Add HeroController to CupertinoApp (#19326)

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

@@ -42,6 +42,9 @@ const TextStyle _kDefaultTextStyle = const TextStyle(
 /// If [home], [routes], [onGenerateRoute], and [onUnknownRoute] are all null,
 /// and [builder] is not null, then no [Navigator] is created.
 ///
+/// This widget also configures the observer of the top-level [Navigator] (if
+/// any) to perform [Hero] animations.
+///
 /// Using this widget with caution on Android since it may produce behaviors
 /// Android users are not expecting such as:
 ///
@@ -171,7 +174,7 @@ class CupertinoApp extends StatefulWidget {
   /// When a named route is pushed with [Navigator.pushNamed], the route name is
   /// looked up in this map. If the name is present, the associated
   /// [WidgetBuilder] is used to construct a [CupertinoPageRoute] that performs
-  /// an appropriate transition to the new route.
+  /// an appropriate transition, including [Hero] animations, to the new route.
   ///
   /// If the app only has one page, then you can specify it using [home] instead.
   ///
@@ -246,8 +249,8 @@ class CupertinoApp extends StatefulWidget {
   ///
   /// Unless a [Navigator] is provided, either implicitly from [builder] being
   /// null, or by a [builder] including its `child` argument, or by a [builder]
-  /// explicitly providing a [Navigator] of its own, APIs such as
+  /// explicitly providing a [Navigator] of its own, widgets and APIs such as
-  /// [Navigator.push] and [Navigator.pop], will not function.
+  /// [Hero], [Navigator.push] and [Navigator.pop], will not function.
   final TransitionBuilder builder;
 
   /// {@macro flutter.widgets.widgetsApp.title}
@@ -317,9 +320,13 @@ class _AlwaysCupertinoScrollBehavior extends ScrollBehavior {
 }
 
 class _CupertinoAppState extends State<CupertinoApp> {
+  HeroController _heroController;
+  List<NavigatorObserver> _navigatorObservers;
+
   @override
   void initState() {
     super.initState();
+    _heroController = new HeroController(); // Linear tweening.
     _updateNavigator();
   }
 
@@ -335,6 +342,9 @@ class _CupertinoAppState extends State<CupertinoApp> {
                      widget.routes.isNotEmpty ||
                      widget.onGenerateRoute != null ||
                      widget.onUnknownRoute != null;
+    _navigatorObservers =
+        new List<NavigatorObserver>.from(widget.navigatorObservers)
+          ..add(_heroController);
   }
 
   Widget defaultBuilder(BuildContext context, Widget child) {
@@ -351,7 +361,7 @@ class _CupertinoAppState extends State<CupertinoApp> {
         routes: widget.routes,
         onGenerateRoute: widget.onGenerateRoute,
         onUnknownRoute: widget.onUnknownRoute,
-        navigatorObservers: widget.navigatorObservers,
+        navigatorObservers: _navigatorObservers,
       );
       if (widget.builder != null) {
         return widget.builder(context, navigator);

packages/flutter/test/cupertino/app_test.dart

@@ -0,0 +1,43 @@
+// Copyright 2018 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/cupertino.dart';
+
+void main() {
+  testWidgets('Heroes work', (WidgetTester tester) async {
+    await tester.pumpWidget(new CupertinoApp(
+      home:
+        new ListView(
+          children: <Widget>[
+            const Hero(tag: 'a', child: const Text('foo')),
+            new Builder(builder: (BuildContext context) {
+              return new CupertinoButton(
+                child: const Text('next'),
+                onPressed: () {
+                  Navigator.push(
+                    context,
+                    new CupertinoPageRoute<void>(
+                      builder: (BuildContext context) {
+                        return const Hero(tag: 'a', child: const Text('foo'));
+                      }
+                    ),
+                  );
+                },
+              );
+            }),
+          ],
+        )
+    ));
+
+    await tester.tap(find.text('next'));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 100));
+
+    // During the hero transition, the hero widget is lifted off of both
+    // page routes and exists as its own overlay on top of both routes.
+    expect(find.widgetWithText(CupertinoPageRoute, 'foo'), findsNothing);
+    expect(find.widgetWithText(Navigator, 'foo'), findsOneWidget);
+  });
+}