Add transitionOnUserGestures to true on SnackBar for back swipe (#106…

…638)

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

@@ -158,6 +158,13 @@ class _SnackBarActionState extends State<SnackBarAction> {
 /// A SnackBar with an action will not time out when TalkBack or VoiceOver are
 /// enabled. This is controlled by [AccessibilityFeatures.accessibleNavigation].
 ///
+/// During page transitions, the [SnackBar] will smoothly animate to its
+/// location on the other page. For example if the [SnackBar.behavior] is set to
+/// [SnackBarBehavior.floating] and the next page has a floating action button,
+/// while the current one does not, the [SnackBar] will smoothly animate above
+/// the floating action button. It also works in the case of a back gesture
+/// transition.
+///
 /// {@tool dartpad}
 /// Here is an example of a [SnackBar] with an [action] button implemented using
 /// [SnackBarAction].
@@ -620,6 +627,7 @@ class _SnackBarState extends State<SnackBar> {
 
     return Hero(
       tag: '<SnackBar Hero tag - ${widget.content}>',
+      transitionOnUserGestures: true,
       child: ClipRect(
         clipBehavior: widget.clipBehavior,
         child: snackBarTransition,

packages/flutter/test/material/snack_bar_test.dart

@@ -1795,6 +1795,82 @@ void main() {
     expect(find.text(secondHeader), findsOneWidget);
   });
 
+  testWidgets('Should have only one SnackBar during back swipe navigation', (WidgetTester tester) async {
+    const String snackBarText = 'hello snackbar';
+    const Key snackTarget = Key('snack-target');
+    const Key transitionTarget = Key('transition-target');
+
+    Widget buildApp() {
+      final PageTransitionsTheme pageTransitionTheme = PageTransitionsTheme(
+        builders: <TargetPlatform, PageTransitionsBuilder>{
+          for(final TargetPlatform platform in TargetPlatform.values)
+            platform: const CupertinoPageTransitionsBuilder(),
+        },
+      );
+      return MaterialApp(
+        theme: ThemeData(pageTransitionsTheme: pageTransitionTheme),
+        initialRoute: '/',
+        routes: <String, WidgetBuilder> {
+          '/': (BuildContext context) {
+            return  Scaffold(
+              body: Center(
+                child: ElevatedButton(
+                  key: transitionTarget,
+                  child: const Text('PUSH'),
+                  onPressed: () {
+                    Navigator.of(context).pushNamed('/second');
+                  },
+                ),
+              ),
+
+            );
+          },
+          '/second': (BuildContext context) {
+            return Scaffold(
+              floatingActionButton: FloatingActionButton(
+                key: snackTarget,
+                onPressed: () async {
+                  ScaffoldMessenger.of(context).showSnackBar(
+                    const SnackBar(
+                      content: Text(snackBarText),
+                    ),
+                  );
+                },
+                child: const Text('X'),
+              ),
+            );
+          },
+        },
+      );
+    }
+    await tester.pumpWidget(buildApp());
+
+    // Transition to second page.
+    await tester.tap(find.byKey(transitionTarget));
+    await tester.pumpAndSettle();
+
+    // 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);
+
+    // Start the gesture at the edge of the screen.
+    final TestGesture gesture =  await tester.startGesture(const Offset(5.0, 200.0));
+    // Trigger the swipe.
+    await gesture.moveBy(const Offset(100.0, 0.0));
+
+    // Back gestures should trigger and draw the hero transition in the very same
+    // frame (since the "from" route has already moved to reveal the "to" route).
+    await tester.pump();
+
+    // We should have only one SnackBar displayed on the screen.
+    expect(find.text(snackBarText), findsOneWidget);
+  });
+
   testWidgets('SnackBars should be shown above the bottomSheet', (WidgetTester tester) async {
     await tester.pumpWidget(const MaterialApp(
       home: Scaffold(