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

sky/sdk/lib/widgets/switch.dart

Index: sky/sdk/lib/widgets/switch.dart
diff --git a/sky/sdk/lib/widgets/switch.dart b/sky/sdk/lib/widgets/switch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..39b2979707a62518cea401c746feff85b98d8163
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+++ b/sky/sdk/lib/widgets/switch.dart
@@ -0,0 +1,69 @@
+// 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 'package:sky/theme2/colors.dart' as colors;
+import 'package:sky/theme2/shadows.dart';
+
+import '../painting/shadows.dart';
+import '../rendering/box.dart';
+import 'basic.dart';
+import 'toggleable.dart';
+
+// TODO(jackson): This should change colors with the theme
+sky.Color _kThumbOnColor = colors.Purple[500];
+const sky.Color _kThumbOffColor = const sky.Color(0xFFFAFAFA);
+sky.Color _kTrackOnColor = new sky.Color(_kThumbOnColor.value & (0x80 << 24));
+const sky.Color _kTrackOffColor = const sky.Color(0x42000000);
+const double _kSwitchWidth = 35.0;
+const double _kThumbRadius = 10.0;
+const double _kSwitchHeight = _kThumbRadius * 2.0;
+const double _kTrackHeight = 14.0;
+const double _kTrackRadius = _kTrackHeight / 2.0;
+const double _kTrackWidth = _kSwitchWidth - (_kThumbRadius - _kTrackRadius) * 2.0;
+
+class Switch extends Toggleable {
+  // TODO(jackson): Hit-test the switch so that it can respond to both taps and swipe gestures
+
+  Switch({
+    Object key,
+    bool value,
+    ValueChanged onChanged
+  }) : super(key: key, value: value, onChanged: onChanged);
+
+  Size get size => const Size(_kSwitchWidth + 2.0, _kSwitchHeight + 2.0);
+
+  void customPaintCallback(sky.Canvas canvas, Size size) {
+    sky.Color thumbColor = value ? _kThumbOnColor : _kThumbOffColor;
+    sky.Color trackColor = value ? _kTrackOnColor : _kTrackOffColor;
+
+    // Draw the track rrect
+    sky.Paint paint = new sky.Paint()..color = trackColor;
+    paint.setStyle(sky.PaintingStyle.fill);
+    sky.Rect rect = new sky.Rect.fromLTRB(
+      0.0,
+      _kSwitchHeight / 2.0 - _kTrackHeight / 2.0,
+      _kTrackWidth,
+      _kSwitchHeight / 2.0 + _kTrackHeight / 2.0
+    );
+    sky.RRect rrect = new sky.RRect()..setRectXY(rect, _kTrackRadius, _kTrackRadius);
+    canvas.drawRRect(rrect, paint);
+
+    // Draw the raised thumb with a shadow
+    paint.color = thumbColor;
+    var builder = new ShadowDrawLooperBuilder();
+    for (BoxShadow boxShadow in shadows[1])
+      builder.addShadow(boxShadow.offset, boxShadow.color, boxShadow.blur);
+    paint.setDrawLooper(builder.build());
+
+    // The thumb contracts slightly during the animation
+    double inset = 2.0 - (toggleAnimation.value - 0.5).abs() * 2.0;
+    Point thumbPos = new Point(
+      _kTrackRadius + toggleAnimation.value * (_kTrackWidth - _kTrackRadius * 2),
+      _kSwitchHeight / 2.0
+    );
+    canvas.drawCircle(thumbPos.x, thumbPos.y, _kThumbRadius - inset, paint);
+  }
+}