Re-land ScaffoldMessenger (#66504)

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

@@ -15,6 +15,7 @@ import 'floating_action_button.dart';
 import 'icons.dart';
 import 'material_localizations.dart';
 import 'page.dart';
+import 'scaffold.dart' show ScaffoldMessenger, ScaffoldMessengerState;
 import 'theme.dart';
 
 /// [MaterialApp] uses this [TextStyle] as its [DefaultTextStyle] to encourage
@@ -166,6 +167,7 @@ class MaterialApp extends StatefulWidget {
   const MaterialApp({
     Key? key,
     this.navigatorKey,
+    this.scaffoldMessengerKey,
     this.home,
     this.routes = const <String, WidgetBuilder>{},
     this.initialRoute,
@@ -214,6 +216,7 @@ 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;
 
@@ -724,7 +735,9 @@ 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
@@ -745,6 +758,7 @@ class _MaterialAppState extends State<MaterialApp> {
               },
             )
           : child!,
+      )
     );
   }
 

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

@@ -135,7 +135,7 @@ 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

@@ -7,7 +7,7 @@ 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.
 ///
@@ -123,3 +123,34 @@ 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.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

@@ -17,6 +17,7 @@ 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';
@@ -59,6 +60,402 @@ 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.
+///
+/// {@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}
+///
+/// See also:
+///
+///  * [SnackBar], which is a temporary notification typically shown near the
+///    bottom of the app using the [ScaffoldMessengerState.showSnackBar] method.
+///  * [debugCheckHasScaffoldMessenger], which asserts that the given context
+///    has a [ScaffoldMessenger] ancestor.
+///  * 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 to 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 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 return null.
+  /// See also:
+  ///
+  ///  * [debugCheckHasScaffoldMessenger], which asserts that the given context
+  ///    has a [ScaffoldMessenger] ancestor.
+  static ScaffoldMessengerState? of(BuildContext context, { bool nullOk = false }) {
+    assert(nullOk != null);
+    assert(context != null);
+
+    assert(nullOk || debugCheckHasScaffoldMessenger(context));
+
+    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();
+    }
+    late 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,
+    required Widget child,
+    required 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].
 ///
@@ -835,11 +1232,11 @@ 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].
@@ -916,7 +1313,7 @@ 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.'),
@@ -1008,8 +1405,6 @@ 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
@@ -1017,7 +1412,6 @@ 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 {
@@ -1389,7 +1783,7 @@ 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,
   ///     );
@@ -1403,14 +1797,32 @@ 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);
+  ///                         },
+  ///                       )
+  ///                     ],
+  ///                   ),
   ///                 ),
   ///               );
   ///             },
+  ///           );
+  ///         },
   ///       ),
   ///     );
   ///   }
@@ -1429,20 +1841,38 @@ 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);
+  ///                             },
+  ///                           )
+  ///                         ],
+  ///                       ),
+  ///                     ),
+  ///                   );
+  ///                 },
+  ///               );
   ///             },
   ///           ),
   ///         );
@@ -1559,7 +1989,7 @@ 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);
@@ -1578,8 +2008,8 @@ 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
@@ -1669,13 +2099,15 @@ 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, and will be deprecated in
+  /// the near future..
   ///
   /// 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
@@ -1684,12 +2116,14 @@ 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}
   ///
@@ -1699,17 +2133,22 @@ 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 & defer to ScaffoldMessenger after customers are migrated.
   ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackbar) {
     _snackBarController ??= SnackBar.createAnimationController(vsync: this)
       ..addStatusListener(_handleSnackBarStatusChange);
@@ -1758,12 +2197,39 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     }
   }
 
-  /// Removes the current [SnackBar] (if any) immediately.
+  /// [ScaffoldMessengerState.removeCurrentSnackBar] removes the current
+  /// [SnackBar] (if any) immediately. This method should not be used, and will
+  /// be depracted in the near future.
   ///
   /// 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 & defer to ScaffoldMessenger after customers are migrated
   void removeCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.remove }) {
     assert(reason != null);
+
+    // SnackBars and SnackBarActions can call to hide and remove themselves, but
+    // they are not aware of who presented them, the Scaffold or the
+    // ScaffoldMessenger. As such, when the SnackBar classes call upon Scaffold
+    // to remove (the current default), we should re-direct to the
+    // ScaffoldMessenger here if that is where the SnackBar originated from.
+    if (_messengerSnackBar != null) {
+      // ScaffoldMessenger is presenting SnackBars.
+      assert(debugCheckHasScaffoldMessenger(context));
+      assert(
+        _scaffoldMessenger != null,
+        'A SnackBar was shown by the ScaffoldMessenger, but has been called upon'
+          'to be removed from a Scaffold that is not registered with a '
+          'ScaffoldMessenger, this can happen if a Scaffold has been rebuilt '
+          'without an ancestor ScaffoldMessenger.',
+      );
+      _scaffoldMessenger!.removeCurrentSnackBar(reason: reason);
+      return;
+    }
+
     if (_snackBars.isEmpty)
       return;
     final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
@@ -1774,11 +2240,38 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     _snackBarController!.value = 0.0;
   }
 
-  /// 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, and will be deprecated in the near future.
   ///
   /// 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 & defer to ScaffoldMessenger after customers are migrated.
   void hideCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.hide }) {
     assert(reason != null);
+
+    // SnackBars and SnackBarActions can call to hide and remove themselves, but
+    // they are not aware of who presented them, the Scaffold or the
+    // ScaffoldMessenger. As such, when the SnackBar classes call upon Scaffold
+    // to remove (the current default), we should re-direct to the
+    // ScaffoldMessenger here if that is where the SnackBar originated from.
+    if (_messengerSnackBar != null) {
+      // ScaffoldMessenger is presenting SnackBars.
+      assert(debugCheckHasScaffoldMessenger(context));
+      assert(
+      _scaffoldMessenger != null,
+      'A SnackBar was shown by the ScaffoldMessenger, but has been called upon'
+        'to be removed from a Scaffold that is not registered with a '
+        'ScaffoldMessenger, this can happen if a Scaffold has been rebuilt '
+        'without an ancestor ScaffoldMessenger.',
+      );
+      _scaffoldMessenger!.hideCurrentSnackBar(reason: reason);
+      return;
+    }
+
     if (_snackBars.isEmpty || _snackBarController!.status == AnimationStatus.dismissed)
       return;
     final MediaQueryData mediaQuery = MediaQuery.of(context)!;
@@ -1797,6 +2290,18 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     _snackBarTimer = null;
   }
 
+  // The _messengerSnackBar represents the current SnackBar being managed by
+  // the ScaffoldMessenger, instead of the Scaffold.
+  ScaffoldFeatureController<SnackBar, SnackBarClosedReason>? _messengerSnackBar;
+
+  // This is used to update the _messengerSnackBar by the ScaffoldMessenger.
+  void _updateSnackBar() {
+    setState(() {
+        _messengerSnackBar = _scaffoldMessenger!._snackBars.isNotEmpty
+          ? _scaffoldMessenger!._snackBars.first
+          : null;
+    });
+  }
 
   // PERSISTENT BOTTOM SHEET API
 
@@ -2010,7 +2515,9 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   ///                     const Text('BottomSheet'),
   ///                     ElevatedButton(
   ///                       child: const Text('Close BottomSheet'),
-  ///                       onPressed: () => Navigator.pop(context),
+  ///                       onPressed: () {
+  ///                         Navigator.pop(context);
+  ///                       }
   ///                     )
   ///                   ],
   ///                 ),
@@ -2198,6 +2705,19 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
   @override
   void didChangeDependencies() {
+    // nullOk is valid here since  both the Scaffold and ScaffoldMessenger are
+    // currently available for managing SnackBars.
+    final ScaffoldMessengerState? _currentScaffoldMessenger = ScaffoldMessenger.of(context, nullOk: true);
+    // If our ScaffoldMessenger has changed, unregister with the old one first.
+    if (_scaffoldMessenger != null &&
+      (_currentScaffoldMessenger == null || _scaffoldMessenger != _currentScaffoldMessenger)) {
+      _scaffoldMessenger?._unregister(this);
+    }
+    // Register with the current ScaffoldMessenger, if there is one.
+    _scaffoldMessenger = _currentScaffoldMessenger;
+    _scaffoldMessenger?._register(this);
+
+    // TODO(Piinks): Remove old SnackBar API after migrating ScaffoldMessenger
     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
@@ -2210,15 +2730,18 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
       hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
     }
     _accessibleNavigation = mediaQuery.accessibleNavigation;
+
     _maybeBuildPersistentBottomSheet();
     super.didChangeDependencies();
   }
 
   @override
   void dispose() {
+    // TODO(Piinks): Remove old SnackBar API after migrating ScaffoldMessenger
     _snackBarController?.dispose();
     _snackBarTimer?.cancel();
     _snackBarTimer = null;
+
     _geometryNotifier.dispose();
     for (final _StandardBottomSheet bottomSheet in _dismissedBottomSheets) {
       bottomSheet.animationController.dispose();
@@ -2228,6 +2751,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     }
     _floatingActionButtonMoveController.dispose();
     _floatingActionButtonVisibilityController.dispose();
+    _scaffoldMessenger?._unregister(this);
     super.dispose();
   }
 
@@ -2341,8 +2865,9 @@ 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;
 
+    // TODO(Piinks): Remove old SnackBar API after migrating ScaffoldMessenger
+    _accessibleNavigation = mediaQuery.accessibleNavigation;
     if (_snackBars.isNotEmpty) {
       final ModalRoute<dynamic>? route = ModalRoute.of(context);
       if (route == null || route.isCurrent) {
@@ -2417,6 +2942,38 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
 
     bool isSnackBarFloating = false;
     double? snackBarWidth;
+    // We should only be using one API for SnackBars. Currently, we can use the
+    // Scaffold, which creates a SnackBar queue (_snackBars), or the
+    // ScaffoldMessenger, which sends a SnackBar to descendant Scaffolds.
+    // (_messengerSnackBar).
+    assert(
+      _snackBars.isEmpty || _messengerSnackBar == null,
+      'Only one API should be used to manage SnackBars. The ScaffoldMessenger is '
+      'the preferred API instead of the Scaffold methods.'
+    );
+
+    // SnackBar set by ScaffoldMessenger
+    if (_messengerSnackBar != null) {
+      final SnackBarBehavior snackBarBehavior = _messengerSnackBar?._widget.behavior
+        ?? themeData.snackBarTheme.behavior
+        ?? SnackBarBehavior.fixed;
+      isSnackBarFloating = snackBarBehavior == SnackBarBehavior.floating;
+      snackBarWidth = _messengerSnackBar?._widget.width;
+
+      _addIfNonNull(
+        children,
+        _messengerSnackBar?._widget,
+        _ScaffoldSlot.snackBar,
+        removeLeftPadding: false,
+        removeTopPadding: true,
+        removeRightPadding: false,
+        removeBottomPadding: widget.bottomNavigationBar != null || widget.persistentFooterButtons != null,
+        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
+      );
+    }
+
+    // SnackBar set by Scaffold
+    // TODO(Piinks): Remove old SnackBar API after migrating ScaffoldMessenger
     if (_snackBars.isNotEmpty) {
       final SnackBarBehavior snackBarBehavior = _snackBars.first._widget.behavior
         ?? themeData.snackBarTheme.behavior
@@ -2590,7 +3147,8 @@ 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.
@@ -150,8 +150,8 @@ 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].
 ///
@@ -160,11 +160,11 @@ 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
@@ -293,7 +293,7 @@ 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>
@@ -305,7 +305,7 @@ 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 }) {
@@ -342,7 +342,6 @@ class SnackBar extends StatefulWidget {
   State<SnackBar> createState() => _SnackBarState();
 }
 
-
 class _SnackBarState extends State<SnackBar> {
   bool _wasVisible = false;
 
@@ -560,6 +559,9 @@ class _SnackBarState extends State<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

@@ -510,6 +510,7 @@ 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.
@@ -567,7 +568,7 @@ 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),
@@ -579,6 +580,7 @@ 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

@@ -2112,7 +2112,7 @@ 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
@@ -2123,19 +2123,38 @@ typedef ElementVisitor = void Function(Element element);
 /// Widget build(BuildContext context) {
 ///   // here, Scaffold.of(context) returns null
 ///   return Scaffold(
-///       appBar: AppBar(title: Text('Demo')),
+///     appBar: const AppBar(title: Text('Demo')),
 ///     body: Builder(
 ///       builder: (BuildContext context) {
 ///         return TextButton(
-///             child: Text('BUTTON'),
+///           child: const 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

@@ -168,6 +168,8 @@ void main() {
       '     _InheritedTheme\n'
       '     Theme\n'
       '     AnimatedTheme\n'
+      '     _ScaffoldMessengerScope\n'
+      '     ScaffoldMessenger\n'
       '     Builder\n'
       '     DefaultTextStyle\n'
       '     CustomPaint\n'
@@ -204,4 +206,83 @@ 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>();
+    final GlobalKey<ScaffoldMessengerState> _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
+    final SnackBar snackBar = SnackBar(
+      content: const Text('Snack'),
+      action: SnackBarAction(label: 'Test', onPressed: () {})
+    );
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: MediaQuery(
+        data: const MediaQueryData(),
+        child: ScaffoldMessenger(
+          key: _scaffoldMessengerKey,
+          child: Builder(
+            builder: (BuildContext context) {
+              return Scaffold(
+                key: _scaffoldKey,
+                body: Container(),
+              );
+            }
+          )
+        ),
+      )
+    ));
+    final List<dynamic> exceptions = <dynamic>[];
+    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      exceptions.add(details.exception);
+    };
+    // ScaffoldMessenger shows SnackBar.
+    _scaffoldMessengerKey.currentState.showSnackBar(snackBar);
+    await tester.pumpAndSettle();
+
+    // Pump widget to rebuild without ScaffoldMessenger
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: MediaQuery(
+        data: const MediaQueryData(),
+        child: Scaffold(
+          key: _scaffoldKey,
+          body: Container(),
+        ),
+      ),
+    ));
+    // The Scaffold should assert we still have an ancestor ScaffoldMessenger in
+    // order to dismiss the SnackBar from the ScaffoldMessenger.
+    await tester.tap(find.text('Test'));
+    FlutterError.onError = oldHandler;
+
+    expect(exceptions.length, 1);
+    expect(exceptions.single.runtimeType, FlutterError);
+    final FlutterError error = exceptions.first as FlutterError;
+    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>#00829]\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/scaffold_test.dart

@@ -2034,6 +2034,117 @@ void main() {
       await tester.pumpAndSettle();
     });
   });
+
+  testWidgets('ScaffoldMessenger.of can return null', (WidgetTester tester) async {
+    ScaffoldMessengerState scaffoldMessenger;
+    const Key tapTarget = Key('tap-target');
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: MediaQuery(
+        data: const MediaQueryData(),
+        child: Scaffold(
+          body: Builder(
+            builder: (BuildContext context) {
+              return GestureDetector(
+                onTap: () {
+                  scaffoldMessenger = ScaffoldMessenger.of(context, nullOk: true);
+                },
+                behavior: HitTestBehavior.opaque,
+                child: Container(
+                  height: 100.0,
+                  width: 100.0,
+                  key: tapTarget,
+                ),
+              );
+            }
+          ),
+        ),
+      ),
+    ));
+    await tester.tap(find.byKey(tapTarget));
+    await tester.pump();
+    expect(scaffoldMessenger, isNull);
+  });
+
+  testWidgets('ScaffoldMessenger.of will assert if !nullOk', (WidgetTester tester) async {
+    const Key tapTarget = Key('tap-target');
+
+    final List<dynamic> exceptions = <dynamic>[];
+    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      exceptions.add(details.exception);
+    };
+
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: MediaQuery(
+        data: const MediaQueryData(),
+        child: Scaffold(
+          body: Builder(
+            builder: (BuildContext context) {
+              return GestureDetector(
+                onTap: () {
+                  ScaffoldMessenger.of(context);
+                },
+                behavior: HitTestBehavior.opaque,
+                child: Container(
+                  height: 100.0,
+                  width: 100.0,
+                  key: tapTarget,
+                ),
+              );
+            }
+          ),
+        ),
+      ),
+    ));
+    await tester.tap(find.byKey(tapTarget));
+    FlutterError.onError = oldHandler;
+
+    expect(exceptions.length, 1);
+    expect(exceptions.single.runtimeType, FlutterError);
+    final FlutterError error = exceptions.first as FlutterError;
+    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'
+      '   Builder widgets require a ScaffoldMessenger widget ancestor.\n'
+      '   The specific widget that could not find a ScaffoldMessenger\n'
+      '   ancestor was:\n'
+      '     Builder\n'
+      '   The ancestors of this widget were:\n'
+      '     _BodyBuilder\n'
+      '     MediaQuery\n'
+      '     LayoutId-[<_ScaffoldSlot.body>]\n'
+      '     CustomMultiChildLayout\n'
+      '     AnimatedBuilder\n'
+      '     DefaultTextStyle\n'
+      '     AnimatedDefaultTextStyle\n'
+      '     _InkFeatures-[GlobalKey#342d0 ink renderer]\n'
+      '     NotificationListener<LayoutChangedNotification>\n'
+      '     PhysicalModel\n'
+      '     AnimatedPhysicalModel\n'
+      '     Material\n'
+      '     PrimaryScrollController\n'
+      '     _ScaffoldScope\n'
+      '     Scaffold\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'
+    ));
+  });
 }
 
 class _GeometryListener extends StatefulWidget {

packages/flutter/test/material/snack_bar_test.dart

@@ -4,6 +4,7 @@
 
 // @dart = 2.8
 
+import 'package:flutter/foundation.dart' show FlutterExceptionHandler;
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -54,6 +55,51 @@ void main() {
     expect(find.text(helloSnackBar), findsNothing);
   });
 
+  testWidgets('SnackBar control test - ScaffoldMessenger', (WidgetTester tester) async {
+    const String helloSnackBar = 'Hello SnackBar';
+    const Key tapTarget = Key('tap-target');
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
+                  content: Text(helloSnackBar),
+                  duration: Duration(seconds: 2),
+                ));
+              },
+              behavior: HitTestBehavior.opaque,
+              child: Container(
+                height: 100.0,
+                width: 100.0,
+                key: tapTarget,
+              ),
+            );
+          }
+        ),
+      ),
+    ));
+    expect(find.text(helloSnackBar), findsNothing);
+    await tester.tap(find.byKey(tapTarget));
+    expect(find.text(helloSnackBar), findsNothing);
+    await tester.pump(); // schedule animation
+    expect(find.text(helloSnackBar), findsOneWidget);
+    await tester.pump(); // begin animation
+    expect(find.text(helloSnackBar), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 0.75s // animation last frame; two second timer starts here
+    expect(find.text(helloSnackBar), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 1.50s
+    expect(find.text(helloSnackBar), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 2.25s
+    expect(find.text(helloSnackBar), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 3.00s // timer triggers to dismiss snackbar, reverse animation is scheduled
+    await tester.pump(); // begin animation
+    expect(find.text(helloSnackBar), findsOneWidget); // frame 0 of dismiss animation
+    await tester.pump(const Duration(milliseconds: 750)); // 3.75s // last frame of animation, snackbar removed from build
+    expect(find.text(helloSnackBar), findsNothing);
+  });
+
   testWidgets('SnackBar twice test', (WidgetTester tester) async {
     int snackBarCount = 0;
     const Key tapTarget = Key('tap-target');
@@ -129,6 +175,81 @@ void main() {
     expect(find.text('bar2'), findsNothing);
   });
 
+  testWidgets('SnackBar twice test - ScaffoldMessenger', (WidgetTester tester) async {
+    int snackBarCount = 0;
+    const Key tapTarget = Key('tap-target');
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                snackBarCount += 1;
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  content: Text('bar$snackBarCount'),
+                  duration: const Duration(seconds: 2),
+                ));
+              },
+              behavior: HitTestBehavior.opaque,
+              child: Container(
+                height: 100.0,
+                width: 100.0,
+                key: tapTarget,
+              ),
+            );
+          }
+        ),
+      ),
+    ));
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+    await tester.tap(find.byKey(tapTarget)); // queue bar1
+    await tester.tap(find.byKey(tapTarget)); // queue bar2
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(); // schedule animation for bar1
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 0.75s // animation last frame; two second timer starts here
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 1.50s
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 2.25s
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 3.00s // timer triggers to dismiss snackbar, reverse animation is scheduled
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 3.75s // last frame of animation, snackbar removed from build, new snack bar put in its place
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 4.50s // animation last frame; two second timer starts here
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 5.25s
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 6.00s
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 6.75s // timer triggers to dismiss snackbar, reverse animation is scheduled
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 7.50s // last frame of animation, snackbar removed from build, new snack bar put in its place
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+  });
+
   testWidgets('SnackBar cancel test', (WidgetTester tester) async {
     int snackBarCount = 0;
     const Key tapTarget = Key('tap-target');
@@ -215,6 +336,92 @@ void main() {
     expect(find.text('bar2'), findsNothing);
   });
 
+  testWidgets('SnackBar cancel test - ScaffoldMessenger', (WidgetTester tester) async {
+    int snackBarCount = 0;
+    const Key tapTarget = Key('tap-target');
+    int time;
+    ScaffoldFeatureController<SnackBar, SnackBarClosedReason> lastController;
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                snackBarCount += 1;
+                lastController = ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  content: Text('bar$snackBarCount'),
+                  duration: Duration(seconds: time),
+                ));
+              },
+              behavior: HitTestBehavior.opaque,
+              child: Container(
+                height: 100.0,
+                width: 100.0,
+                key: tapTarget,
+              ),
+            );
+          }
+        ),
+      ),
+    ));
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+    time = 1000;
+    await tester.tap(find.byKey(tapTarget)); // queue bar1
+    final ScaffoldFeatureController<SnackBar, SnackBarClosedReason> firstController = lastController;
+    time = 2;
+    await tester.tap(find.byKey(tapTarget)); // queue bar2
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(); // schedule animation for bar1
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 0.75s // animation last frame; two second timer starts here
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 1.50s
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 2.25s
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 10000)); // 12.25s
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+
+    firstController.close(); // snackbar is manually dismissed
+
+    await tester.pump(const Duration(milliseconds: 750)); // 13.00s // reverse animation is scheduled
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 13.75s // last frame of animation, snackbar removed from build, new snack bar put in its place
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 14.50s // animation last frame; two second timer starts here
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 15.25s
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 16.00s
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 16.75s // timer triggers to dismiss snackbar, reverse animation is scheduled
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+    await tester.pump(const Duration(milliseconds: 750)); // 17.50s // last frame of animation, snackbar removed from build, new snack bar put in its place
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+  });
+
   testWidgets('SnackBar dismiss test', (WidgetTester tester) async {
     int snackBarCount = 0;
     const Key tapTarget = Key('tap-target');
@@ -260,6 +467,51 @@ void main() {
     expect(find.text('bar2'), findsOneWidget);
   });
 
+  testWidgets('SnackBar dismiss test - ScaffoldMessenger', (WidgetTester tester) async {
+    int snackBarCount = 0;
+    const Key tapTarget = Key('tap-target');
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                snackBarCount += 1;
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  content: Text('bar$snackBarCount'),
+                  duration: const Duration(seconds: 2),
+                ));
+              },
+              behavior: HitTestBehavior.opaque,
+              child: Container(
+                height: 100.0,
+                width: 100.0,
+                key: tapTarget,
+              ),
+            );
+          }
+        ),
+      ),
+    ));
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+    await tester.tap(find.byKey(tapTarget)); // queue bar1
+    await tester.tap(find.byKey(tapTarget)); // queue bar2
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(); // schedule animation for bar1
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(); // begin animation
+    expect(find.text('bar1'), findsOneWidget);
+    expect(find.text('bar2'), findsNothing);
+    await tester.pump(const Duration(milliseconds: 750)); // 0.75s // animation last frame; two second timer starts here
+    await tester.drag(find.text('bar1'), const Offset(0.0, 50.0));
+    await tester.pump(); // bar1 dismissed, bar2 begins animating
+    expect(find.text('bar1'), findsNothing);
+    expect(find.text('bar2'), findsOneWidget);
+  });
+
   testWidgets('SnackBar cannot be tapped twice', (WidgetTester tester) async {
     int tapCount = 0;
     await tester.pumpWidget(MaterialApp(
@@ -299,6 +551,45 @@ void main() {
     expect(tapCount, equals(1));
   });
 
+  testWidgets('SnackBar cannot be tapped twice - ScaffoldMessenger', (WidgetTester tester) async {
+    int tapCount = 0;
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  content: const Text('I am a snack bar.'),
+                  duration: const Duration(seconds: 2),
+                  action: SnackBarAction(
+                    label: 'ACTION',
+                    onPressed: () {
+                      ++tapCount;
+                    },
+                  ),
+                ));
+              },
+              child: const Text('X'),
+            );
+          }
+        ),
+      ),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    expect(tapCount, equals(0));
+    await tester.tap(find.text('ACTION'));
+    expect(tapCount, equals(1));
+    await tester.tap(find.text('ACTION'));
+    expect(tapCount, equals(1));
+    await tester.pump();
+    await tester.tap(find.text('ACTION'));
+    expect(tapCount, equals(1));
+  });
+
   testWidgets('Light theme SnackBar has dark background', (WidgetTester tester) async {
     final ThemeData lightTheme = ThemeData.light();
     await tester.pumpWidget(
@@ -467,6 +758,53 @@ void main() {
     );
   });
 
+  testWidgets('SnackbarBehavior.floating is positioned within safe area - ScaffoldMessenger', (WidgetTester tester) async {
+    const double viewPadding = 50.0;
+    const double floatingSnackBarDefaultBottomMargin = 10.0;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: MediaQuery(
+          data: const MediaQueryData(
+            // Simulate non-safe area.
+            viewPadding: EdgeInsets.only(bottom: viewPadding),
+          ),
+          child: Scaffold(
+            body: Builder(
+              builder: (BuildContext context) {
+                return GestureDetector(
+                  onTap: () {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        content: Text('I am a snack bar.'),
+                        behavior: SnackBarBehavior.floating,
+                      ),
+                    );
+                  },
+                  child: const Text('X'),
+                );
+              }
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('X'));
+    await tester.pump(); // Start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    final Finder materialFinder = find.descendant(
+      of: find.byType(SnackBar),
+      matching: find.byType(Material),
+    );
+    final Offset snackBarBottomLeft = tester.getBottomLeft(materialFinder);
+    expect(
+      snackBarBottomLeft.dy,
+      // Device height is 600.
+      600 - viewPadding - floatingSnackBarDefaultBottomMargin,
+    );
+  });
+
   testWidgets('Snackbar padding can be customized', (WidgetTester tester) async {
     const double padding = 20.0;
     await tester.pumpWidget(
@@ -476,7 +814,7 @@ 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 +858,7 @@ 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 +896,7 @@ 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 +949,7 @@ 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 +1004,7 @@ 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 +1055,7 @@ 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 +1109,7 @@ 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 +1167,7 @@ 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 +1199,18 @@ 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 +1255,14 @@ 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));
 
@@ -979,6 +1317,49 @@ void main() {
     expect(find.text('ACTION'), findsNothing);
   });
 
+  testWidgets('accessible navigation behavior with action - ScaffoldMessenger', (WidgetTester tester) async {
+    final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
+
+    await tester.pumpWidget(MaterialApp(
+      home: MediaQuery(
+        data: const MediaQueryData(accessibleNavigation: true),
+        child: ScaffoldMessenger(
+          child: Builder(
+            builder: (BuildContext context) {
+              return Scaffold(
+                key: scaffoldKey,
+                body: GestureDetector(
+                  onTap: () {
+                    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                      content: const Text('snack'),
+                      duration: const Duration(seconds: 1),
+                      action: SnackBarAction(
+                        label: 'ACTION',
+                        onPressed: () { },
+                      ),
+                    ));
+                  },
+                  child: const Text('X'),
+                ),
+              );
+            }
+          )
+        ),
+      ),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump();
+    // Find action immediately
+    expect(find.text('ACTION'), findsOneWidget);
+    // Snackbar doesn't close
+    await tester.pump(const Duration(seconds: 10));
+    expect(find.text('ACTION'), findsOneWidget);
+    await tester.tap(find.text('ACTION'));
+    await tester.pump();
+    // Snackbar closes immediately
+    expect(find.text('ACTION'), findsNothing);
+  });
+
   testWidgets('contributes dismiss semantics', (WidgetTester tester) async {
     final SemanticsHandle handle = tester.ensureSemantics();
     final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
@@ -988,8 +1369,7 @@ void main() {
         data: const MediaQueryData(accessibleNavigation: true),
         child: Scaffold(
           key: scaffoldKey,
-                body: Builder(
-                  builder: (BuildContext context) {
+          body: Builder(builder: (BuildContext context) {
             return GestureDetector(
               onTap: () {
                 Scaffold.of(context).showSnackBar(SnackBar(
@@ -1003,10 +1383,52 @@ void main() {
               },
               child: const Text('X'),
             );
-                  },
+          }),
+        ),
+      )
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pumpAndSettle();
+
+    expect(tester.getSemantics(find.text('snack')), matchesSemantics(
+      isLiveRegion: true,
+      hasDismissAction: true,
+      hasScrollDownAction: true,
+      hasScrollUpAction: true,
+      label: 'snack',
+      textDirection: TextDirection.ltr,
+    ));
+    handle.dispose();
+  });
+
+  testWidgets('contributes dismiss semantics - ScaffoldMessenger', (WidgetTester tester) async {
+    final SemanticsHandle handle = tester.ensureSemantics();
+    final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
+
+    await tester.pumpWidget(MaterialApp(
+      home: MediaQuery(
+        data: const MediaQueryData(accessibleNavigation: true),
+        child: ScaffoldMessenger(
+          child: Builder(builder: (BuildContext context) {
+            return Scaffold(
+              key: scaffoldKey,
+              body: GestureDetector(
+                onTap: () {
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                    content: const Text('snack'),
+                    duration: const Duration(seconds: 1),
+                    action: SnackBarAction(
+                      label: 'ACTION',
+                      onPressed: () { },
                     ),
+                  ));
+                },
+                child: const Text('X'),
               ),
+            );
+          }),
         ),
+      )
     ));
     await tester.tap(find.text('X'));
     await tester.pumpAndSettle();
@@ -1031,7 +1453,7 @@ void main() {
                 builder: (BuildContext context) {
                   return GestureDetector(
                       onTap: () {
-                        Scaffold.of(context).showSnackBar(const SnackBar(
+                        ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
                             content: Text(helloSnackBar),
                         ));
                       },
@@ -1118,6 +1540,54 @@ void main() {
     expect(find.text('test'), findsNothing);
   });
 
+  testWidgets('SnackBar handles updates to accessibleNavigation - ScaffoldMessenger', (WidgetTester tester) async {
+    Future<void> boilerplate({ bool accessibleNavigation }) {
+      return tester.pumpWidget(MaterialApp(
+          home: MediaQuery(
+              data: MediaQueryData(accessibleNavigation: accessibleNavigation),
+              child: Scaffold(
+                  body: Builder(
+                      builder: (BuildContext context) {
+                        return GestureDetector(
+                            onTap: () {
+                              ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                                  content: const Text('test'),
+                                  action: SnackBarAction(label: 'foo', onPressed: () { }),
+                              ));
+                            },
+                            behavior: HitTestBehavior.opaque,
+                            child: const Text('X'),
+                        );
+                      }
+                  ),
+              ),
+          ),
+      ));
+    }
+
+    await boilerplate(accessibleNavigation: false);
+    expect(find.text('test'), findsNothing);
+    await tester.tap(find.text('X'));
+    await tester.pump(); // schedule animation
+    expect(find.text('test'), findsOneWidget);
+    await tester.pump(); // begin animation
+    await tester.pump(const Duration(milliseconds: 4750)); // 4.75s
+    expect(find.text('test'), findsOneWidget);
+
+    // Enabled accessible navigation
+    await boilerplate(accessibleNavigation: true);
+
+    await tester.pump(const Duration(milliseconds: 4000)); // 8.75s
+    await tester.pump();
+    expect(find.text('test'), findsOneWidget);
+
+    // disable accessible navigation
+    await boilerplate(accessibleNavigation: false);
+    await tester.pumpAndSettle(const Duration(milliseconds: 5750));
+
+    expect(find.text('test'), findsNothing);
+  });
+
   testWidgets('Snackbar asserts if passed a null duration', (WidgetTester tester) async {
     const Key tapTarget = Key('tap-target');
     await tester.pumpWidget(MaterialApp(
@@ -1126,7 +1596,7 @@ 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 +1626,7 @@ 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 +1663,14 @@ 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 +1717,8 @@ 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 +1748,8 @@ 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 +1768,8 @@ 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 +1779,7 @@ 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 +1843,8 @@ 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 +1879,7 @@ 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 +1921,8 @@ 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 +1958,8 @@ 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 +1975,107 @@ void main() {
       },
     );
   });
+
+  testWidgets('SnackBars hero across transitions when using ScaffoldMessenger', (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);
+  });
+
+  testWidgets('SnackBars cannot be used by the Scaffold and ScaffoldMessenger at the same time', (WidgetTester tester) async {
+    await tester.pumpWidget(const MaterialApp(
+      home: Scaffold(),
+    ));
+
+    final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+    scaffoldMessengerState.showSnackBar(SnackBar(
+      content: const Text('ScaffoldMessenger'),
+      duration: const Duration(seconds: 2),
+      action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+      behavior: SnackBarBehavior.floating,
+    ));
+    final ScaffoldState scaffoldState = tester.state(find.byType(Scaffold));
+    scaffoldState.showSnackBar(SnackBar(
+      content: const Text('Scaffold'),
+      duration: const Duration(seconds: 2),
+      action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+      behavior: SnackBarBehavior.floating,
+    ));
+
+    final List<dynamic> exceptions = <dynamic>[];
+    final FlutterExceptionHandler oldHandler = FlutterError.onError;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      exceptions.add(details.exception);
+    };
+    await tester.pump();
+    FlutterError.onError = oldHandler;
+
+    expect(exceptions.length, 1);
+    final AssertionError error = exceptions.first as AssertionError;
+    expect(error.message, contains('Only one API should be used to manage SnackBars.'));
+  });
 }

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: () {}),
@@ -111,7 +111,7 @@ 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: () {}),
@@ -155,7 +155,7 @@ void main() {
             builder: (BuildContext context) {
               return GestureDetector(
                 onTap: () {
-                  Scaffold.of(context).showSnackBar(SnackBar(
+                  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                     backgroundColor: backgroundColor,
                     elevation: elevation,
                     shape: shape,
@@ -202,7 +202,7 @@ 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: () {}),
@@ -244,7 +244,7 @@ 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: () {}),