AnimatedContainer: generalized Container widget that handles animating values

sky/sdk/lib/animation/animation_performance.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_value.dart';
 import 'curves.dart';
 
 // TODO(mpcomplete): merge this stuff with AnimatedValue somehow. We shouldn't
 // have 2 different ways to animate values.
 abstract class AnimatedVariable {
   void setFraction(double t);
 }
 
 class AnimatedType<T extends dynamic> extends AnimatedVariable {
   T value;
-  final T begin, end;
+  T begin, end;
   final Curve curve;
 
-  AnimatedType(this.begin, this.end, {this.curve: linear}) {
+  AnimatedType(this.begin, {this.end, this.curve: linear}) {
     value = begin;
   }
 
   void setFraction(double t) {
-    // TODO(mpcomplete): Reverse the timeline and curve.
-    value = begin + (end - begin) * curve.transform(t);
+    if (end != null) {
+      // TODO(mpcomplete): Reverse the timeline and curve.
+      value = begin + (end - begin) * curve.transform(t);
+    }
   }
 }
 
 // This class manages a "performance" - a collection of values that change
 // based on a timeline. For example, a performance may handle an animation
 // of a menu opening by sliding and fading in (changing Y value and opacity)
 // over .5 seconds. The performance can move forwards (present) or backwards
 // (dismiss). A consumer may also take direct control of the timeline by
 // manipulating |progress|, or |fling| the timeline causing a physics-based
 // simulation to take over the progression.
@@ skipping 44 matching lines @@
   // time.
   void fling({double velocity: 1.0}) {
     double target = velocity.sign < 0.0 ? 0.0 : 1.0;
     double distance = (target - timeline.value).abs();
     double duration = distance / velocity.abs();
 
     if (distance > 0.0)
       timeline.animateTo(target, duration, curve: linear);
   }
 }