Improve the openning animation for PopupMenu

sky/framework/components/popup_menu.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 '../animation/animated_value.dart';
 import '../fn.dart';
 import '../theme/colors.dart';
+import '../theme/view-configuration.dart';
+import 'dart:math' as math;
+import 'dart:sky' as sky;
 import 'material.dart';
 import 'popup_menu_item.dart';
 
-const double _kItemInitialOpacity = 0.0;
-const double _kItemFinalOpacity = 1.0;
-const double _kItemFadeDuration = 500.0;
-const double _kItemFadeDelay = 200.0;
+const double _kItemFadeDuration = 300.0;
+const double _kItemFadeDelay = 100.0;
+const double _kMenuExpandDuration = 300.0;
+
+class PopupMenuController {
+  bool isOpen = false;
+  AnimatedValue position = new AnimatedValue(0.0);
+
+  void open() {
+    isOpen = true;
+    position.animateTo(1.0, _kMenuExpandDuration);
+  }
+
+  void close() {
+    position.animateTo(0.0, _kMenuExpandDuration);
+    // TODO(abarth): We shouldn't mark the menu as closed until the animation
+    // completes.
+    isOpen = false;
+  }
+}
 
 class PopupMenu extends Component {
   static final Style _style = new Style('''
     border-radius: 2px;
     padding: 8px 0;
+    box-sizing: border-box;
     background-color: ${Grey[50]};'''
   );
 
   List<List<Node>> items;
   int level;
+  PopupMenuController controller;
+
+  AnimatedValueListener _position;
   List<AnimatedValue> _opacities;
+  int _width;
+  int _height;
 
-  PopupMenu({ Object key, this.items, this.level }) : super(key: key) {
-    _opacities = new List.from(items.map(
-        (item) => new AnimatedValue(_kItemInitialOpacity)));
+  PopupMenu({ Object key, this.controller, this.items, this.level })
+      : super(key: key) {
+    _position = new AnimatedValueListener(this, controller.position);
   }
 
-  // TODO(abarth): Rather than using didMount, we should have the parent
-  // component kick off these animations.
-  void didMount() {
+  void _ensureItemAnimations() {
+    if (_opacities != null && controller.isOpen)
+      return;
+    _opacities = new List.from(items.map((_) => new AnimatedValue(0.0)));
     int i = 0;
     _opacities.forEach((opacity) {
-      opacity.animateTo(_kItemFinalOpacity, _kItemFadeDuration,
-                        initialDelay: _kItemFadeDelay * i++);
+      opacity.animateTo(1.0, _kItemFadeDuration,
+                        initialDelay: _kItemFadeDelay * ++i);
     });
   }
 
+  String _inlineStyle() {
+    double value = _position.value;
+    if (value == null || value == 1.0 || _height == null || _width == null)
+      return null;
+    return '''
+      opacity: ${math.min(1.0, value * 1.5)};
+      width: ${math.min(_width, _width * value * 3.0)}px;
+      height: ${_height * value}px;''';
+  }
+
+  void didMount() {
+    setState(() {
+      var root = getRoot();
+      _width = root.clientWidth;
+      _height = root.clientHeight;
+    });
+  }
+
+  void didUnmount() {
+    _position.stopListening();
+  }
+
   Node build() {
+    _position.ensureListening();
+    _ensureItemAnimations();
+
     List<Node> children = [];
-    int i = 0;
-    items.forEach((List<Node> item) {
-      children.add(
-          new PopupMenuItem(key: i, children: item, opacity: _opacities[i]));
-      ++i;
-    });
+
+    if (controller.isOpen) {
+      int i = 0;
+      items.forEach((List<Node> item) {
+        children.add(
+            new PopupMenuItem(key: i, children: item, opacity: _opacities[i]));
+        ++i;
+      });
+    }
 
     return new Material(
       style: _style,
+      inlineStyle: _inlineStyle(),
       children: children,
       level: level
     );
   }
 }