[Effen] Make the drawer not be included in the build output when the drawer is not shown.

sky/framework/components/drawer.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 'animated_component.dart';
 import '../animation/animated_value.dart';
 import '../animation/curves.dart';
 import '../fn.dart';
 import '../theme/colors.dart';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'material.dart';
 
 const double _kWidth = 304.0;
 const double _kMinFlingVelocity = 0.4;
 const double _kBaseSettleDurationMS = 246.0;
 const double _kMaxSettleDurationMS = 600.0;
 const Curve _kAnimationCurve = parabolicRise;
 
+typedef void DrawerStatusChangeHandler (bool showing);
+
 class DrawerController {
-  final AnimatedValue position = new AnimatedValue(-_kWidth);
+
+  DrawerController(this.onStatusChange) {
+    position = new AnimatedValue(-_kWidth, onChange: _checkValue);
+  }
+  final DrawerStatusChangeHandler onStatusChange;
+  AnimatedValue position;
+
+  bool _oldClosedState = true;
+  void _checkValue() {
+    var newClosedState = isClosed;
+    if (onStatusChange != null && _oldClosedState != newClosedState) {
+      onStatusChange(!newClosedState);
+      _oldClosedState = newClosedState;
+    }
+  }
 
   bool get isClosed => position.value == -_kWidth;
-
   bool get _isMostlyClosed => position.value <= -_kWidth / 2;
-
   void toggle(_) => _isMostlyClosed ? _open() : _close();
 
   void handleMaskTap(_) => _close();
-
   void handlePointerDown(_) => position.stop();
 
   void handlePointerMove(sky.PointerEvent event) {
     if (position.isAnimating)
       return;
     position.value = math.min(0.0, math.max(position.value + event.dx, -_kWidth));
   }
 
   void handlePointerUp(_) {
     if (!position.isAnimating)
@@ skipping 23 matching lines @@
   void handleFlingStart(event) {
     double direction = event.velocityX.sign;
     double velocityX = event.velocityX.abs() / 1000;
     if (velocityX < _kMinFlingVelocity)
       return;
 
     double targetPosition = direction < 0.0 ? -_kWidth : 0.0;
     double distance = (targetPosition - position.value).abs();
     double duration = distance / velocityX;
 
-    position.animateTo(targetPosition, duration, curve: linear);
+    if (distance > 0)
+      position.animateTo(targetPosition, duration, curve: linear);
   }
 }
 
 class Drawer extends AnimatedComponent {
   // TODO(abarth): We need a better way to become a container for absolutely
   // positioned elements.
   static final Style _style = new Style('''
     transform: translateX(0);''');
 
   static final Style _maskStyle = new Style('''
@@ skipping 25 matching lines @@
   Drawer({
     Object key,
     this.controller,
     this.children,
     this.level: 0
   }) : super(key: key) {
     animateField(controller.position, #_position);
   }
 
   UINode build() {
-    bool isClosed = _position <= -_kWidth;
-    String inlineStyle = 'display: ${isClosed ? 'none' : ''}';
     String maskInlineStyle = 'opacity: ${(_position / _kWidth + 1) * 0.5}';
     String contentInlineStyle = 'transform: translateX(${_position}px)';
 
     var mask = new EventListenerNode(
       new Container(
         style: _maskStyle,
         inlineStyle: maskInlineStyle
       ),
       onGestureTap: controller.handleMaskTap,
       onGestureFlingStart: controller.handleFlingStart
     );
 
     Material content = new Material(
       content: new Container(
         style: _contentStyle,
         inlineStyle: contentInlineStyle,
         children: children
       ),
       level: level);
 
     return new EventListenerNode(
       new Container(
         style: _style,
-        inlineStyle: inlineStyle,
         children: [ mask, content ]
       ),
       onPointerDown: controller.handlePointerDown,
       onPointerMove: controller.handlePointerMove,
       onPointerUp: controller.handlePointerUp,
       onPointerCancel: controller.handlePointerCancel
     );
   }
 }