ScaffoldMessenger (flutter#64101)

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;
+}