Re-land ScaffoldMessenger (#65416)

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

@@ -17,6 +17,7 @@ import 'floating_action_button.dart';
 import 'icons.dart';
 import 'material_localizations.dart';
 import 'page.dart';
+import 'scaffold.dart';
 import 'theme.dart';
 
 /// [MaterialApp] uses this [TextStyle] as its [DefaultTextStyle] to encourage
@@ -168,6 +169,7 @@ class MaterialApp extends StatefulWidget {
   const MaterialApp({
     Key key,
     this.navigatorKey,
+    this.scaffoldMessengerKey,
     this.home,
     this.routes = const <String, WidgetBuilder>{},
     this.initialRoute,
@@ -215,6 +217,7 @@ class MaterialApp extends StatefulWidget {
   /// Creates a [MaterialApp] that uses the [Router] instead of a [Navigator].
   const MaterialApp.router({
     Key key,
+    this.scaffoldMessengerKey,
     this.routeInformationProvider,
     @required this.routeInformationParser,
     @required this.routerDelegate,
@@ -263,6 +266,14 @@ class MaterialApp extends StatefulWidget {
   /// {@macro flutter.widgets.widgetsApp.navigatorKey}
   final GlobalKey<NavigatorState> navigatorKey;
 
+  /// A key to use when building the [ScaffoldMessenger].
+  ///
+  /// If a [scaffoldMessengerKey] is specified, the [ScaffoldMessenger] can be
+  /// directly manipulated without first obtaining it from a [BuildContext] via
+  /// [ScaffoldMessenger.of]: from the [scaffoldMessengerKey], use the
+  /// [GlobalKey.currentState] getter.
+  final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey;
+
   /// {@macro flutter.widgets.widgetsApp.home}
   final Widget home;
 
@@ -722,27 +733,30 @@ class _MaterialAppState extends State<MaterialApp> {
     }
     theme ??= widget.theme ?? ThemeData.light();
 
-    return AnimatedTheme(
-      data: theme,
-      isMaterialAppTheme: true,
-      child: widget.builder != null
-        ? Builder(
-            builder: (BuildContext context) {
-              // Why are we surrounding a builder with a builder?
-              //
-              // The widget.builder may contain code that invokes
-              // Theme.of(), which should return the theme we selected
-              // above in AnimatedTheme. However, if we invoke
-              // widget.builder() directly as the child of AnimatedTheme
-              // then there is no Context separating them, and the
-              // widget.builder() will not find the theme. Therefore, we
-              // surround widget.builder with yet another builder so that
-              // a context separates them and Theme.of() correctly
-              // resolves to the theme we passed to AnimatedTheme.
-              return widget.builder(context, child);
-            },
-          )
-        : child,
+    return ScaffoldMessenger(
+      key: widget.scaffoldMessengerKey,
+      child: AnimatedTheme(
+        data: theme,
+        isMaterialAppTheme: true,
+        child: widget.builder != null
+          ? Builder(
+              builder: (BuildContext context) {
+                // Why are we surrounding a builder with a builder?
+                //
+                // The widget.builder may contain code that invokes
+                // Theme.of(), which should return the theme we selected
+                // above in AnimatedTheme. However, if we invoke
+                // widget.builder() directly as the child of AnimatedTheme
+                // then there is no Context separating them, and the
+                // widget.builder() will not find the theme. Therefore, we
+                // surround widget.builder with yet another builder so that
+                // a context separates them and Theme.of() correctly
+                // resolves to the theme we passed to AnimatedTheme.
+                return widget.builder(context, child);
+              },
+            )
+          : child,
+      )
     );
   }
 

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

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

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

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

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

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

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

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

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

@@ -2084,33 +2084,52 @@ 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
 /// been found, and the [Scaffold.of] function would have returned null.
 ///
 /// ```dart
-///   @override
-///   Widget build(BuildContext context) {
-///     // here, Scaffold.of(context) returns null
-///     return Scaffold(
-///       appBar: AppBar(title: Text('Demo')),
-///       body: Builder(
-///         builder: (BuildContext context) {
-///           return TextButton(
-///             child: Text('BUTTON'),
-///             onPressed: () {
-///               // here, Scaffold.of(context) returns the locally created Scaffold
-///               Scaffold.of(context).showSnackBar(SnackBar(
-///                 content: Text('Hello.')
-///               ));
-///             }
-///           );
-///         }
-///       )
-///     );
-///   }
+/// @override
+/// Widget build(BuildContext context) {
+///   // here, Scaffold.of(context) returns null
+///   return Scaffold(
+///     appBar: AppBar(title: Text('Demo')),
+///     body: Builder(
+///       builder: (BuildContext context) {
+///         return TextButton(
+///           child: Text('BUTTON'),
+///           onPressed: () {
+///             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),
+///                           },
+///                         )
+///                       ],
+///                     ),
+///                   ),
+///                 );
+///               },
+///             );
+///           },
+///         );
+///       },
+///     )
+///   );
+/// }
 /// ```
 ///
 /// The [BuildContext] for a particular widget can change location over time as

packages/flutter/test/material/debug_test.dart

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

packages/flutter/test/material/floating_action_button_location_test.dart

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

packages/flutter/test/material/snack_bar_theme_test.dart

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