Handle back during pushReplacement (#10016)

Previously, we would strand the old route in the overlay even though it had
been removed from the history.

packages/flutter/lib/src/material/tab_controller.dart

@@ -102,7 +102,10 @@ class TabController extends ChangeNotifier {
       notifyListeners(); // Because the value of indexIsChanging may have changed.
       _animationController
         .animateTo(_index.toDouble(), duration: duration, curve: curve)
-        .orCancel.then<Null>(_indexChanged, onError: _indexChanged);
+        .whenCompleteOrCancel(() {
+          _indexIsChangingCount -= 1;
+          notifyListeners();
+        });
     } else {
       _indexIsChangingCount += 1;
       _animationController.value = _index.toDouble();
@@ -111,12 +114,6 @@ class TabController extends ChangeNotifier {
     }
   }
 
-  Null _indexChanged(dynamic value) {
-    _indexIsChangingCount -= 1;
-    notifyListeners();
-    return null;
-  }
-
   /// The index of the currently selected tab. Changing the index also updates
   /// [previousIndex], sets the [animation]'s value to index, resets
   /// [indexIsChanging] to false, and notifies listeners.

packages/flutter/lib/src/scheduler/ticker.dart

@@ -333,6 +333,9 @@ class Ticker {
 /// [orCancel], which returns a derivative [Future] that completes with an error
 /// if the [Ticker] that returned the [TickerFuture] was stopped with `canceled`
 /// set to true, or if it was disposed without being stopped.
+///
+/// To run a callback when either this future resolves or when the tricker is
+/// canceled, use [whenCompleteOrCancel].
 class TickerFuture implements Future<Null> {
   TickerFuture._();
 
@@ -364,6 +367,16 @@ class TickerFuture implements Future<Null> {
     _secondaryCompleter?.completeError(new TickerCanceled(ticker));
   }
 
+  /// Calls `callback` either when this future resolves or when the ticker is
+  /// canceled.
+  void whenCompleteOrCancel(VoidCallback callback) {
+    Null thunk(dynamic value) {
+      callback();
+      return null;
+    }
+    orCancel.then(thunk, onError: thunk);
+  }
+
   /// A future that resolves when this future resolves or throws when the ticker
   /// is canceled.
   Future<Null> get orCancel {

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

@@ -58,7 +58,7 @@ abstract class Route<T> {
   ///
   /// The returned value resolves when the push transition is complete.
   @protected
-  Future<Null> didPush() => new Future<Null>.value();
+  TickerFuture didPush() => new TickerFuture.complete();
 
   /// When this route is popped (see [Navigator.pop]) if the result isn't
   /// specified or if it's null, this value will be used instead.
@@ -894,7 +894,7 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
       newRoute._navigator = this;
       newRoute.install(_currentOverlayEntry);
       _history[index] = newRoute;
-      newRoute.didPush().then<Null>((Null value) {
+      newRoute.didPush().whenCompleteOrCancel(() {
         // The old route's exit is not animated. We're assuming that the
         // new route completely obscures the old one.
         if (mounted) {

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

@@ -165,7 +165,7 @@ abstract class TransitionRoute<T> extends OverlayRoute<T> {
   }
 
   @override
-  Future<Null> didPush() {
+  TickerFuture didPush() {
     _animation.addStatusListener(_handleStatusChanged);
     return _controller.forward();
   }
@@ -704,7 +704,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   }
 
   @override
-  Future<Null> didPush() {
+  TickerFuture didPush() {
     navigator.focusScopeNode.setFirstFocus(focusScopeNode);
     return super.didPush();
   }

packages/flutter/test/widgets/navigator_replacement_test.dart

@@ -0,0 +1,42 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+
+void main() {
+  testWidgets('Back during pushReplacement',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: const Material(child: const Text("home")),
+      routes: <String, WidgetBuilder> {
+        '/a': (BuildContext context) => const Material(child: const Text("a")),
+        '/b': (BuildContext context) => const Material(child: const Text("b")),
+      },
+    ));
+
+    final NavigatorState navigator = tester.state(find.byType(Navigator));
+    navigator.pushNamed('/a');
+    await tester.pumpAndSettle();
+
+    expect(find.text('a'), findsOneWidget);
+    expect(find.text('home'), findsNothing);
+
+    navigator.pushReplacementNamed('/b');
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 10));
+
+    expect(find.text('a'), findsOneWidget);
+    expect(find.text('b'), findsOneWidget);
+    expect(find.text('home'), findsNothing);
+
+    navigator.pop();
+
+    await tester.pumpAndSettle();
+
+    expect(find.text('a'), findsNothing);
+    expect(find.text('b'), findsNothing);
+    expect(find.text('home'), findsOneWidget);
+  });
+}

packages/flutter/test/widgets/routes_test.dart

@@ -38,7 +38,7 @@ class TestRoute extends LocalHistoryRoute<String> {
   }
 
   @override
-  Future<Null> didPush() {
+  TickerFuture didPush() {
     log('didPush');
     return super.didPush();
   }