Polish the menu animation

sky/sdk/lib/widgets/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 'dart:math' as math;
 import 'dart:sky' as sky;
 
 import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/painting/box_painter.dart';
 import 'package:sky/theme/colors.dart';
 import 'package:sky/theme/shadows.dart';
 import 'package:sky/widgets/animated_component.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/popup_menu_item.dart';
 import 'package:sky/widgets/scrollable_viewport.dart';
 
-const Duration _kMenuOpenDuration = const Duration(milliseconds: 300);
-const Duration _kMenuCloseDuration = const Duration(milliseconds: 200);
-const Duration _kMenuCloseDelay = const Duration(milliseconds: 100);
+const Duration _kMenuDuration = const Duration(milliseconds: 300);
+double _kMenuCloseIntervalEnd = 2.0 / 3.0;
 const double _kMenuWidthStep = 56.0;
 const double _kMenuMargin = 16.0; // 24.0 on tablet
 const double _kMenuMinWidth = 2.0 * _kMenuWidthStep;
 const double _kMenuMaxWidth = 5.0 * _kMenuWidthStep;
 const double _kMenuHorizontalPadding = 16.0;
 const double _kMenuVerticalPadding = 8.0;
 
 enum PopupMenuStatus {
   active,
   inactive,
@@ skipping 13 matching lines @@
 
   bool showing;
   PopupMenuStatusChangedCallback onStatusChanged;
   List<PopupMenuItem> items;
   int level;
 
   AnimatedType<double> _opacity;
   AnimatedType<double> _width;
   AnimatedType<double> _height;
   List<AnimatedType<double>> _itemOpacities;
+  AnimatedList _animationList;
   AnimationPerformance _performance;
 
   void initState() {
     _performance = new AnimationPerformance()
+      ..duration = _kMenuDuration
       ..addListener(_checkForStateChanged);
     _updateAnimationVariables();
     watch(_performance);
     _updateBoxPainter();
     if (showing)
       _open();
   }
 
   void syncFields(PopupMenu source) {
     if (showing != source.showing) {
       showing = source.showing;
       if (showing)
         _open();
       else
         _close();
     }
     onStatusChanged = source.onStatusChanged;
     if (level != source.level) {
       level = source.level;
       _updateBoxPainter();
     }
     if (items.length != source.items.length)
       _updateAnimationVariables();
     items = source.items;
     super.syncFields(source);
   }
 
   void _updateAnimationVariables() {
-    double unit = 1.0 / (items.length + 1);
+    double unit = 1.0 / (items.length + 1.5); // 1.0 for the width and 0.5 for the last item's fade.
     _opacity = new AnimatedType<double>(0.0, end: 1.0);
     _width = new AnimatedType<double>(0.0, end: 1.0, interval: new Interval(0.0, unit));
-    _height = new AnimatedType<double>(0.0, end: 1.0, interval: new Interval(0.0, 0.5));
+    _height = new AnimatedType<double>(0.0, end: 1.0, interval: new Interval(0.0, unit * items.length));
     _itemOpacities = new List<AnimatedType<double>>();
     for (int i = 0; i < items.length; ++i) {
       double start = (i + 1) * unit;
       double end = (start + 1.5 * unit).clamp(0.0, 1.0);
       _itemOpacities.add(new AnimatedType<double>(
           0.0, end: 1.0, interval: new Interval(start, end)));
     }
     List<AnimatedVariable> variables = new List<AnimatedVariable>()
       ..add(_opacity)
       ..add(_width)
       ..add(_height)
       ..addAll(_itemOpacities);
-    _performance.variable = new AnimatedList(variables);
+    _animationList = new AnimatedList(variables);
+    _performance.variable = _animationList;
   }
 
   void _updateBoxPainter() {
     _painter = new BoxPainter(new BoxDecoration(
       backgroundColor: Grey[50],
       borderRadius: 2.0,
       boxShadow: shadows[level]));
   }
 
   PopupMenuStatus get _status => _opacity.value != 0.0 ? PopupMenuStatus.active : PopupMenuStatus.inactive;
 
   PopupMenuStatus _lastStatus;
   void _checkForStateChanged() {
     PopupMenuStatus status = _status;
     if (_lastStatus != null && status != _lastStatus && onStatusChanged != null)
       onStatusChanged(status);
     _lastStatus = status;
   }
 
   void _open() {
-    _performance
-      ..duration = _kMenuOpenDuration
-      ..play();
+    _animationList.interval = null;
+    _performance.play();
   }
 
   void _close() {
-    _performance
-      ..duration = _kMenuCloseDuration
-      ..reverse();
+    _animationList.interval = new Interval(0.0, _kMenuCloseIntervalEnd);
+    _performance.reverse();
   }
 
   BoxPainter _painter;
 
   Widget build() {
     int i = 0;
     List<Widget> children = new List.from(items.map((Widget item) {
       return new Opacity(opacity: _itemOpacities[i++].value, child: item);
     }));
 
@@ skipping 24 matching lines @@
                 )
               )
             )
           )
         )
       )
     );
   }
 
 }