Add widgets for reparenting widgets

Wrap widgets you want to reparent in a Mimicable widget and assign the
Mimicable widget a global key. Then, given the same global key to a Mimic
widget to make it appear elsewhere in the view hierarchy.

packages/flutter/example/widgets/pop_out_and_expand.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/painting/box_painter.dart';
+import 'package:sky/widgets.dart';
+
+class Circle extends Component {
+  Circle({ this.child });
+
+  Widget child;
+
+  Widget build() {
+    return new Container(
+      height: 100.0,
+      margin: new EdgeDims.symmetric(horizontal: 20.0, vertical: 4.0),
+      decoration: new BoxDecoration(
+        backgroundColor: const Color(0xFF0000FF)
+      ),
+      child: new Center(
+        child: child
+      )
+    );
+  }
+}
+
+class CircleData {
+  final GlobalKey key;
+  final String content;
+
+  CircleData({ this.key, this.content });
+}
+
+class ExampleApp extends App {
+  ExampleApp() {
+    for (int i = 0; i < 20; ++i) {
+      _data.add(new CircleData(
+        key: new GlobalKey(),
+        content: '$i'
+      ));
+    }
+  }
+
+  final List<CircleData> _data = new List<CircleData>();
+
+  GlobalKey _keyToMimic;
+
+  Widget _buildCircle(CircleData circleData) {
+    return new Mimicable(
+      key: circleData.key,
+      child: new Listener(
+        child: new Circle(
+          child: new Text(circleData.content)
+        ),
+        onGestureTap: (_) {
+          setState(() {
+            _keyToMimic = circleData.key;
+          });
+        }
+      )
+    );
+  }
+
+  Widget build() {
+    List<Widget> circles = new List<Widget>();
+    for (int i = 0; i < 20; ++i) {
+      circles.add(_buildCircle(_data[i]));
+    }
+
+    List<Widget> layers = new List<Widget>();
+    layers.add(new ScrollableBlock(circles));
+
+    if (_keyToMimic != null) {
+      layers.add(
+        new Positioned(
+          top: 50.0,
+          left: 50.0,
+          child: new Mimic(
+            original: _keyToMimic)
+        )
+      );
+    }
+
+    return new Stack(layers);
+  }
+}
+
+void main() {
+  runApp(new ExampleApp());
+}

packages/flutter/lib/widgets.dart

@@ -21,11 +21,14 @@ export 'widgets/drawer_item.dart';
 export 'widgets/flat_button.dart';
 export 'widgets/floating_action_button.dart';
 export 'widgets/focus.dart';
-export 'widgets/icon_button.dart';
+export 'widgets/framework.dart';
 export 'widgets/icon.dart';
+export 'widgets/icon_button.dart';
 export 'widgets/ink_well.dart';
-export 'widgets/material_button.dart';
 export 'widgets/material.dart';
+export 'widgets/material_button.dart';
+export 'widgets/mimic.dart';
+export 'widgets/mimic.dart';
 export 'widgets/modal_overlay.dart';
 export 'widgets/navigator.dart';
 export 'widgets/popup_menu.dart';
@@ -41,4 +44,3 @@ export 'widgets/task_description.dart';
 export 'widgets/theme.dart';
 export 'widgets/tool_bar.dart';
 export 'widgets/transitions.dart';
-export 'widgets/framework.dart';

packages/flutter/lib/widgets/framework.dart

@@ -110,6 +110,11 @@ abstract class GlobalKey extends Key {
     assert(removed);
   }
 
+  static Widget getWidget(GlobalKey key) {
+    assert(key != null);
+    return _registry[key];
+  }
+
   static void _notifyListeners() {
     assert(!_inRenderDirtyComponents);
     assert(!Widget._notifyingMountStatus);

packages/flutter/lib/widgets/mimic.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';
+
+class Mimic extends StatefulComponent {
+  Mimic({ Key key, this.original }) : super(key: key);
+
+  GlobalKey original;
+
+  void initState() {
+    _requestToStartMimic();
+  }
+
+  void syncFields(Mimic source) {
+    if (original != source.original) {
+      _stopMimic();
+      original = source.original;
+      _requestToStartMimic();
+    }
+  }
+
+  void didMount() {
+    super.didMount();
+    // TODO(abarth): Why is didMount being called without a call to didUnmount?
+    if (_mimicable == null)
+      _requestToStartMimic();
+  }
+
+  void didUnmount() {
+    super.didUnmount();
+    _stopMimic();
+  }
+
+  Mimicable _mimicable;
+  Size _size;
+
+  void _requestToStartMimic() {
+    assert(_mimicable == null);
+    assert(_size == null);
+    if (original == null)
+      return;
+    _mimicable = GlobalKey.getWidget(original) as Mimicable;
+    assert(_mimicable != null);
+    _mimicable._requestToStartMimic(this);
+  }
+
+  void _startMimic(GlobalKey key, Size size) {
+    assert(key == original);
+    setState(() {
+      _size = size;
+    });
+  }
+
+  void _stopMimic() {
+    if (_mimicable != null)
+      _mimicable._didStopMimic(this);
+    _mimicable = null;
+    _size = null;
+  }
+
+  Widget build() {
+    if (_size == null || !_mimicable.mounted)
+      return new Container();
+    return new ConstrainedBox(
+      constraints: new BoxConstraints.tight(_size),
+      child: _mimicable.child
+    );
+  }
+}
+
+class Mimicable extends StatefulComponent {
+  Mimicable({ GlobalKey key, this.child }) : super(key: key);
+
+  Widget child;
+
+  Size _size;
+  Size get size => _size;
+
+  Mimic _mimic;
+  bool _didStartMimic = false;
+
+  void syncFields(Mimicable source) {
+    child = source.child;
+  }
+
+  void _requestToStartMimic(Mimic mimic) {
+    assert(mounted);
+    if (_mimic != null)
+      return;
+    setState(() {
+      _mimic = mimic;
+      _didStartMimic = false;
+    });
+  }
+
+  void _didStopMimic(Mimic mimic) {
+    assert(_mimic != null);
+    assert(mimic == _mimic);
+    setState(() {
+      _mimic = null;
+      _didStartMimic = false;
+    });
+  }
+
+  void _handleSizeChanged(Size size) {
+    setState(() {
+      _size = size;
+    });
+  }
+
+  void _startMimicIfNeeded() {
+    if (_didStartMimic)
+      return;
+    assert(_mimic != null);
+    _mimic._startMimic(key, _size);
+    _didStartMimic = true;
+  }
+
+  Widget build() {
+    if (_mimic != null) {
+      _startMimicIfNeeded();
+      return new ConstrainedBox(constraints: new BoxConstraints.tight(_size));
+    }
+    return new SizeObserver(
+      callback: _handleSizeChanged,
+      child: child
+    );
+  }
+}