Really basic Switch implementation in Sky. Could definitely use some polish

sky/sdk/lib/widgets/toggleable.dart

Index: sky/sdk/lib/widgets/toggleable.dart
diff --git a/sky/sdk/lib/widgets/toggleable.dart b/sky/sdk/lib/widgets/toggleable.dart
new file mode 100644
index 0000000000000000000000000000000000000000..621804c29d6cac778bad1cfb3f249e87536c11a1
--- /dev/null
+++ b/sky/sdk/lib/widgets/toggleable.dart
@@ -0,0 +1,74 @@
+// 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:sky' as sky;
+
+import 'animated_component.dart';
+import 'basic.dart';
+import '../framework/animation/animated_value.dart';
+import '../framework/animation/curves.dart';
+
+typedef void ValueChanged(value);
+
+const double _kCheckDuration = 200.0;
+
+abstract class Toggleable extends AnimatedComponent {
+
+  Toggleable({
+    Object key,
+    this.value,
+    this.onChanged
+  }) : super(key: key) {
+    toggleAnimation = new AnimatedValue(value ? 1.0 : 0.0);
+  }
+
+  bool value;
+  AnimatedValue toggleAnimation;
+  ValueChanged onChanged;
+
+  void syncFields(Toggleable source) {
+    onChanged = source.onChanged;
+    if (value != source.value) {
+      value = source.value;
+      double targetValue = value ? 1.0 : 0.0;
+      double difference = (toggleAnimation.value - targetValue).abs();
+      if (difference > 0) {
+        toggleAnimation.stop();
+        double t = difference * duration;
+        Curve curve = targetValue > toggleAnimation.value ? curveUp : curveDown;
+        toggleAnimation.animateTo(targetValue, t, curve: curve);
+      }
+    }
+    super.syncFields(source);
+  }
+
+  void _handleClick(sky.Event e) {
+    onChanged(!value);
+  }
+
+  // Override these methods to draw yourself
+  void customPaintCallback(sky.Canvas canvas, Size size) {
+    assert(false);
+  }
+  Size get size => const Size.zero;
+  EdgeDims get margin => const EdgeDims.symmetric(horizontal: 5.0);
+  double get duration => 200.0;
+  Curve get curveUp => easeIn;
+  Curve get curveDown => easeOut;
+
+  UINode build() {
+    return new EventListenerNode(
+      new Container(
+        margin: margin,
+        width: size.width,
+        height: size.height,
+        child: new CustomPaint(
+          token: toggleAnimation.value,
+          callback: customPaintCallback
+        )
+      ),
+      onGestureTap: _handleClick
+    );
+  }
+}