Add TabNavigator

sky/sdk/lib/widgets/tabs.dart

Index: sky/sdk/lib/widgets/tabs.dart
diff --git a/sky/sdk/lib/widgets/tabs.dart b/sky/sdk/lib/widgets/tabs.dart
index 37f19cbf5b8dc0040b31dc4195e4ff7534c4f5cf..13cfaa35ed2abd3fd01020e32bc72940e2922946 100644
--- a/sky/sdk/lib/widgets/tabs.dart
+++ b/sky/sdk/lib/widgets/tabs.dart
@@ -261,7 +261,7 @@ class TabBar extends Component {
     this.onChanged
   }) : super(key: key);
 
-  final List<TabLabel> labels;
+  final Iterable<TabLabel> labels;
   final int selectedIndex;
   final SelectedIndexChanged onChanged;
 
@@ -286,9 +286,10 @@ class TabBar extends Component {
     assert(labels != null && labels.isNotEmpty);
     List<Widget> tabs = <Widget>[];
     bool textAndIcons = false;
-    for (int tabIndex = 0; tabIndex < labels.length; tabIndex++) {
-      tabs.add(_toTab(labels[tabIndex], tabIndex));
-      if (labels[tabIndex].text != null && labels[tabIndex].icon != null)
+    int tabIndex = 0;
+    for (TabLabel label in labels) {
+      tabs.add(_toTab(label, tabIndex++));
+      if (label.text != null && label.icon != null)
         textAndIcons = true;
     }
     return new TabBarWrapper(
@@ -300,3 +301,51 @@ class TabBar extends Component {
     );
   }
 }
+
+class TabNavigatorView {
+  TabNavigatorView({ this.label, this.builder });
+
+  final TabLabel label;
+  final Builder builder;
+
+  Widget buildContent() {
+    assert(builder != null);
+    Widget content = builder();
+    assert(content != null);
+    return content;
+  }
+}
+
+class TabNavigator extends Component {
+  TabNavigator({
+    String key,
+    this.views,
+    this.selectedIndex: 0,
+    this.onChanged
+  }) : super(key: key);
+
+  final List<TabNavigatorView> views;
+  final int selectedIndex;
+  final SelectedIndexChanged onChanged;
+
+  void _handleSelectedIndexChanged(int tabIndex) {
+    if (onChanged != null)
+      onChanged(tabIndex);
+  }
+
+  Widget build() {
+    assert(views != null && views.isNotEmpty);
+    assert(selectedIndex >= 0 && selectedIndex < views.length);
+
+    TabBar tabBar = new TabBar(
+      labels: views.map((view) => view.label),
+      onChanged: _handleSelectedIndexChanged,
+      selectedIndex: selectedIndex
+    );
+
+    Widget content = views[selectedIndex].buildContent();
+    return new Flex([tabBar, new Flexible(child: content)],
+      direction: FlexDirection.vertical
+    );
+  }
+}