Support aborted hero flights (#8805)

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

@@ -222,6 +222,7 @@ class _HeroFlight {
   ProxyAnimation _proxyAnimation;
   _HeroFlightManifest manifest;
   OverlayEntry overlayEntry;
+  bool _aborted = false;
 
   RectTween _doCreateRectTween(Rect begin, Rect end) {
     if (manifest.createRectTween != null)
@@ -237,9 +238,10 @@ class _HeroFlight {
       child: manifest.toHero.config,
       builder: (BuildContext context, Widget child) {
         final RenderBox toHeroBox = manifest.toHero.context?.findRenderObject();
-        if (toHeroBox == null || !toHeroBox.attached) {
-          // The toHero no longer exists. Continue flying while fading out.
-          if (_heroOpacity == kAlwaysCompleteAnimation) {
+        if (_aborted || toHeroBox == null || !toHeroBox.attached) {
+          // The toHero no longer exists or it's no longer the flight's destination.
+          // Continue flying while fading out.
+          if (_heroOpacity.isCompleted) {
             _heroOpacity = new Tween<double>(begin: 1.0, end: 0.0)
               .chain(new CurveTween(curve: new Interval(_proxyAnimation.value, 1.0)))
               .animate(_proxyAnimation);
@@ -294,6 +296,7 @@ class _HeroFlight {
 
   // The simple case: we're either starting a push or a pop animation.
   void start(_HeroFlightManifest initialManifest) {
+    assert(!_aborted);
     assert(() {
       final Animation<double> initial = initialManifest.animation;
       switch (initialManifest.type) {
@@ -305,6 +308,7 @@ class _HeroFlight {
     });
 
     manifest = initialManifest;
+
     if (manifest.type == _HeroFlightType.pop)
       _proxyAnimation.parent = new ReverseAnimation(manifest.animation);
     else
@@ -388,9 +392,14 @@ class _HeroFlight {
       newManifest.toHero.startFlight();
     }
 
+    _aborted = false;
     manifest = newManifest;
   }
 
+  void abort() {
+    _aborted = true;
+  }
+
   @override
   String toString() {
     final RouteSettings from = manifest.fromRoute.settings;
@@ -418,8 +427,7 @@ class HeroController extends NavigatorObserver {
 
   // All of the heroes that are currently in the overlay and in motion.
   // Indexed by the hero tag.
-  // TBD: final?
-  Map<Object, _HeroFlight> _flights = <Object, _HeroFlight>{};
+  final Map<Object, _HeroFlight> _flights = <Object, _HeroFlight>{};
 
   @override
   void didPush(Route<dynamic> to, Route<dynamic> from) {
@@ -506,6 +514,8 @@ class HeroController extends NavigatorObserver {
             _flights[tag].restart(manifest);
         else
           _flights[tag] = new _HeroFlight(_handleFlightEnded)..start(manifest);
+      } else if (_flights[tag] != null) {
+        _flights[tag].abort();
       }
     }
   }

packages/flutter/test/widgets/heroes_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 
 Key firstKey = const Key('first');
 Key secondKey = const Key('second');
@@ -804,4 +805,110 @@ void main() {
     expect(find.byKey(routeHeroKey), findsNothing);
   });
 
+  testWidgets('Aborted flight', (WidgetTester tester) async {
+    // See https://github.com/flutter/flutter/issues/5798
+    final Key heroABKey = const Key('AB hero');
+    final Key heroBCKey = const Key('BC hero');
+
+    // Show a 150x150 Hero tagged 'BC'
+    final MaterialPageRoute<Null> routeC = new MaterialPageRoute<Null>(
+      builder: (BuildContext context) {
+        return new Material(
+          child: new ListView(
+            children: <Widget>[
+              // This container will appear at Y=0
+              new Container(
+                child: new Hero(tag: 'BC', child: new Container(key: heroBCKey, height: 150.0))
+              ),
+              new SizedBox(height: 800.0),
+            ],
+          )
+        );
+      },
+    );
+
+    // Show a height=200 Hero tagged 'AB' and a height=50 Hero tagged 'BC'
+    final MaterialPageRoute<Null> routeB = new MaterialPageRoute<Null>(
+      builder: (BuildContext context) {
+        return new Material(
+          child: new ListView(
+            children: <Widget>[
+              new SizedBox(height: 100.0),
+              // This container will appear at Y=100
+              new Container(
+                child: new Hero(tag: 'AB', child: new Container(key: heroABKey, height: 200.0))
+              ),
+              new FlatButton(
+                child: new Text('PUSH C'),
+                onPressed: () { Navigator.push(context, routeC); }
+              ),
+              new Container(
+                child: new Hero(tag: 'BC', child: new Container(height: 150.0))
+              ),
+              new SizedBox(height: 800.0),
+            ],
+          )
+        );
+      },
+    );
+
+    // Show a 100x100 Hero tagged 'AB' with key heroABKey
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Scaffold(
+          body: new Builder(
+            builder: (BuildContext context) { // Navigator.push() needs context
+              return new ListView(
+                children: <Widget> [
+                  new SizedBox(height: 200.0),
+                  // This container will appear at Y=200
+                  new Container(
+                    child: new Hero(tag: 'AB', child: new Container(height: 100.0, width: 100.0)),
+                  ),
+                  new FlatButton(
+                    child: new Text('PUSH B'),
+                    onPressed: () { Navigator.push(context, routeB); }
+                  ),
+                ],
+              );
+            },
+          ),
+        ),
+      )
+    );
+
+    // Pushes routeB
+    await tester.tap(find.text('PUSH B'));
+    await tester.pump();
+    await tester.pump();
+
+    final double initialY = tester.getTopLeft(find.byKey(heroABKey)).y;
+    expect(initialY, 200.0);
+
+    await tester.pump(const Duration(milliseconds: 200));
+    final double yAt200ms = tester.getTopLeft(find.byKey(heroABKey)).y;
+    // Hero AB is mid flight.
+    expect(yAt200ms, lessThan(200.0));
+    expect(yAt200ms, greaterThan(100.0));
+
+    // Pushes route C, causes hero AB's flight to abort, hero BC's flight to start
+    await tester.tap(find.text('PUSH C'));
+    await tester.pump();
+    await tester.pump();
+
+    // Hero AB's aborted flight finishes where it was expected although
+    // it's been faded out.
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(tester.getTopLeft(find.byKey(heroABKey)).y, 100.0);
+
+    // One Opacity widget per Hero, only one now has opacity 0.0
+    final Iterable<RenderOpacity> renderers = tester.renderObjectList(find.byType(Opacity));
+    final Iterable<double> opacities = renderers.map((RenderOpacity r) => r.opacity);
+    expect(opacities.singleWhere((double opacity) => opacity == 0.0), 0.0);
+
+    // Hero BC's flight finishes normally.
+    await tester.pump(const Duration(milliseconds: 300));
+    expect(tester.getTopLeft(find.byKey(heroBCKey)).y, 0.0);
+  });
+
 }