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flutter · fluttergithubbot · Sep 3, 2020 · Aug 26, 2020 · Aug 26, 2020 · Aug 26, 2020
commit c4632bdcb614aefcdcb35be99e6f3268650e10c1
diff --git a/packages/flutter/lib/src/material/debug.dart b/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;
+}