Add liftToOverlay to Mimicable

This patch integrates Mimicable with Overlay such that you can tell a Mimicable
to lift up into the overlay and animate towards another widget identified by a
global key.

examples/widgets/mimic_demo.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:flutter/animation.dart';
+import 'package:flutter/material.dart';
+
+const double _kHeight = 150.0;
+const Duration _kEffectDuration = const Duration(seconds: 1);
+
+class MimicDemo extends StatefulComponent {
+  _MimicDemoState createState() => new _MimicDemoState();
+}
+
+class _MimicDemoState extends State<MimicDemo> {
+  GlobalKey<MimicableState> _orange = new GlobalKey<MimicableState>();
+  GlobalKey _targetContainer = new GlobalKey();
+
+  bool _slotForOrangeOnTop = false;
+  bool _orangeOnTop = false;
+
+  void _handleTap() {
+    if (_slotForOrangeOnTop)
+      return;
+    setState(() {
+      _slotForOrangeOnTop = true;
+    });
+    MimicOverlayEntry entry = _orange.currentState.liftToOverlay();
+    entry.animateTo(targetKey: _targetContainer, duration: _kEffectDuration, curve: Curves.ease).then((_) {
+      setState(() {
+        _orangeOnTop = true;
+      });
+      entry.dispose();
+    });
+  }
+
+  void _reset() {
+    setState(() {
+      _slotForOrangeOnTop = false;
+      _orangeOnTop = false;
+    });
+  }
+
+  Widget _buildOrange() {
+    return new Mimicable(
+      key: _orange,
+      child: new Container(
+        height: _kHeight,
+        decoration: new BoxDecoration(
+          backgroundColor: Colors.deepOrange[500]
+        )
+      )
+    );
+  }
+
+  Widget build(BuildContext context) {
+    List<Widget> children = <Widget>[
+      new Container(
+        height: _kHeight,
+        decoration: new BoxDecoration(
+          backgroundColor: Colors.amber[500]
+        )
+      ),
+      new AnimatedContainer(
+        key: _targetContainer,
+        height: _slotForOrangeOnTop ? _kHeight : 0.0,
+        duration: _kEffectDuration,
+        curve: Curves.ease,
+        child: _orangeOnTop ? _buildOrange() : null
+      ),
+      new Container(
+        height: _kHeight,
+        decoration: new BoxDecoration(
+          backgroundColor: Colors.green[500]
+        )
+      ),
+      new Container(
+        height: _kHeight,
+        decoration: new BoxDecoration(
+          backgroundColor: Colors.blue[500]
+        )
+      ),
+    ];
+
+    if (!_orangeOnTop)
+      children.add(_buildOrange());
+
+    return new GestureDetector(
+      onTap: _handleTap,
+      onLongPress: _reset,
+      child: new Block(children)
+    );
+  }
+}
+
+void main() {
+  runApp(new MaterialApp(
+    title: 'Mimic Demo',
+    routes: {
+      '/': (_) => new MimicDemo()
+    }
+  ));
+}

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

@@ -136,6 +136,8 @@ class _AnimatedContainerState extends State<AnimatedContainer> {
   }
 
   bool _configVariable(AnimatedValue variable, dynamic targetValue) {
+    if (targetValue == variable.end)
+      return false;
     dynamic currentValue = variable.value;
     variable.end = targetValue;
     variable.begin = currentValue;

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

@@ -2,10 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+import 'dart:ui' as ui;
+
+import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
 import 'framework.dart';
+import 'overlay.dart';
 
 class MimicableKey {
   MimicableKey._(this._state);
@@ -19,6 +24,84 @@ class MimicableKey {
   }
 }
 
+class MimicOverlayEntry {
+  MimicOverlayEntry._(this._key) {
+    _overlayEntry = new OverlayEntry(builder: _build);
+    _initialGlobalBounds = _key.globalBounds;
+  }
+
+  Rect _initialGlobalBounds;
+
+  MimicableKey _key;
+  OverlayEntry _overlayEntry;
+
+  // Animation state
+  GlobalKey _targetKey;
+  Curve _curve;
+  Performance _performance;
+
+  Future animateTo({
+    GlobalKey targetKey,
+    Duration duration,
+    Curve curve: Curves.linear
+  }) {
+    assert(_key != null);
+    assert(_overlayEntry != null);
+    assert(targetKey != null);
+    assert(duration != null);
+    assert(curve != null);
+    _targetKey = targetKey;
+    _curve = curve;
+    // TODO(abarth): Support changing the animation target when in flight.
+    assert(_performance == null);
+    _performance = new Performance(duration: duration)
+      ..addListener(_overlayEntry.markNeedsBuild);
+    return _performance.play();
+  }
+
+  void dispose() {
+    _targetKey = null;
+    _curve = null;
+    _performance?.stop();
+    _performance = null;
+    _key.stopMimic();
+    _key = null;
+    _overlayEntry.remove();
+    _overlayEntry = null;
+  }
+
+  Widget _build(BuildContext context) {
+    assert(_key != null);
+    assert(_overlayEntry != null);
+    Rect globalBounds = _initialGlobalBounds;
+    Point globalPosition = globalBounds.topLeft;
+    if (_targetKey != null) {
+      assert(_performance != null);
+      assert(_curve != null);
+      RenderBox box = _targetKey.currentContext?.findRenderObject();
+      if (box != null) {
+        // TODO(abarth): Handle the case where the transform here isn't just a translation.
+        Point localPosition = box.localToGlobal(Point.origin);
+        double t = _curve.transform(_performance.progress);
+        // TODO(abarth): Add Point.lerp.
+        globalPosition = new Point(ui.lerpDouble(globalPosition.x, localPosition.x, t),
+                                 ui.lerpDouble(globalPosition.y, localPosition.y, t));
+      }
+    }
+
+    RenderBox stack = context.ancestorRenderObjectOfType(RenderStack);
+    // TODO(abarth): Handle the case where the transform here isn't just a translation.
+    Point localPosition = stack == null ? globalPosition: stack.globalToLocal(globalPosition);
+    return new Positioned(
+      left: localPosition.x,
+      top: localPosition.y,
+      width: globalBounds.width,
+      height: globalBounds.height,
+      child: new Mimic(original: _key)
+    );
+  }
+}
+
 class Mimic extends StatelessComponent {
   Mimic({ Key key, this.original }) : super(key: key);
 
@@ -52,6 +135,14 @@ class MimicableState extends State<Mimicable> {
     return new MimicableKey._(this);
   }
 
+  MimicOverlayEntry liftToOverlay() {
+    OverlayState overlay = Overlay.of(context);
+    assert(overlay != null); // You need an overlay to lift into.
+    MimicOverlayEntry entry = new MimicOverlayEntry._(startMimic());
+    overlay.insert(entry._overlayEntry);
+    return entry;
+  }
+
   void _stopMimic() {
     assert(_beingMimicked);
     if (!mounted) {