Back swipe hero (#23320)

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

@@ -316,6 +316,10 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
   /// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
   /// with [transitionBetweenRoutes] set to true.
   ///
+  /// This transition will also occur on edge back swipe gestures like on iOS
+  /// but only if the previous page below has `maintainState` set to true on the
+  /// [PageRoute].
+  ///
   /// When set to true, only one navigation bar can be present per route unless
   /// [heroTag] is also set.
   ///
@@ -398,6 +402,7 @@ class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
       createRectTween: _linearTranslateWithLargestRectSizeTween,
       placeholderBuilder: _navBarHeroLaunchPadBuilder,
       flightShuttleBuilder: _navBarHeroFlightShuttleBuilder,
+      transitionOnUserGestures: true,
       child: _TransitionableNavigationBar(
         componentsKeys: keys,
         backgroundColor: widget.backgroundColor,
@@ -732,6 +737,7 @@ class _LargeTitleNavigationBarSliverDelegate
       createRectTween: _linearTranslateWithLargestRectSizeTween,
       flightShuttleBuilder: _navBarHeroFlightShuttleBuilder,
       placeholderBuilder: _navBarHeroLaunchPadBuilder,
+      transitionOnUserGestures: true,
       // This is all the way down here instead of being at the top level of
       // CupertinoSliverNavigationBar like CupertinoNavigationBar because it
       // needs to wrap the top level RenderBox rather than a RenderSliver.

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

@@ -253,7 +253,7 @@ class CupertinoPageRoute<T> extends PageRoute<T> {
   }
 
   // Called by _CupertinoBackGestureDetector when a pop ("back") drag start
-  // gesture is detected. The returned controller handles all of the subsquent
+  // gesture is detected. The returned controller handles all of the subsequent
   // drag events.
   static _CupertinoBackGestureController<T> _startPopGesture<T>(PageRoute<T> route) {
     assert(!_popGestureInProgress.contains(route));

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

@@ -123,8 +123,10 @@ class Hero extends StatefulWidget {
     this.createRectTween,
     this.flightShuttleBuilder,
     this.placeholderBuilder,
+    this.transitionOnUserGestures = false,
     @required this.child,
   }) : assert(tag != null),
+       assert(transitionOnUserGestures != null),
        assert(child != null),
        super(key: key);
 
@@ -176,14 +178,31 @@ class Hero extends StatefulWidget {
   /// left in place once the Hero shuttle has taken flight.
   final TransitionBuilder placeholderBuilder;
 
+  /// Whether to perform the hero transition if the [PageRoute] transition was
+  /// triggered by a user gesture, such as a back swipe on iOS.
+  ///
+  /// If [Hero]s with the same [tag] on both the from and the to routes have
+  /// [transitionOnUserGestures] set to true, a back swipe gesture will
+  /// trigger the same hero animation as a programmatically triggered push or
+  /// pop.
+  ///
+  /// The route being popped to or the bottom route must also have
+  /// [PageRoute.maintainState] set to true for a gesture triggered hero
+  /// transition to work.
+  ///
+  /// Defaults to false and cannot be null.
+  final bool transitionOnUserGestures;
+
   // Returns a map of all of the heroes in context, indexed by hero tag.
-  static Map<Object, _HeroState> _allHeroesFor(BuildContext context) {
+  static Map<Object, _HeroState> _allHeroesFor(BuildContext context, bool isUserGestureTransition) {
     assert(context != null);
+    assert(isUserGestureTransition != null);
     final Map<Object, _HeroState> result = <Object, _HeroState>{};
     void visitor(Element element) {
       if (element.widget is Hero) {
         final StatefulElement hero = element;
         final Hero heroWidget = element.widget;
+        if (!isUserGestureTransition || heroWidget.transitionOnUserGestures) {
           final Object tag = heroWidget.tag;
           assert(tag != null);
           assert(() {
@@ -202,6 +221,7 @@ class Hero extends StatefulWidget {
           final _HeroState heroState = hero.state;
           result[tag] = heroState;
         }
+      }
       // Don't perform transitions across different Navigators.
       if (element.widget is Navigator) {
         return;
@@ -274,6 +294,7 @@ class _HeroFlightManifest {
     @required this.toHero,
     @required this.createRectTween,
     @required this.shuttleBuilder,
+    @required this.isUserGestureTransition,
   }) : assert(fromHero.widget.tag == toHero.widget.tag);
 
   final HeroFlightDirection type;
@@ -285,6 +306,7 @@ class _HeroFlightManifest {
   final _HeroState toHero;
   final CreateRectTween createRectTween;
   final HeroFlightShuttleBuilder shuttleBuilder;
+  final bool isUserGestureTransition;
 
   Object get tag => fromHero.widget.tag;
 
@@ -410,7 +432,12 @@ class _HeroFlight {
       assert(type != null);
       switch (type) {
         case HeroFlightDirection.pop:
-          return initial.value == 1.0 && initial.status == AnimationStatus.reverse;
+          return initial.value == 1.0 && initialManifest.isUserGestureTransition
+              // During user gesture transitions, the animation controller isn't
+              // driving the reverse transition, but should still be in a previously
+              // completed stage with the initial value at 1.0.
+              ? initial.status == AnimationStatus.completed
+              : initial.status == AnimationStatus.reverse;
         case HeroFlightDirection.push:
           return initial.value == 0.0 && initial.status == AnimationStatus.forward;
       }
@@ -532,14 +559,11 @@ class HeroController extends NavigatorObserver {
   /// linear [Tween<Rect>].
   HeroController({ this.createRectTween });
 
-  /// Used to create [RectTween]s that interpolate the position of heros in flight.
+  /// Used to create [RectTween]s that interpolate the position of heroes in flight.
   ///
   /// If null, the controller uses a linear [RectTween].
   final CreateRectTween createRectTween;
 
-  // Disable Hero animations while a user gesture is controlling the navigation.
-  bool _questsEnabled = true;
-
   // All of the heroes that are currently in the overlay and in motion.
   // Indexed by the hero tag.
   final Map<Object, _HeroFlight> _flights = <Object, _HeroFlight>{};
@@ -548,37 +572,49 @@ class HeroController extends NavigatorObserver {
   void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
     assert(navigator != null);
     assert(route != null);
-    _maybeStartHeroTransition(previousRoute, route, HeroFlightDirection.push);
+    _maybeStartHeroTransition(previousRoute, route, HeroFlightDirection.push, false);
   }
 
   @override
   void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
     assert(navigator != null);
     assert(route != null);
-    _maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop);
-  }
-
-  @override
-  void didStartUserGesture() {
-    _questsEnabled = false;
+    _maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop, false);
   }
 
   @override
-  void didStopUserGesture() {
-    _questsEnabled = true;
+  void didStartUserGesture(Route<dynamic> route, Route<dynamic> previousRoute) {
+    assert(navigator != null);
+    assert(route != null);
+    _maybeStartHeroTransition(route, previousRoute, HeroFlightDirection.pop, true);
   }
 
   // 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(Route<dynamic> fromRoute, Route<dynamic> toRoute, HeroFlightDirection flightType) {
-    if (_questsEnabled && toRoute != fromRoute && toRoute is PageRoute<dynamic> && fromRoute is PageRoute<dynamic>) {
+  void _maybeStartHeroTransition(
+    Route<dynamic> fromRoute,
+    Route<dynamic> toRoute,
+    HeroFlightDirection flightType,
+    bool isUserGestureTransition,
+  ) {
+    if (toRoute != fromRoute && toRoute is PageRoute<dynamic> && fromRoute is PageRoute<dynamic>) {
       final PageRoute<dynamic> from = fromRoute;
       final PageRoute<dynamic> to = toRoute;
       final Animation<double> animation = (flightType == HeroFlightDirection.push) ? to.animation : from.animation;
 
       // A user gesture may have already completed the pop.
-      if (flightType == HeroFlightDirection.pop && animation.status == AnimationStatus.dismissed)
+      if (flightType == HeroFlightDirection.pop && animation.status == AnimationStatus.dismissed) {
         return;
+      }
+
+      // For pop transitions driven by a user gesture: if the "to" page has
+      // maintainState = true, then the hero's final dimensions can be measured
+      // immediately because their page's layout is still valid.
+      if (isUserGestureTransition && flightType == HeroFlightDirection.pop && to.maintainState) {
+        _startHeroTransition(from, to, animation, flightType, isUserGestureTransition);
+      } else {
+        // Otherwise, delay measuring until the end of the next frame to allow
+        // the 'to' route to build and layout.
 
         // Putting a route offstage changes its animation value to 1.0. Once this
         // frame completes, we'll know where the heroes in the `to` route are
@@ -586,18 +622,20 @@ class HeroController extends NavigatorObserver {
         to.offstage = to.animation.value == 0.0;
 
         WidgetsBinding.instance.addPostFrameCallback((Duration value) {
-        _startHeroTransition(from, to, animation, flightType);
+          _startHeroTransition(from, to, animation, flightType, isUserGestureTransition);
         });
       }
     }
+  }
 
-  // Find the matching pairs of heros in from and to and either start or a new
+  // Find the matching pairs of heroes in from and to and either start or a new
   // hero flight, or divert an existing one.
   void _startHeroTransition(
     PageRoute<dynamic> from,
     PageRoute<dynamic> to,
     Animation<double> animation,
     HeroFlightDirection flightType,
+    bool isUserGestureTransition,
   ) {
     // If the navigator or one of the routes subtrees was removed before this
     // end-of-frame callback was called, then don't actually start a transition.
@@ -609,8 +647,8 @@ class HeroController extends NavigatorObserver {
     final Rect navigatorRect = _globalBoundingBoxFor(navigator.context);
 
     // At this point the toHeroes may have been built and laid out for the first time.
-    final Map<Object, _HeroState> fromHeroes = Hero._allHeroesFor(from.subtreeContext);
-    final Map<Object, _HeroState> toHeroes = Hero._allHeroesFor(to.subtreeContext);
+    final Map<Object, _HeroState> fromHeroes = Hero._allHeroesFor(from.subtreeContext, isUserGestureTransition);
+    final Map<Object, _HeroState> toHeroes = Hero._allHeroesFor(to.subtreeContext, isUserGestureTransition);
 
     // If the `to` route was offstage, then we're implicitly restoring its
     // animation value back to what it was before it was "moved" offstage.
@@ -632,6 +670,7 @@ class HeroController extends NavigatorObserver {
           createRectTween: createRectTween,
           shuttleBuilder:
               toShuttleBuilder ?? fromShuttleBuilder ?? _defaultHeroFlightShuttleBuilder,
+          isUserGestureTransition: isUserGestureTransition,
         );
 
         if (_flights[tag] != null)

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

@@ -348,11 +348,18 @@ class NavigatorObserver {
   /// The [Navigator] replaced `oldRoute` with `newRoute`.
   void didReplace({ Route<dynamic> newRoute, Route<dynamic> oldRoute }) { }
 
-  /// The [Navigator]'s routes are being moved by a user gesture.
+  /// The [Navigator]'s route `route` is being moved by a user gesture.
   ///
-  /// For example, this is called when an iOS back gesture starts, and is used
-  /// to disabled hero animations during such interactions.
-  void didStartUserGesture() { }
+  /// For example, this is called when an iOS back gesture starts.
+  ///
+  /// Paired with a call to [didStopUserGesture] when the route is no longer
+  /// being manipulated via user gesture.
+  ///
+  /// If present, the route immediately below `route` is `previousRoute`.
+  /// Though the gesture may not necessarily conclude at `previousRoute` if
+  /// the gesture is canceled. In that case, [didStopUserGesture] is still
+  /// called but a follow-up [didPop] is not.
+  void didStartUserGesture(Route<dynamic> route, Route<dynamic> previousRoute) { }
 
   /// User gesture is no longer controlling the [Navigator].
   ///
@@ -1911,8 +1918,15 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
   void didStartUserGesture() {
     _userGesturesInProgress += 1;
     if (_userGesturesInProgress == 1) {
+      final Route<dynamic> route = _history.last;
+      final Route<dynamic> previousRoute = !route.willHandlePopInternally && _history.length > 1
+          ? _history[_history.length - 2]
+          : null;
+      // Don't operate the _history list since the gesture may be cancelled.
+      // In case of a back swipe, the gesture controller will call .pop() itself.
+
       for (NavigatorObserver observer in widget.observers)
-        observer.didStartUserGesture();
+        observer.didStartUserGesture(route, previousRoute);
     }
   }
 

packages/flutter/test/cupertino/nav_bar_transition_test.dart

@@ -1015,4 +1015,110 @@ void main() {
     expect(bottomBuildTimes, 2);
     expect(topBuildTimes, 3);
   });
+
+  testWidgets('Back swipe gesture transitions',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    // Go to the next page.
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // 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();
+
+    // Page 2, which is the middle of the top route, start to fly back to the right.
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(352.5802058875561, 13.5),
+    );
+
+    // Page 1 is in transition in 2 places. Once as the top back label and once
+    // as the bottom middle.
+    expect(flying(tester, find.text('Page 1')), findsNWidgets(2));
+
+    // Past the halfway point now.
+    await gesture.moveBy(const Offset(500.0, 0.0));
+    await gesture.up();
+
+    await tester.pump();
+
+    // Transition continues.
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(654.2055835723877, 13.5),
+    );
+    await tester.pump(const Duration(milliseconds: 50));
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(720.8727767467499, 13.5),
+    );
+
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // Cleans up properly
+    expect(() => flying(tester, find.text('Page 1')), throwsAssertionError);
+    expect(() => flying(tester, find.text('Page 2')), throwsAssertionError);
+    // Just the bottom route's middle now.
+    expect(find.text('Page 1'), findsOneWidget);
+  });
+
+  testWidgets('Back swipe gesture cancels properly with transition',
+      (WidgetTester tester) async {
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+      toTitle: 'Page 2',
+    );
+
+    // Go to the next page.
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // 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();
+
+    // Page 2, which is the middle of the top route, start to fly back to the right.
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(352.5802058875561, 13.5),
+    );
+
+    await gesture.up();
+    await tester.pump();
+
+    // Transition continues from the point we let off.
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(352.5802058875561, 13.5),
+    );
+    await tester.pump(const Duration(milliseconds: 50));
+    expect(
+      tester.getTopLeft(flying(tester, find.text('Page 2'))),
+      const Offset(350.00985169410706, 13.5),
+    );
+
+    // Finish the snap back animation.
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // Cleans up properly
+    expect(() => flying(tester, find.text('Page 1')), throwsAssertionError);
+    expect(() => flying(tester, find.text('Page 2')), throwsAssertionError);
+    // Back to page 2.
+    expect(find.text('Page 2'), findsOneWidget);
+  });
 }

packages/flutter/test/widgets/heroes_test.dart

@@ -14,13 +14,19 @@ Key homeRouteKey = const Key('homeRoute');
 Key routeTwoKey = const Key('routeTwo');
 Key routeThreeKey = const Key('routeThree');
 
+bool transitionFromUserGestures = false;
+
 final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
   '/': (BuildContext context) => Material(
     child: ListView(
       key: homeRouteKey,
       children: <Widget>[
         Container(height: 100.0, width: 100.0),
-        Card(child: Hero(tag: 'a', child: Container(height: 100.0, width: 100.0, key: firstKey))),
+        Card(child: Hero(
+          tag: 'a',
+          transitionOnUserGestures: transitionFromUserGestures,
+          child: Container(height: 100.0, width: 100.0, key: firstKey),
+        )),
         Container(height: 100.0, width: 100.0),
         FlatButton(
           child: const Text('two'),
@@ -42,7 +48,11 @@ final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
           onPressed: () { Navigator.pop(context); }
         ),
         Container(height: 150.0, width: 150.0),
-        Card(child: Hero(tag: 'a', child: Container(height: 150.0, width: 150.0, key: secondKey))),
+        Card(child: Hero(
+          tag: 'a',
+          transitionOnUserGestures: transitionFromUserGestures,
+          child: Container(height: 150.0, width: 150.0, key: secondKey),
+        )),
         Container(height: 150.0, width: 150.0),
         FlatButton(
           child: const Text('three'),
@@ -67,7 +77,11 @@ final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
         Card(
           child: Padding(
             padding: const EdgeInsets.only(left: 50.0),
-            child: Hero(tag: 'a', child: Container(height: 150.0, width: 150.0, key: secondKey))
+            child: Hero(
+              tag: 'a',
+              transitionOnUserGestures: transitionFromUserGestures,
+              child: Container(height: 150.0, width: 150.0, key: secondKey),
+            )
           ),
         ),
         Container(height: 150.0, width: 150.0),
@@ -78,7 +92,6 @@ final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
       ]
     )
   ),
-
 };
 
 class ThreeRoute extends MaterialPageRoute<void> {
@@ -121,6 +134,10 @@ class MyStatefulWidgetState extends State<MyStatefulWidget> {
 }
 
 void main() {
+  setUp(() {
+    transitionFromUserGestures = false;
+  });
+
   testWidgets('Heroes animate', (WidgetTester tester) async {
 
     await tester.pumpWidget(MaterialApp(routes: routes));
@@ -1335,4 +1352,89 @@ void main() {
     expect(find.text('Venom'), findsOneWidget);
     expect(find.text('Joker'), findsOneWidget);
   });
+
+  testWidgets('Heroes do not transition on back gestures by default', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(
+        platform: TargetPlatform.iOS,
+      ),
+      routes: routes,
+    ));
+
+    expect(find.byKey(firstKey), isOnstage);
+    expect(find.byKey(firstKey), isInCard);
+    expect(find.byKey(secondKey), findsNothing);
+
+    await tester.tap(find.text('two'));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(find.byKey(firstKey), findsNothing);
+    expect(find.byKey(secondKey), isOnstage);
+    expect(find.byKey(secondKey), isInCard);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5.0, 200.0));
+    await gesture.moveBy(const Offset(200.0, 0.0));
+
+    await tester.pump();
+
+    // Both Heros exist and seated in their normal parents.
+    expect(find.byKey(firstKey), isOnstage);
+    expect(find.byKey(firstKey), isInCard);
+    expect(find.byKey(secondKey), isOnstage);
+    expect(find.byKey(secondKey), isInCard);
+
+    // To make sure the hero had all chances of starting.
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(find.byKey(firstKey), isOnstage);
+    expect(find.byKey(firstKey), isInCard);
+    expect(find.byKey(secondKey), isOnstage);
+    expect(find.byKey(secondKey), isInCard);
+  });
+
+  testWidgets('Heroes can transition on gesture in one frame', (WidgetTester tester) async {
+    transitionFromUserGestures = true;
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(
+        platform: TargetPlatform.iOS,
+      ),
+      routes: routes,
+    ));
+
+    await tester.tap(find.text('two'));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(find.byKey(firstKey), findsNothing);
+    expect(find.byKey(secondKey), isOnstage);
+    expect(find.byKey(secondKey), isInCard);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5.0, 200.0));
+    await gesture.moveBy(const Offset(200.0, 0.0));
+    await tester.pump();
+
+    // We're going to page 1 so page 1's Hero is lifted into flight.
+    expect(find.byKey(firstKey), isOnstage);
+    expect(find.byKey(firstKey), isNotInCard);
+    expect(find.byKey(secondKey), findsNothing);
+
+    // Move further along.
+    await gesture.moveBy(const Offset(500.0, 0.0));
+    await tester.pump();
+
+    // Same results.
+    expect(find.byKey(firstKey), isOnstage);
+    expect(find.byKey(firstKey), isNotInCard);
+    expect(find.byKey(secondKey), findsNothing);
+
+    await gesture.up();
+    // Finish transition.
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    // Hero A is back in the card.
+    expect(find.byKey(firstKey), isOnstage);
+    expect(find.byKey(firstKey), isInCard);
+    expect(find.byKey(secondKey), findsNothing);
+  });
 }

packages/flutter/test/widgets/navigator_test.dart

@@ -96,6 +96,7 @@ class TestObserver extends NavigatorObserver {
   OnObservation onPopped;
   OnObservation onRemoved;
   OnObservation onReplaced;
+  OnObservation onStartUserGesture;
 
   @override
   void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
@@ -122,6 +123,12 @@ class TestObserver extends NavigatorObserver {
     if (onReplaced != null)
       onReplaced(newRoute, oldRoute);
   }
+
+  @override
+  void didStartUserGesture(Route<dynamic> route, Route<dynamic> previousRoute) {
+    if (onStartUserGesture != null)
+      onStartUserGesture(route, previousRoute);
+  }
 }
 
 void main() {
@@ -715,6 +722,38 @@ void main() {
     expect(log, <String>['pushed / (previous is <none>)', 'pushed B (previous is /)', 'pushed C (previous is B)', 'replaced B with D']);
   });
 
+  testWidgets('didStartUserGesture observable',
+      (WidgetTester tester) async {
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+       '/': (BuildContext context) => OnTapPage(id: '/', onTap: () { Navigator.pushNamed(context, '/A'); }),
+      '/A': (BuildContext context) => OnTapPage(id: 'A', onTap: () { Navigator.pop(context); }),
+    };
+
+    Route<dynamic> observedRoute;
+    Route<dynamic> observedPreviousRoute;
+    final TestObserver observer = TestObserver()
+      ..onStartUserGesture = (Route<dynamic> route, Route<dynamic> previousRoute) {
+        observedRoute = route;
+        observedPreviousRoute = previousRoute;
+      };
+
+    await tester.pumpWidget(MaterialApp(
+      routes: routes,
+      navigatorObservers: <NavigatorObserver>[observer],
+    ));
+
+    await tester.tap(find.text('/'));
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+    expect(find.text('/'), findsNothing);
+    expect(find.text('A'), findsOneWidget);
+
+    tester.state<NavigatorState>(find.byType(Navigator)).didStartUserGesture();
+
+    expect(observedRoute.settings.name, '/A');
+    expect(observedPreviousRoute.settings.name, '/');
+  });
+
   testWidgets('ModalRoute.of sets up a route to rebuild if its state changes', (WidgetTester tester) async {
     final GlobalKey<NavigatorState> key = GlobalKey<NavigatorState>();
     final List<String> log = <String>[];