Fix user gesture in CupertinoPageRoute (#39590)

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

@@ -731,6 +731,33 @@ class HeroController extends NavigatorObserver {
     _maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop, true);
   }
 
+  @override
+  void didStopUserGesture() {
+    if (navigator.userGestureInProgress)
+      return;
+
+    // If the user horizontal drag gesture initiated the flight (i.e. the back swipe)
+    // didn't move towards the pop direction at all, the animation will not play
+    // and thus the status update callback _handleAnimationUpdate will never be
+    // called when the gesture finishes. In this case the initiated flight needs
+    // to be manually invalidated.
+    bool isInvalidFlight(_HeroFlight flight) {
+      return flight.manifest.isUserGestureTransition
+          && flight.manifest.type == HeroFlightDirection.pop
+          && flight._proxyAnimation.isDismissed;
+    }
+
+    final List<_HeroFlight> invalidFlights = _flights.values
+      .where(isInvalidFlight)
+      .toList(growable: false);
+
+    // Treat these invalidated flights as dismissed. Calling _handleAnimationUpdate
+    // will also remove the flight from _flights.
+    for (_HeroFlight flight in invalidFlights) {
+      flight._handleAnimationUpdate(AnimationStatus.dismissed);
+    }
+  }
+
   // If we're transitioning between different page routes, start a hero transition
   // after the toRoute has been laid out with its animation's value at 1.0.
   void _maybeStartHeroTransition(

packages/flutter/test/widgets/heroes_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui;
 
+import 'package:flutter/gestures.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
@@ -25,6 +26,7 @@ Future<ui.Image> createTestImage() {
 Key firstKey = const Key('first');
 Key secondKey = const Key('second');
 Key thirdKey = const Key('third');
+Key simpleKey = const Key('simple');
 
 Key homeRouteKey = const Key('homeRoute');
 Key routeTwoKey = const Key('routeTwo');
@@ -52,6 +54,10 @@ final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
           child: const Text('twoInset'),
           onPressed: () { Navigator.pushNamed(context, '/twoInset'); },
         ),
+        FlatButton(
+          child: const Text('simple'),
+          onPressed: () { Navigator.pushNamed(context, '/simple'); },
+        ),
       ],
     ),
   ),
@@ -108,6 +114,19 @@ final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
       ],
     ),
   ),
+  // This route is the same as /two except that Hero 'a' is shifted to the right by
+  // 50 pixels. When the hero's in-flight bounds between / and /twoInset are animated
+  // using MaterialRectArcTween (the default) they'll follow a different path
+  // then when the flight starts at /twoInset and returns to /.
+  '/simple': (BuildContext context) => CupertinoPageScaffold(
+    child: Center(
+      child: Hero(
+        tag: 'a',
+        transitionOnUserGestures: transitionFromUserGestures,
+        child: Container(height: 150.0, width: 150.0, key: simpleKey),
+      ),
+    ),
+  ),
 };
 
 class ThreeRoute extends MaterialPageRoute<void> {
@@ -2257,6 +2276,38 @@ Future<void> main() async {
     },
   );
 
+  // Regression test for https://github.com/flutter/flutter/issues/38183.
+  testWidgets('Remove user gesture driven flights when the gesture is invalid', (WidgetTester tester) async {
+    transitionFromUserGestures = true;
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(
+        platform: TargetPlatform.iOS,
+      ),
+      routes: routes,
+    ));
+
+    await tester.tap(find.text('simple'));
+    await tester.pump();
+    await tester.pumpAndSettle();
+
+    expect(find.byKey(simpleKey), findsOneWidget);
+
+    // Tap once to trigger a flight.
+    await tester.tapAt(const Offset(10, 200));
+    await tester.pumpAndSettle();
+
+    // Wait till the previous gesture is accepted.
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // Tap again to trigger another flight, see if it throws.
+    await tester.tapAt(const Offset(10, 200));
+    await tester.pumpAndSettle();
+
+    // The simple route should still be on top.
+    expect(find.byKey(simpleKey), findsOneWidget);
+    expect(tester.takeException(), isNull);
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/40239.
   testWidgets(
     'In a pop transition, when fromHero is null, the to hero should eventually become visible',