Adds a RouteObserver (#11236)

* Adds a PageRouteObserver

Required for https://github.com/flutter/plugins/pull/174.
See also https://github.com/flutter/flutter/issues/11216.

* formatting fixes

* Make RouteObserver generic

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

@@ -967,3 +967,85 @@ abstract class PopupRoute<T> extends ModalRoute<T> {
     super.didChangeNext(nextRoute);
   }
 }
+
+/// A [Navigator] observer that notifies [RouteAware]s of changes to the
+/// state of their [Route].
+///
+/// To make a [StatefulWidget] aware of its current [Route] state, implement
+/// [RouteAware] in its [State] and subscribe it to the [RouteObserver]:
+///
+/// ```dart
+/// class RouteAwareWidgetState extends State<RouteAwareWidget> with RouteAware {
+///   @override
+///   void didChangeDependencies() {
+///     super.didChangeDependencies();
+///     routeObserver.subscribe(this, ModalRoute.of(context));
+///   }
+///
+///   @override
+///   void dispose() {
+///     routeObserver.unsubscribe(this);
+///     super.dispose();
+///   }
+///
+///   @override
+///   void didPush() {
+///     // Do something
+///   }
+///
+///   @override
+///   void didPopNext() {
+///     // Do something
+///   }
+///
+/// }
+/// ```
+class RouteObserver<T extends Route<dynamic>> extends NavigatorObserver {
+  final Map<T, RouteAware> _listeners = <T, RouteAware>{};
+
+  void subscribe(RouteAware routeAware, T route) {
+    assert(routeAware != null);
+    assert(route != null);
+    if (!_listeners.containsKey(route)) {
+      routeAware.didPush();
+      _listeners[route] = routeAware;
+    }
+  }
+
+  void unsubscribe(RouteAware routeAware) {
+    assert(routeAware != null);
+    _listeners.remove(routeAware);
+  }
+
+  @override
+  void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
+    if (route is T && previousRoute is T) {
+      _listeners[previousRoute]?.didPopNext();
+      _listeners[route]?.didPop();
+    }
+  }
+
+  @override
+  void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
+    if (route is T && previousRoute is T) {
+      _listeners[previousRoute]?.didPushNext();
+    }
+  }
+}
+
+/// A interface that is aware of its current Route.
+abstract class RouteAware {
+  /// Called when the top route has been popped off, and the current route
+  /// shows up.
+  void didPopNext() { }
+
+  /// Called when the current route has been pushed.
+  void didPush() { }
+
+  /// Called when the current route has been popped off.
+  void didPop() { }
+
+  /// Called when a new route has been pushed, and the current route is no
+  /// longer visible.
+  void didPushNext() { }
+}
\ No newline at end of file

packages/flutter/pubspec.yaml

@@ -18,6 +18,7 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  mockito: "^2.0.2"
 
 environment:
   sdk: '>=1.19.0 <2.0.0'

packages/flutter/test/widgets/routes_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:collection';
 
+import 'package:mockito/mockito.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
@@ -397,4 +398,45 @@ void main() {
       ]
     );
   });
+
+  group('PageRouteObserver', () {
+    test('calls correct listeners', () {
+      final RouteObserver<PageRoute<dynamic>> observer = new RouteObserver<PageRoute<dynamic>>();
+      final RouteAware pageRouteAware1 = new MockRouteAware();
+      final MockPageRoute route1 = new MockPageRoute();
+      observer.subscribe(pageRouteAware1, route1);
+      verify(pageRouteAware1.didPush()).called(1);
+
+      final RouteAware pageRouteAware2 = new MockRouteAware();
+      final MockPageRoute route2 = new MockPageRoute();
+      observer.didPush(route2, route1);
+      verify(pageRouteAware1.didPushNext()).called(1);
+
+      observer.subscribe(pageRouteAware2, route2);
+      verify(pageRouteAware2.didPush()).called(1);
+
+      observer.didPop(route2, route1);
+      verify(pageRouteAware2.didPop()).called(1);
+      verify(pageRouteAware1.didPopNext()).called(1);
+    });
+
+    test('does not call listeners for non-PageRoute', () {
+      final RouteObserver<PageRoute<dynamic>> observer = new RouteObserver<PageRoute<dynamic>>();
+      final RouteAware pageRouteAware = new MockRouteAware();
+      final MockPageRoute pageRoute = new MockPageRoute();
+      final MockRoute route = new MockRoute();
+      observer.subscribe(pageRouteAware, pageRoute);
+      verify(pageRouteAware.didPush());
+
+      observer.didPush(route, pageRoute);
+      observer.didPop(route, pageRoute);
+      verifyNoMoreInteractions(pageRouteAware);
+    });
+  });
 }
+
+class MockPageRoute extends Mock implements PageRoute<dynamic> { }
+
+class MockRoute extends Mock implements Route<dynamic> { }
+
+class MockRouteAware extends Mock implements RouteAware { }