See also:
+  ///
+  ///   * [ScaffoldMessenger], this should be used instead to manage [SnackBar]s.
+  // TODO(Piinks): Deprecate after customers are migrated
   ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackbar) {
-    _snackBarController ??= SnackBar.createAnimationController(vsync: this)
-      ..addStatusListener(_handleSnackBarStatusChange);
-    if (_snackBars.isEmpty) {
-      assert(_snackBarController.isDismissed);
-      _snackBarController.forward();
-    }
-    ScaffoldFeatureController<SnackBar, SnackBarClosedReason> controller;
-    controller = ScaffoldFeatureController<SnackBar, SnackBarClosedReason>._(
-      // We provide a fallback key so that if back-to-back snackbars happen to
-      // match in structure, material ink splashes and highlights don't survive
-      // from one to the next.
-      snackbar.withAnimation(_snackBarController, fallbackKey: UniqueKey()),
-      Completer<SnackBarClosedReason>(),
-      () {
-        assert(_snackBars.first == controller);
-        hideCurrentSnackBar(reason: SnackBarClosedReason.hide);
-      },
-      null, // SnackBar doesn't use a builder function so setState() wouldn't rebuild it
-    );
-    setState(() {
-      _snackBars.addLast(controller);
-    });
-    return controller;
+    assert(debugCheckHasScaffoldMessenger(context));
+    return _scaffoldMessenger.showSnackBar(snackbar);
   }
 
-  void _handleSnackBarStatusChange(AnimationStatus status) {
-    switch (status) {
-      case AnimationStatus.dismissed:
-        assert(_snackBars.isNotEmpty);
-        setState(() {
-          _snackBars.removeFirst();
-        });
-        if (_snackBars.isNotEmpty)
-          _snackBarController.forward();
-        break;
-      case AnimationStatus.completed:
-        setState(() {
-          assert(_snackBarTimer == null);
-          // build will create a new timer if necessary to dismiss the snack bar
-        });
-        break;
-      case AnimationStatus.forward:
-      case AnimationStatus.reverse:
-        break;
-    }
-  }
-
-  /// Removes the current [SnackBar] (if any) immediately.
+  /// [ScaffoldMessengerState.removeCurrentSnackBar] removes the current
+  /// [SnackBar] (if any) immediately. This method should not be used.
   ///
   /// The removed snack bar does not run its normal exit animation. If there are
   /// any queued snack bars, they begin their entrance animation immediately.
+  ///
+  /// See also:
+  ///
+  ///   * [ScaffoldMessenger], this should be used instead to manage [SnackBar]s.
+  // TODO(Piinks): Deprecate after customers are migrated
   void removeCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.remove }) {
-    assert(reason != null);
-    if (_snackBars.isEmpty)
-      return;
-    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
-    if (!completer.isCompleted)
-      completer.complete(reason);
-    _snackBarTimer?.cancel();
-    _snackBarTimer = null;
-    _snackBarController.value = 0.0;
+    assert(debugCheckHasScaffoldMessenger(context));
+    _scaffoldMessenger.removeCurrentSnackBar(reason: reason);
   }
 
-  /// Removes the current [SnackBar] by running its normal exit animation.
+  /// [ScaffoldMessengerState.hideCurrentSnackBar] removes the current
+  /// [SnackBar] by running its normal exit animation. This method should not be
+  /// used.
   ///
   /// The closed completer is called after the animation is complete.
+  ///
+  /// See also:
+  ///
+  ///   * [ScaffoldMessenger], this should be used instead to manage [SnackBar]s.
+  // TODO(Piinks): Deprecate after customers are migrated.
   void hideCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.hide }) {
-    assert(reason != null);
-    if (_snackBars.isEmpty || _snackBarController.status == AnimationStatus.dismissed)
-      return;
-    final MediaQueryData mediaQuery = MediaQuery.of(context);
-    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
-    if (mediaQuery.accessibleNavigation) {
-      _snackBarController.value = 0.0;
-      completer.complete(reason);
-    } else {
-      _snackBarController.reverse().then<void>((void value) {
-        assert(mounted);
-        if (!completer.isCompleted)
-          completer.complete(reason);
-      });
-    }
-    _snackBarTimer?.cancel();
-    _snackBarTimer = null;
+    assert(debugCheckHasScaffoldMessenger(context));
+    _scaffoldMessenger.hideCurrentSnackBar(reason: reason);
   }
 
+  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> _snackBar;
+
+  void _updateSnackBar() {
+    setState(() {
+        _snackBar = _scaffoldMessenger._snackBars.isNotEmpty
+          ? _scaffoldMessenger._snackBars.first
+          : null;
+    });
+  }
 
   // PERSISTENT BOTTOM SHEET API
 
@@ -2016,7 +2378,9 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   ///                     const Text('BottomSheet'),
   ///                     ElevatedButton(
   ///                       child: const Text('Close BottomSheet'),
-  ///                       onPressed: () => Navigator.pop(context),
+  ///                       onPressed: () {
+  ///                         Navigator.pop(context);
+  ///                       }
   ///                     )
   ///                   ],
   ///                 ),
@@ -2204,27 +2568,14 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
   @override
   void didChangeDependencies() {
-    final MediaQueryData mediaQuery = MediaQuery.of(context);
-    // If we transition from accessible navigation to non-accessible navigation
-    // and there is a SnackBar that would have timed out that has already
-    // completed its timer, dismiss that SnackBar. If the timer hasn't finished
-    // yet, let it timeout as normal.
-    if (_accessibleNavigation == true
-      && !mediaQuery.accessibleNavigation
-      && _snackBarTimer != null
-      && !_snackBarTimer.isActive) {
-      hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
-    }
-    _accessibleNavigation = mediaQuery.accessibleNavigation;
+    _scaffoldMessenger = ScaffoldMessenger.of(context);
+    _scaffoldMessenger?._register(this);
     _maybeBuildPersistentBottomSheet();
     super.didChangeDependencies();
   }
 
   @override
   void dispose() {
-    _snackBarController?.dispose();
-    _snackBarTimer?.cancel();
-    _snackBarTimer = null;
     _geometryNotifier.dispose();
     for (final _StandardBottomSheet bottomSheet in _dismissedBottomSheets) {
       bottomSheet.animationController?.dispose();
@@ -2234,6 +2585,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     }
     _floatingActionButtonMoveController.dispose();
     _floatingActionButtonVisibilityController.dispose();
+    _scaffoldMessenger?._unregister(this);
     super.dispose();
   }
 
@@ -2347,28 +2699,6 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     final MediaQueryData mediaQuery = MediaQuery.of(context);
     final ThemeData themeData = Theme.of(context);
     final TextDirection textDirection = Directionality.of(context);
-    _accessibleNavigation = mediaQuery.accessibleNavigation;
-
-    if (_snackBars.isNotEmpty) {
-      final ModalRoute<dynamic> route = ModalRoute.of(context);
-      if (route == null || route.isCurrent) {
-        if (_snackBarController.isCompleted && _snackBarTimer == null) {
-          final SnackBar snackBar = _snackBars.first._widget;
-          _snackBarTimer = Timer(snackBar.duration, () {
-            assert(_snackBarController.status == AnimationStatus.forward ||
-                   _snackBarController.status == AnimationStatus.completed);
-            // Look up MediaQuery again in case the setting changed.
-            final MediaQueryData mediaQuery = MediaQuery.of(context);
-            if (mediaQuery.accessibleNavigation && snackBar.action != null)
-              return;
-            hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
-          });
-        }
-      } else {
-        _snackBarTimer?.cancel();
-        _snackBarTimer = null;
-      }
-    }
 
     final List<LayoutId> children = <LayoutId>[];
     _addIfNonNull(
@@ -2423,16 +2753,16 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
     bool isSnackBarFloating = false;
     double snackBarWidth;
-    if (_snackBars.isNotEmpty) {
-      final SnackBarBehavior snackBarBehavior = _snackBars.first._widget.behavior
+    if (_snackBar != null) {
+      final SnackBarBehavior snackBarBehavior = _snackBar._widget.behavior
         ?? themeData.snackBarTheme.behavior
         ?? SnackBarBehavior.fixed;
       isSnackBarFloating = snackBarBehavior == SnackBarBehavior.floating;
-      snackBarWidth = _snackBars.first._widget.width;
+      snackBarWidth = _snackBar._widget.width;
 
       _addIfNonNull(
         children,
-        _snackBars.first._widget,
+        _snackBar._widget,
         _ScaffoldSlot.snackBar,
         removeLeftPadding: false,
         removeTopPadding: true,
@@ -2596,7 +2926,8 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
 /// An interface for controlling a feature of a [Scaffold].
 ///
-/// Commonly obtained from [ScaffoldState.showSnackBar] or [ScaffoldState.showBottomSheet].
+/// Commonly obtained from [ScaffoldMessengerState.showSnackBar] or
+/// [ScaffoldState.showBottomSheet].
 class ScaffoldFeatureController<T extends Widget, U> {
   const ScaffoldFeatureController._(this._widget, this._completer, this.close, this.setState);
   final T _widget;

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

@@ -32,7 +32,7 @@ const Curve _snackBarFadeOutCurve = Interval(0.72, 1.0, curve: Curves.fastOutSlo
 
 /// Specify how a [SnackBar] was closed.
 ///
-/// The [ScaffoldState.showSnackBar] function returns a
+/// The [ScaffoldMessengerState.showSnackBar] function returns a
 /// [ScaffoldFeatureController]. The value of the controller's closed property
 /// is a Future that resolves to a SnackBarClosedReason. Applications that need
 /// to know how a snackbar was closed can use this value.
@@ -40,7 +40,7 @@ const Curve _snackBarFadeOutCurve = Interval(0.72, 1.0, curve: Curves.fastOutSlo
 /// Example:
 ///
 /// ```dart
-/// Scaffold.of(context).showSnackBar(
+/// ScaffoldMessenger.of(context).showSnackBar(
 ///   SnackBar( ... )
 /// ).closed.then((SnackBarClosedReason reason) {
 ///    ...
@@ -57,10 +57,10 @@ enum SnackBarClosedReason {
   swipe,
 
   /// The snack bar was closed by the [ScaffoldFeatureController] close callback
-  /// or by calling [ScaffoldState.hideCurrentSnackBar] directly.
+  /// or by calling [ScaffoldMessengerState.hideCurrentSnackBar] directly.
   hide,
 
-  /// The snack bar was closed by an call to [ScaffoldState.removeCurrentSnackBar].
+  /// The snack bar was closed by an call to [ScaffoldMessengerState.removeCurrentSnackBar].
   remove,
 
   /// The snack bar was closed because its timer expired.
@@ -123,7 +123,7 @@ class _SnackBarActionState extends State<SnackBarAction> {
       _haveTriggeredAction = true;
     });
     widget.onPressed();
-    Scaffold.of(context).hideCurrentSnackBar(reason: SnackBarClosedReason.action);
+    ScaffoldMessenger.of(context).hideCurrentSnackBar(reason: SnackBarClosedReason.action);
   }
 
   @override
@@ -146,8 +146,8 @@ class _SnackBarActionState extends State<SnackBarAction> {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=zpO6n_oZWw0}
 ///
-/// To display a snack bar, call `Scaffold.of(context).showSnackBar()`, passing
-/// an instance of [SnackBar] that describes the message.
+/// To display a snack bar, call `ScaffoldMessenger.of(context).showSnackBar()`,
+/// passing an instance of [SnackBar] that describes the message.
 ///
 /// To control how long the [SnackBar] remains visible, specify a [duration].
 ///
@@ -156,11 +156,11 @@ class _SnackBarActionState extends State<SnackBarAction> {
 ///
 /// See also:
 ///
-///  * [Scaffold.of], to obtain the current [ScaffoldState], which manages the
-///    display and animation of snack bars.
-///  * [ScaffoldState.showSnackBar], which displays a [SnackBar].
-///  * [ScaffoldState.removeCurrentSnackBar], which abruptly hides the currently
-///    displayed snack bar, if any, and allows the next to be displayed.
+///  * [ScaffoldMessenger.of], to obtain the current [ScaffoldMessengerState],
+///    which manages the display and animation of snack bars.
+///  * [ScaffoldMessengerState.showSnackBar], which displays a [SnackBar].
+///  * [ScaffoldMessengerState.removeCurrentSnackBar], which abruptly hides the
+///    currently displayed snack bar, if any, and allows the next to be displayed.
 ///  * [SnackBarAction], which is used to specify an [action] button to show
 ///    on the snack bar.
 ///  * [SnackBarThemeData], to configure the default property values for
@@ -289,7 +289,7 @@ class SnackBar extends StatefulWidget {
   ///
   /// See also:
   ///
-  ///  * [ScaffoldState.removeCurrentSnackBar], which abruptly hides the
+  ///  * [ScaffoldMessengerState.removeCurrentSnackBar], which abruptly hides the
   ///    currently displayed snack bar, if any, and allows the next to be
   ///    displayed.
   ///  * <https://material.io/design/components/snackbars.html>
@@ -301,7 +301,7 @@ class SnackBar extends StatefulWidget {
   /// Called the first time that the snackbar is visible within a [Scaffold].
   final VoidCallback onVisible;
 
-  // API for Scaffold.showSnackBar():
+  // API for ScaffoldMessengerState.showSnackBar():
 
   /// Creates an animation controller useful for driving a snack bar's entrance and exit animation.
   static AnimationController createAnimationController({ @required TickerProvider vsync }) {
@@ -516,14 +516,14 @@ class _SnackBarState extends State<SnackBar> {
       container: true,
       liveRegion: true,
       onDismiss: () {
-        Scaffold.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.dismiss);
+        ScaffoldMessenger.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.dismiss);
       },
       child: Dismissible(
         key: const Key('dismissible'),
         direction: DismissDirection.down,
         resizeDuration: null,
         onDismissed: (DismissDirection direction) {
-          Scaffold.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
+          ScaffoldMessenger.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
         },
         child: snackBar,
       ),
@@ -550,7 +550,9 @@ class _SnackBarState extends State<SnackBar> {
         child: snackBar,
       );
     }
-
-    return ClipRect(child: snackBarTransition);
+    return Hero(
+      child: ClipRect(child: snackBarTransition),
+      tag: '<SnackBar Hero tag - ${widget.content}>',
+    );
   }
 }

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

@@ -512,6 +512,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 /// class _SliverAnimatedListSampleState extends State<SliverAnimatedListSample> {
 ///   final GlobalKey<SliverAnimatedListState> _listKey = GlobalKey<SliverAnimatedListState>();
 ///   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+///   final GlobalKey<ScaffoldMessengerState> _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
 ///   ListModel<int> _list;
 ///   int _selectedItem;
 ///   int _nextItem; // The next item inserted when the user presses the '+' button.
@@ -569,7 +570,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 ///         _selectedItem = null;
 ///       });
 ///     } else {
-///       _scaffoldKey.currentState.showSnackBar(SnackBar(
+///       _scaffoldMessengerKey.currentState.showSnackBar(SnackBar(
 ///         content: Text(
 ///           'Select an item to remove from the list.',
 ///           style: TextStyle(fontSize: 20),
@@ -581,6 +582,7 @@ class AnimatedListState extends State<AnimatedList> with TickerProviderStateMixi
 ///   @override
 ///   Widget build(BuildContext context) {
 ///     return MaterialApp(
+///       scaffoldMessengerKey: _scaffoldMessengerKey,
 ///       home: Scaffold(
 ///         key: _scaffoldKey,
 ///         body: CustomScrollView(

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

@@ -2084,7 +2084,7 @@ typedef ElementVisitor = void Function(Element element);
 /// widget can be used: the build context passed to the [Builder.builder]
 /// callback will be that of the [Builder] itself.
 ///
-/// For example, in the following snippet, the [ScaffoldState.showSnackBar]
+/// For example, in the following snippet, the [ScaffoldState.showBottomSheet]
 /// method is called on the [Scaffold] widget that the build method itself
 /// creates. If a [Builder] had not been used, and instead the `context`
 /// argument of the build method itself had been used, no [Scaffold] would have
@@ -2101,13 +2101,32 @@ typedef ElementVisitor = void Function(Element element);
 ///         return TextButton(
 ///           child: Text('BUTTON'),
 ///           onPressed: () {
-///               // here, Scaffold.of(context) returns the locally created Scaffold
-///               Scaffold.of(context).showSnackBar(SnackBar(
-///                 content: Text('Hello.')
-///               ));
-///             }
+///             Scaffold.of(context).showBottomSheet<void>(
+///               (BuildContext context) {
+///                 return Container(
+///                   alignment: Alignment.center,
+///                   height: 200,
+///                   color: Colors.amber,
+///                   child: Center(
+///                     child: Column(
+///                       mainAxisSize: MainAxisSize.min,
+///                       children: <Widget>[
+///                         const Text('BottomSheet'),
+///                         ElevatedButton(
+///                           child: const Text('Close BottomSheet'),
+///                           onPressed: () {
+///                             Navigator.pop(context),
+///                           },
+///                         )
+///                       ],
+///                     ),
+///                   ),
 ///                 );
-///         }
+///               },
+///             );
+///           },
+///         );
+///       },
 ///     )
 ///   );
 /// }

packages/flutter/test/material/debug_test.dart

@@ -164,6 +164,8 @@ void main() {
       '     _InheritedTheme\n'
       '     Theme\n'
       '     AnimatedTheme\n'
+      '     _ScaffoldMessengerScope\n'
+      '     ScaffoldMessenger\n'
       '     Builder\n'
       '     DefaultTextStyle\n'
       '     CustomPaint\n'
@@ -196,4 +198,50 @@ void main() {
       '   or WidgetsApp widget at the top of your application widget tree.\n',
     ));
   });
+
+  testWidgets('debugCheckHasScaffoldMessenger control test', (WidgetTester tester) async {
+    final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: MediaQuery(
+        data: const MediaQueryData(),
+        child: Scaffold(
+          key: _scaffoldKey,
+          body: Container(),
+        ),
+      ),
+    ));
+    FlutterError error;
+    try {
+      _scaffoldKey.currentState.showSnackBar(const SnackBar(content: Text('Something is missing here')));
+    } on FlutterError catch (e) {
+      error = e;
+    } finally {
+      expect(error.diagnostics.length, 5);
+      expect(error.diagnostics[2], isA<DiagnosticsProperty<Element>>());
+      expect(error.diagnostics[3], isA<DiagnosticsBlock>());
+      expect(error.diagnostics[4].level, DiagnosticLevel.hint);
+      expect(
+        error.diagnostics[4].toStringDeep(),
+        equalsIgnoringHashCodes(
+          'Typically, the ScaffoldMessenger widget is introduced by the\n'
+          'MaterialApp at the top of your application widget tree.\n',
+        ),
+      );
+      expect(error.toStringDeep(), equalsIgnoringHashCodes(
+        'FlutterError\n'
+          '   No ScaffoldMessenger widget found.\n'
+          '   Scaffold widgets require a ScaffoldMessenger widget ancestor.\n'
+          '   The specific widget that could not find a ScaffoldMessenger\n'
+          '   ancestor was:\n'
+          '     Scaffold-[LabeledGlobalKey<ScaffoldState>#d60fa]\n'
+          '   The ancestors of this widget were:\n'
+          '     MediaQuery\n'
+          '     Directionality\n'
+          '     [root]\n'
+          '   Typically, the ScaffoldMessenger widget is introduced by the\n'
+          '   MaterialApp at the top of your application widget tree.\n'
+      ));
+    }
+  });
 }

packages/flutter/test/material/floating_action_button_location_test.dart

@@ -649,7 +649,7 @@ void main() {
               builder: (BuildContext context) {
                 return FloatingActionButton(
                   onPressed: () {
-                    Scaffold.of(context).showSnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(
                       const SnackBar(content: Text('Snacky!')),
                     );
                   },

packages/flutter/test/material/snack_bar_test.dart

@@ -18,7 +18,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(const SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
                   content: Text(helloSnackBar),
                   duration: Duration(seconds: 2),
                 ));
@@ -64,7 +64,7 @@ void main() {
             return GestureDetector(
               onTap: () {
                 snackBarCount += 1;
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: Text('bar$snackBarCount'),
                   duration: const Duration(seconds: 2),
                 ));
@@ -141,7 +141,7 @@ void main() {
             return GestureDetector(
               onTap: () {
                 snackBarCount += 1;
-                lastController = Scaffold.of(context).showSnackBar(SnackBar(
+                lastController = ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: Text('bar$snackBarCount'),
                   duration: Duration(seconds: time),
                 ));
@@ -225,7 +225,7 @@ void main() {
             return GestureDetector(
               onTap: () {
                 snackBarCount += 1;
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: Text('bar$snackBarCount'),
                   duration: const Duration(seconds: 2),
                 ));
@@ -268,7 +268,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('I am a snack bar.'),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(
@@ -309,7 +309,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(
                       SnackBar(
                         content: const Text('I am a snack bar.'),
                         duration: const Duration(seconds: 2),
@@ -351,7 +351,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(
                     SnackBar(
                       content: const Text('I am a snack bar.'),
                       duration: const Duration(seconds: 2),
@@ -389,7 +389,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(
                       SnackBar(
                         content: const Text('I am a snack bar.'),
                         margin: const EdgeInsets.all(padding),
@@ -476,7 +476,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(
                       const SnackBar(
                         content: Text('I am a snack bar.'),
                         padding: EdgeInsets.all(padding),
@@ -520,7 +520,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(
                       SnackBar(
                         content: const Text('I am a snack bar.'),
                         width: width,
@@ -558,7 +558,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(
                     SnackBar(
                       content: const Text('I am a snack bar.'),
                       duration: const Duration(seconds: 2),
@@ -611,7 +611,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     content: const Text('I am a snack bar.'),
                     duration: const Duration(seconds: 2),
                     action: SnackBarAction(label: 'ACTION', onPressed: () { }),
@@ -666,7 +666,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(SnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                       content: const Text('I am a snack bar.'),
                       duration: const Duration(seconds: 2),
                       action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -717,7 +717,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     content: const Text('I am a snack bar.'),
                     duration: const Duration(seconds: 2),
                     action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -771,7 +771,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     content: const Text('I am a snack bar.'),
                     duration: const Duration(seconds: 2),
                     action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -829,7 +829,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(SnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                       content: const Text('I am a snack bar.'),
                       duration: const Duration(seconds: 2),
                       action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -861,18 +861,18 @@ void main() {
     });
 
   testWidgets('SnackBarClosedReason', (WidgetTester tester) async {
-    final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
+    final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
     bool actionPressed = false;
     SnackBarClosedReason closedReason;
 
     await tester.pumpWidget(MaterialApp(
+      scaffoldMessengerKey: scaffoldMessengerKey,
       home: Scaffold(
-        key: scaffoldKey,
         body: Builder(
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('snack'),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(
@@ -917,14 +917,14 @@ void main() {
     // Pop up the snack bar and then remove it.
     await tester.tap(find.text('X'));
     await tester.pump(const Duration(milliseconds: 750));
-    scaffoldKey.currentState.removeCurrentSnackBar();
+    scaffoldMessengerKey.currentState.removeCurrentSnackBar();
     await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.remove));
 
     // Pop up the snack bar and then hide it.
     await tester.tap(find.text('X'));
     await tester.pump(const Duration(milliseconds: 750));
-    scaffoldKey.currentState.hideCurrentSnackBar();
+    scaffoldMessengerKey.currentState.hideCurrentSnackBar();
     await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.hide));
 
@@ -944,13 +944,14 @@ void main() {
     await tester.pumpWidget(MaterialApp(
       home: MediaQuery(
         data: const MediaQueryData(accessibleNavigation: true),
-        child: Scaffold(
-          key: scaffoldKey,
-          body: Builder(
+        child: ScaffoldMessenger(
+          child: Builder(
             builder: (BuildContext context) {
-              return GestureDetector(
+              return Scaffold(
+                key: scaffoldKey,
+                body: GestureDetector(
                   onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                       content: const Text('snack'),
                       duration: const Duration(seconds: 1),
                       action: SnackBarAction(
@@ -960,9 +961,10 @@ void main() {
                     ));
                   },
                   child: const Text('X'),
-              );
-            },
                 ),
+              );
+            }
+          )
         ),
       ),
     ));
@@ -986,13 +988,13 @@ void main() {
     await tester.pumpWidget(MaterialApp(
       home: MediaQuery(
         data: const MediaQueryData(accessibleNavigation: true),
-            child: Scaffold(
+        child: ScaffoldMessenger(
+          child: Builder(builder: (BuildContext context) {
+            return Scaffold(
               key: scaffoldKey,
-                body: Builder(
-                  builder: (BuildContext context) {
-                    return GestureDetector(
+              body: GestureDetector(
                 onTap: () {
-                          Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     content: const Text('snack'),
                     duration: const Duration(seconds: 1),
                     action: SnackBarAction(
@@ -1002,11 +1004,11 @@ void main() {
                   ));
                 },
                 child: const Text('X'),
-                    );
-                  },
-                ),
               ),
+            );
+          }),
         ),
+      )
     ));
     await tester.tap(find.text('X'));
     await tester.pumpAndSettle();
@@ -1031,7 +1033,7 @@ void main() {
                 builder: (BuildContext context) {
                   return GestureDetector(
                       onTap: () {
-                        Scaffold.of(context).showSnackBar(const SnackBar(
+                        ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
                             content: Text(helloSnackBar),
                         ));
                       },
@@ -1080,7 +1082,7 @@ void main() {
                       builder: (BuildContext context) {
                         return GestureDetector(
                             onTap: () {
-                              Scaffold.of(context).showSnackBar(SnackBar(
+                              ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                                   content: const Text('test'),
                                   action: SnackBarAction(label: 'foo', onPressed: () { }),
                               ));
@@ -1126,7 +1128,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: Text(nonconst('hello')),
                   duration: null,
                 ));
@@ -1156,7 +1158,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('hello'),
                   duration: const Duration(seconds: 1),
                   onVisible: () {
@@ -1193,14 +1195,14 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('hello'),
                   duration: const Duration(seconds: 1),
                   onVisible: () {
                     called += 1;
                   },
                 ));
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('hello 2'),
                   duration: const Duration(seconds: 1),
                   onVisible: () {
@@ -1247,8 +1249,8 @@ void main() {
             ),
           );
 
-          final ScaffoldState scaffoldState = tester.state(find.byType(Scaffold));
-          scaffoldState.showSnackBar(snackBar);
+          final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+          scaffoldMessengerState.showSnackBar(snackBar);
 
           await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
 
@@ -1278,8 +1280,8 @@ void main() {
             ),
           );
 
-          final ScaffoldState scaffoldState = tester.state(find.byType(Scaffold));
-          scaffoldState.showSnackBar(snackBar);
+          final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+          scaffoldMessengerState.showSnackBar(snackBar);
 
           await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
 
@@ -1298,9 +1300,8 @@ void main() {
       testWidgets(
         'Padding of $behavior is not consumed by viewInsets',
         (WidgetTester tester) async {
-          final Widget child = Directionality(
-            textDirection: TextDirection.ltr,
-            child: Scaffold(
+          final Widget child = MaterialApp(
+            home: Scaffold(
               resizeToAvoidBottomInset: false,
               floatingActionButton: FloatingActionButton(
                 child: const Icon(Icons.send),
@@ -1310,7 +1311,7 @@ void main() {
                 builder: (BuildContext context) {
                   return GestureDetector(
                     onTap: () {
-                      Scaffold.of(context).showSnackBar(
+                      ScaffoldMessenger.of(context).showSnackBar(
                         SnackBar(
                           content: const Text('I am a snack bar.'),
                           duration: const Duration(seconds: 2),
@@ -1374,8 +1375,8 @@ void main() {
           ),
         );
 
-        final ScaffoldState scaffoldState = tester.state(find.byType(Scaffold));
-        scaffoldState.showSnackBar(
+        final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+        scaffoldMessengerState.showSnackBar(
           const SnackBar(
             content: Text('Snackbar text'),
             behavior: SnackBarBehavior.fixed,
@@ -1410,7 +1411,7 @@ void main() {
               builder: (BuildContext context) {
                 return GestureDetector(
                   onTap: () {
-                    Scaffold.of(context).showSnackBar(SnackBar(
+                    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                       content: const Text('I am a snack bar.'),
                       duration: const Duration(seconds: 2),
                       action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -1452,8 +1453,8 @@ void main() {
           ),
         );
 
-        final ScaffoldState scaffoldState = tester.state(find.byType(Scaffold));
-        scaffoldState.showSnackBar(
+        final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+        scaffoldMessengerState.showSnackBar(
           const SnackBar(
             content: Text('SnackBar text'),
             behavior: SnackBarBehavior.fixed,
@@ -1489,8 +1490,8 @@ void main() {
           ),
         );
 
-        final ScaffoldState scaffoldState = tester.state(find.byType(Scaffold));
-        scaffoldState.showSnackBar(
+        final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+        scaffoldMessengerState.showSnackBar(
           const SnackBar(
             content: Text('SnackBar text'),
             behavior: SnackBarBehavior.floating,
@@ -1506,4 +1507,74 @@ void main() {
       },
     );
   });
+
+  testWidgets('SnackBars hero across transitions', (WidgetTester tester) async {
+    const String snackBarText = 'hello snackbar';
+    const String firstHeader = 'home';
+    const String secondHeader = 'second';
+    const Key snackTarget = Key('snack-target');
+    const Key transitionTarget = Key('transition-target');
+
+    Widget _buildApp() {
+      return MaterialApp(
+        routes: <String, WidgetBuilder> {
+          '/': (BuildContext context) {
+            return Scaffold(
+              appBar: AppBar(title: const Text(firstHeader)),
+              body: Center(
+                child: ElevatedButton(
+                  key: transitionTarget,
+                  child: const Text('PUSH'),
+                  onPressed: () {
+                    Navigator.of(context).pushNamed('/second');
+                  },
+                ),
+              ),
+              floatingActionButton: FloatingActionButton(
+                key: snackTarget,
+                onPressed: () async {
+                  ScaffoldMessenger.of(context).showSnackBar(
+                    const SnackBar(
+                      content: Text(snackBarText),
+                    ),
+                  );
+                },
+                child: const Text('X'),
+              ),
+            );
+          },
+          '/second': (BuildContext context) => Scaffold(appBar: AppBar(title: const Text(secondHeader)),
+      ),
+        }
+      );
+    }
+    await tester.pumpWidget(_buildApp());
+
+    expect(find.text(snackBarText), findsNothing);
+    expect(find.text(firstHeader), findsOneWidget);
+    expect(find.text(secondHeader), findsNothing);
+
+    // Present SnackBar
+    await tester.tap(find.byKey(snackTarget));
+    await tester.pump(); // schedule animation
+    expect(find.text(snackBarText), findsOneWidget);
+    await tester.pump(); // begin animation
+    expect(find.text(snackBarText), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750));
+    expect(find.text(snackBarText), findsOneWidget);
+    // Push new route
+    await tester.tap(find.byKey(transitionTarget));
+    await tester.pump();
+    expect(find.text(snackBarText), findsOneWidget);
+    expect(find.text(firstHeader), findsOneWidget);
+    expect(find.text(secondHeader, skipOffstage: false), findsOneWidget);
+    await tester.pump();
+    expect(find.text(snackBarText), findsOneWidget);
+    expect(find.text(firstHeader), findsOneWidget);
+    expect(find.text(secondHeader), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750));
+    expect(find.text(snackBarText), findsOneWidget);
+    expect(find.text(firstHeader), findsNothing);
+    expect(find.text(secondHeader), findsOneWidget);
+  });
 }

packages/flutter/test/material/snack_bar_theme_test.dart

@@ -73,7 +73,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text(text),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -110,7 +110,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     content: const Text(text),
                     duration: const Duration(seconds: 2),
                     action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -153,7 +153,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     backgroundColor: backgroundColor,
                     elevation: elevation,
                     shape: shape,
@@ -200,7 +200,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('I am a snack bar.'),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),
@@ -242,7 +242,7 @@ void main() {
           builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
-                Scaffold.of(context).showSnackBar(SnackBar(
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                   content: const Text('I am a snack bar.'),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),