Handle back swipe completed->completed or completed->dismissed transitions (#27866)

packages/flutter/lib/src/cupertino/route.dart

@@ -634,18 +634,22 @@ class _CupertinoBackGestureController<T> {
       controller.animateBack(0.0, duration: Duration(milliseconds: droppedPageBackAnimationTime), curve: animationCurve);
     }
 
-    assert(controller.isAnimating);
-    assert(controller.status != AnimationStatus.completed);
-    assert(controller.status != AnimationStatus.dismissed);
+    if (controller.isAnimating) {
+      // Don't end the gesture until the transition completes.
+      _animating = true;
+      controller.addStatusListener(_handleStatusChanged);
+    } else {
+      // Animate calls could return inline if already at the target destination
+      // value.
+      return _handleStatusChanged(controller.status);
+    }
 
-    // Don't end the gesture until the transition completes.
-    _animating = true;
-    controller.addStatusListener(_handleStatusChanged);
   }
 
   void _handleStatusChanged(AnimationStatus status) {
-    assert(_animating);
-    controller.removeStatusListener(_handleStatusChanged);
+    if (_animating) {
+      controller.removeStatusListener(_handleStatusChanged);
+    }
     _animating = false;
     if (status == AnimationStatus.dismissed)
       navigator.pop<T>(); // this will cause the route to get disposed, which will dispose us

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

@@ -155,7 +155,6 @@ abstract class TransitionRoute<T> extends OverlayRoute<T> {
           overlayEntries.first.opaque = false;
         break;
       case AnimationStatus.dismissed:
-        assert(!overlayEntries.first.opaque);
         // We might still be the current route if a subclass is controlling the
         // the transition and hits the dismissed status. For example, the iOS
         // back gesture drives this animation to the dismissed status before

packages/flutter/test/cupertino/page_test.dart

@@ -427,6 +427,74 @@ void main() {
     expect(find.text('Page 1'), isOnstage);
     expect(find.text('Page 2'), isOnstage);
   });
+
+  testWidgets('test edge swipe then drop back at starting point works', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        onGenerateRoute: (RouteSettings settings) {
+          return CupertinoPageRoute<void>(
+            settings: settings,
+            builder: (BuildContext context) {
+              final String pageNumber = settings.name == '/' ? '1' : '2';
+              return Center(child: Text('Page $pageNumber'));
+            }
+          );
+        },
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5, 200));
+    await gesture.moveBy(const Offset(300, 0));
+    await tester.pump();
+    // Bring it exactly back such that there's nothing to animate when releasing.
+    await gesture.moveBy(const Offset(-300, 0));
+    await gesture.up();
+    await tester.pump();
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+  });
+
+  testWidgets('test edge swipe then drop back at ending point works', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        onGenerateRoute: (RouteSettings settings) {
+          return CupertinoPageRoute<void>(
+            settings: settings,
+            builder: (BuildContext context) {
+              final String pageNumber = settings.name == '/' ? '1' : '2';
+              return Center(child: Text('Page $pageNumber'));
+            }
+          );
+        },
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5, 200));
+    // The width of the page.
+    await gesture.moveBy(const Offset(800, 0));
+    await gesture.up();
+    await tester.pump();
+
+    expect(find.text('Page 1'), isOnstage);
+    expect(find.text('Page 2'), findsNothing);
+  });
 }
 
 class RtlOverrideWidgetsDelegate extends LocalizationsDelegate<WidgetsLocalizations> {

packages/flutter/test/material/page_test.dart

@@ -470,4 +470,74 @@ void main() {
     // An exception should've been thrown because the `builder` returned null.
     expect(tester.takeException(), isInstanceOf<FlutterError>());
   });
+
+  testWidgets('test iOS edge swipe then drop back at starting point works', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(platform: TargetPlatform.iOS),
+        onGenerateRoute: (RouteSettings settings) {
+          return MaterialPageRoute<void>(
+            settings: settings,
+            builder: (BuildContext context) {
+              final String pageNumber = settings.name == '/' ? '1' : '2';
+              return Center(child: Text('Page $pageNumber'));
+            }
+          );
+        },
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5, 200));
+    await gesture.moveBy(const Offset(300, 0));
+    await tester.pump();
+    // Bring it exactly back such that there's nothing to animate when releasing.
+    await gesture.moveBy(const Offset(-300, 0));
+    await gesture.up();
+    await tester.pump();
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+  });
+
+  testWidgets('test iOS edge swipe then drop back at ending point works', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(platform: TargetPlatform.iOS),
+        onGenerateRoute: (RouteSettings settings) {
+          return MaterialPageRoute<void>(
+            settings: settings,
+            builder: (BuildContext context) {
+              final String pageNumber = settings.name == '/' ? '1' : '2';
+              return Center(child: Text('Page $pageNumber'));
+            }
+          );
+        },
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5, 200));
+    // The width of the page.
+    await gesture.moveBy(const Offset(800, 0));
+    await gesture.up();
+    await tester.pump();
+
+    expect(find.text('Page 1'), isOnstage);
+    expect(find.text('Page 2'), findsNothing);
+  });
 }