Simple support for routing in Sky

R=abarth@chromium.org, abarth, hixie

Review URL: https://codereview.chromium.org/1186273002.

examples/raw/navigation.dart

+// Copyright 2015 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:sky/widgets/basic.dart';
+import 'package:sky/widgets/navigator.dart';
+import 'package:sky/widgets/raised_button.dart';
+
+List<Route> routes = [
+  new Route(
+    name: 'safety',
+    builder: (navigator) => new RaisedButton(
+      child: new Text('PRESS FORWARD'),
+      onPressed: () => navigator.pushNamedRoute('adventure')
+    )
+  ),
+  new Route(
+    name: 'adventure',
+    builder: (navigator) => new RaisedButton(
+      child: new Text('NO WAIT! GO BACK!'),
+      onPressed: () => navigator.pushRoute(routes[0])
+    )
+  )
+];
+
+class NavigationExampleApp extends App {
+  UINode build() {
+    return new Navigator(routes: routes);
+  }
+}
+
+void main() {
+  App app = new NavigationExampleApp();
+}

sdk/BUILD.gn

@@ -114,6 +114,7 @@ dart_pkg("sdk") {
     "lib/widgets/menu_divider.dart",
     "lib/widgets/menu_item.dart",
     "lib/widgets/modal_overlay.dart",
+    "lib/widgets/navigator.dart",
     "lib/widgets/popup_menu.dart",
     "lib/widgets/popup_menu_item.dart",
     "lib/widgets/radio.dart",

sdk/lib/widgets/navigator.dart

+// Copyright 2015 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 'basic.dart';
+
+typedef UINode Builder(Navigator navigator);
+
+abstract class RouteBase {
+  RouteBase({this.name});
+  final String name;
+  UINode build(Navigator navigator);
+}
+
+class Route extends RouteBase {
+  Route({String name, this.builder}) : super(name: name);
+  final Builder builder;
+  UINode build(Navigator navigator) => builder(navigator);
+}
+
+class Navigator extends Component {
+  Navigator({Object key, this.currentRoute, this.routes})
+    : super(key: key, stateful: true);
+
+  RouteBase currentRoute;
+  List<RouteBase> routes;
+ 
+  void syncFields(Navigator source) {
+    currentRoute = source.currentRoute;
+    routes = source.routes;
+  }
+
+  void pushNamedRoute(String name) {
+    assert(routes != null);
+    for (RouteBase route in routes) {
+      if (route.name == name) {
+        setState(() {
+          currentRoute = route;
+        });
+        return;
+      }
+    }
+    assert(false);  // route not found
+  }
+
+  void pushRoute(RouteBase route) {
+    setState(() {
+      currentRoute = route;
+    });
+  }
+
+  UINode build() {
+    Route route = currentRoute == null ? routes[0] : currentRoute;
+    assert(route != null);
+    return route.build(this);
+  }
+}
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