Fix curve for popping hero (#39844)

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

@@ -413,6 +413,7 @@ class _HeroFlightManifest {
     @required this.createRectTween,
     @required this.shuttleBuilder,
     @required this.isUserGestureTransition,
+    @required this.isDiverted,
   }) : assert(fromHero.widget.tag == toHero.widget.tag);
 
   final HeroFlightDirection type;
@@ -425,6 +426,7 @@ class _HeroFlightManifest {
   final CreateRectTween createRectTween;
   final HeroFlightShuttleBuilder shuttleBuilder;
   final bool isUserGestureTransition;
+  final bool isDiverted;
 
   Object get tag => fromHero.widget.tag;
 
@@ -432,6 +434,7 @@ class _HeroFlightManifest {
     return CurvedAnimation(
       parent: (type == HeroFlightDirection.push) ? toRoute.animation : fromRoute.animation,
       curve: Curves.fastOutSlowIn,
+      reverseCurve: isDiverted ? null : Curves.fastOutSlowIn.flipped,
     );
   }
 
@@ -805,6 +808,7 @@ class HeroController extends NavigatorObserver {
       if (toHeroes[tag] != null) {
         final HeroFlightShuttleBuilder fromShuttleBuilder = fromHeroes[tag].widget.flightShuttleBuilder;
         final HeroFlightShuttleBuilder toShuttleBuilder = toHeroes[tag].widget.flightShuttleBuilder;
+        final bool isDiverted = _flights[tag] != null;
 
         final _HeroFlightManifest manifest = _HeroFlightManifest(
           type: flightType,
@@ -818,9 +822,10 @@ class HeroController extends NavigatorObserver {
           shuttleBuilder:
               toShuttleBuilder ?? fromShuttleBuilder ?? _defaultHeroFlightShuttleBuilder,
           isUserGestureTransition: isUserGestureTransition,
+          isDiverted: isDiverted,
         );
 
-        if (_flights[tag] != null)
+        if (isDiverted)
           _flights[tag].divert(manifest);
         else
           _flights[tag] = _HeroFlight(_handleFlightEnded)..start(manifest);

packages/flutter/test/widgets/heroes_test.dart

@@ -1254,17 +1254,17 @@ Future<void> main() async {
 
     await tester.pump(duration * 0.25);
     actualHeroCenter = tester.getCenter(find.byKey(firstKey));
-    predictedHeroCenter = popCenterTween.lerp(curve.flipped.transform(0.25));
+    predictedHeroCenter = popCenterTween.lerp(curve.transform(0.25));
     expect(actualHeroCenter, within<Offset>(distance: epsilon, from: predictedHeroCenter));
 
     await tester.pump(duration * 0.25);
     actualHeroCenter = tester.getCenter(find.byKey(firstKey));
-    predictedHeroCenter = popCenterTween.lerp(curve.flipped.transform(0.5));
+    predictedHeroCenter = popCenterTween.lerp(curve.transform(0.5));
     expect(actualHeroCenter, within<Offset>(distance: epsilon, from: predictedHeroCenter));
 
     await tester.pump(duration * 0.25);
     actualHeroCenter = tester.getCenter(find.byKey(firstKey));
-    predictedHeroCenter = popCenterTween.lerp(curve.flipped.transform(0.75));
+    predictedHeroCenter = popCenterTween.lerp(curve.transform(0.75));
     expect(actualHeroCenter, within<Offset>(distance: epsilon, from: predictedHeroCenter));
 
     await tester.pumpAndSettle();
@@ -1375,17 +1375,17 @@ Future<void> main() async {
 
     await tester.pump(duration * 0.25);
     actualHeroRect = tester.getRect(find.byKey(firstKey));
-    predictedHeroRect = popRectTween.lerp(curve.flipped.transform(0.25));
+    predictedHeroRect = popRectTween.lerp(curve.transform(0.25));
     expect(actualHeroRect, within<Rect>(distance: epsilon, from: predictedHeroRect));
 
     await tester.pump(duration * 0.25);
     actualHeroRect = tester.getRect(find.byKey(firstKey));
-    predictedHeroRect = popRectTween.lerp(curve.flipped.transform(0.5));
+    predictedHeroRect = popRectTween.lerp(curve.transform(0.5));
     expect(actualHeroRect, within<Rect>(distance: epsilon, from: predictedHeroRect));
 
     await tester.pump(duration * 0.25);
     actualHeroRect = tester.getRect(find.byKey(firstKey));
-    predictedHeroRect = popRectTween.lerp(curve.flipped.transform(0.75));
+    predictedHeroRect = popRectTween.lerp(curve.transform(0.75));
     expect(actualHeroRect, within<Rect>(distance: epsilon, from: predictedHeroRect));
 
     await tester.pumpAndSettle();
@@ -2255,4 +2255,83 @@ Future<void> main() async {
       moreOrLessEquals(tester.getTopLeft(find.text('1')).dx, epsilon: 0.01)
     );
   });
+
+  testWidgets('popped hero uses fastOutSlowIn curve', (WidgetTester tester) async {
+    final Key container1 = UniqueKey();
+    final Key container2 = UniqueKey();
+    final GlobalKey<NavigatorState> navigator = GlobalKey<NavigatorState>();
+
+    final Animatable<Size> tween = SizeTween(
+      begin: const Size(200, 200),
+      end: const Size(100, 100),
+    ).chain(CurveTween(curve: Curves.fastOutSlowIn));
+
+
+    await tester.pumpWidget(
+      MaterialApp(
+        navigatorKey: navigator,
+        home: Scaffold(
+          body: Center(
+            child: Hero(
+              tag: 'test',
+              createRectTween: (Rect begin, Rect end) {
+                return RectTween(begin: begin, end: end);
+              },
+              child: Container(
+                key: container1,
+                height: 100,
+                width: 100,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    final Size originalSize = tester.getSize(find.byKey(container1));
+    expect(originalSize, const Size(100, 100));
+
+    navigator.currentState.push(MaterialPageRoute<void>(builder: (BuildContext context) {
+      return Scaffold(
+        body: Center(
+          child: Hero(
+            tag: 'test',
+            createRectTween: (Rect begin, Rect end) {
+              return RectTween(begin: begin, end: end);
+            },
+            child: Container(
+              key: container2,
+              height: 200,
+              width: 200,
+            ),
+          ),
+        ),
+      );
+    }));
+    await tester.pumpAndSettle();
+    final Size newSize = tester.getSize(find.byKey(container2));
+    expect(newSize, const Size(200, 200));
+
+    navigator.currentState.pop();
+    await tester.pump();
+
+    // Jump 25% into the transition (total length = 300ms)
+    await tester.pump(const Duration(milliseconds: 75)); // 25% of 300ms
+    Size heroSize = tester.getSize(find.byKey(container1));
+    expect(heroSize, tween.transform(0.25));
+
+    // Jump to 50% into the transition.
+    await tester.pump(const Duration(milliseconds: 75)); // 25% of 300ms
+    heroSize = tester.getSize(find.byKey(container1));
+    expect(heroSize, tween.transform(0.50));
+
+    // Jump to 75% into the transition.
+    await tester.pump(const Duration(milliseconds: 75)); // 25% of 300ms
+    heroSize = tester.getSize(find.byKey(container1));
+    expect(heroSize, tween.transform(0.75));
+
+    // Jump to 100% into the transition.
+    await tester.pump(const Duration(milliseconds: 75)); // 25% of 300ms
+    heroSize = tester.getSize(find.byKey(container1));
+    expect(heroSize, tween.transform(1.0));
+  });
 }