Heroes don't reverse any more

We need to use the performance for the "from" route when going backwards. Also,
fix a bug in the HeroParty where it would call the quest finished callback
multiple times.

Fixes #1958

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

@@ -20,9 +20,23 @@ class HeroPageRoute extends PageRoute {
   }) : super(builder: builder, settings: settings);
 
   final HeroController heroController;
+  NavigatorState _navigator;
+
+  void didPush(OverlayState overlay, OverlayEntry insertionPoint) {
+    super.didPush(overlay, insertionPoint);
+    // TODO(abarth): Pass the NavigatorState explicitly.
+    if (overlay != null) {
+      _navigator = Navigator.of(overlay.context);
+      heroController?.didPush(_navigator, this);
+    }
+  }
 
-  void didMakeCurrent() {
-    heroController?.didMakeCurrent(this);
+  void didPop(dynamic result) {
+    super.didPop(result);
+    if (_navigator != null) {
+      heroController?.didPop(_navigator, this);
+      _navigator = null;
+    }
   }
 }
 
@@ -32,29 +46,47 @@ class HeroController {
   }
 
   HeroParty _party;
+  PerformanceView _performance;
   HeroPageRoute _from;
   HeroPageRoute _to;
 
   final List<OverlayEntry> _overlayEntries = new List<OverlayEntry>();
 
-  void didMakeCurrent(PageRoute current) {
-    assert(current != null);
-    assert(current.performance != null);
-    if (_from == null) {
-      _from = current;
-      return;
+  void didPush(NavigatorState navigator, HeroPageRoute route) {
+    assert(route != null);
+    assert(route.performance != null);
+    Route from = navigator.currentRoute;
+    if (from is HeroPageRoute)
+      _from = from;
+    _to = route;
+    _performance = route.performance;
+    _checkForHeroQuest();
+  }
+
+  void didPop(NavigatorState navigator, HeroPageRoute route) {
+    assert(route != null);
+    assert(route.performance != null);
+    Route to = navigator.currentRoute;
+    if (to is HeroPageRoute) {
+      _to = to;
+      _from = route;
+      _performance = route.performance;
+      _checkForHeroQuest();
     }
-    _to = current;
-    if (_from != _to) {
-      current.offstage = current.performance.status != PerformanceStatus.completed;
+  }
+
+  void _checkForHeroQuest() {
+    if (_from != null && _to != null && _from != _to) {
+      _to.offstage = _to.performance.status != PerformanceStatus.completed;
       scheduler.requestPostFrameCallback(_updateQuest);
     }
   }
 
   void _handleQuestFinished() {
     _removeHeroesFromOverlay();
-    _from = _to;
+    _from = null;
     _to = null;
+    _performance = null;
   }
 
   Rect _getAnimationArea(BuildContext context) {
@@ -97,7 +129,7 @@ class HeroController {
     Map<Object, HeroHandle> heroesTo = Hero.of(context, mostValuableKeys);
     _to.offstage = false;
 
-    PerformanceView performance = _to.performance;
+    PerformanceView performance = _performance;
     Curve curve = Curves.ease;
     if (performance.status == PerformanceStatus.reverse) {
       performance = new ReversePerformance(performance);

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

@@ -366,14 +366,17 @@ class HeroParty {
 
   PerformanceView _currentPerformance;
 
+  void _clearCurrentPerformance() {
+    _currentPerformance?.removeStatusListener(_handleUpdate);
+    _currentPerformance = null;
+  }
+
   Iterable<Widget> getWidgets(BuildContext context, PerformanceView performance) sync* {
     assert(performance != null || _heroes.length == 0);
     if (performance != _currentPerformance) {
-      if (_currentPerformance != null)
-        _currentPerformance.removeStatusListener(_handleUpdate);
+      _clearCurrentPerformance();
       _currentPerformance = performance;
-      if (_currentPerformance != null)
-        _currentPerformance.addStatusListener(_handleUpdate);
+      _currentPerformance?.addStatusListener(_handleUpdate);
     }
     for (_HeroQuestState hero in _heroes)
       yield hero.build(context, performance);
@@ -389,7 +392,7 @@ class HeroParty {
           source._resetChild();
       }
       _heroes.clear();
-      _currentPerformance = null;
+      _clearCurrentPerformance();
       if (onQuestFinished != null)
         onQuestFinished();
     }

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

@@ -9,7 +9,6 @@ abstract class Route {
   List<OverlayEntry> get overlayEntries;
 
   void didPush(OverlayState overlay, OverlayEntry insertionPoint);
-  void didMakeCurrent();
   void didPop(dynamic result);
 }
 
@@ -95,13 +94,11 @@ class NavigatorState extends State<Navigator> {
     _popAllEphemeralRoutes();
     route.didPush(overlay, _currentOverlay);
     _modal.add(route);
-    route.didMakeCurrent();
   }
 
   void pushEphemeral(Route route) {
     route.didPush(overlay, _currentOverlay);
     _ephemeral.add(route);
-    route.didMakeCurrent();
   }
 
   void _popAllEphemeralRoutes() {
@@ -114,7 +111,6 @@ class NavigatorState extends State<Navigator> {
 
   void pop([dynamic result]) {
     _removeCurrentRoute().didPop(result);
-    currentRoute.didMakeCurrent();
   }
 
   Widget build(BuildContext context) {

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

@@ -17,7 +17,6 @@ class StateRoute extends Route {
   List<OverlayEntry> get overlayEntries => const <OverlayEntry>[];
 
   void didPush(OverlayState overlay, OverlayEntry insertionPoint) { }
-  void didMakeCurrent() { }
   void didPop(dynamic result) {
     if (onPop != null)
       onPop();
@@ -38,8 +37,6 @@ class OverlayRoute extends Route {
     }
   }
 
-  void didMakeCurrent() { }
-
   void didPop(dynamic result) {
     for (OverlayEntry entry in _overlayEntries)
       entry.remove();