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

sky/sdk/lib/widgets/checkbox.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:sky' as sky;
 
 import 'package:sky/theme2/colors.dart' as colors;
+import 'package:sky/theme2/shadows.dart';
 
 import '../framework/animation/animated_value.dart';
 import '../framework/animation/curves.dart';
+import '../painting/shadows.dart';
 import '../rendering/box.dart';
 import 'animated_component.dart';
 import 'basic.dart';
 
 typedef void ValueChanged(value);
 
 const double _kMidpoint = 0.5;
 const double _kCheckDuration = 200.0;
 const sky.Color _kUncheckedColor = const sky.Color(0x8A000000);
 // TODO(jackson): This should change colors with the theme
@@ skipping 33 matching lines @@
         _checkedAnimation.animateTo(targetValue, duration, curve: curve);
       }
     }
     super.syncFields(source);
   }
 
   void _handleClick(sky.Event e) {
     onChanged(!checked);
   }
 
+  void _customPaintCallback(sky.Canvas canvas, Size size) {
+    // Choose a color between grey and the theme color
+    sky.Paint paint = new sky.Paint()..strokeWidth = 2.0
+                                     ..color = _kUncheckedColor;
+
+    // The rrect contracts slightly during the animation
+    double inset = 2.0 - (_checkedAnimation.value - _kMidpoint).abs() * 2.0;
+    sky.Rect rect = new sky.Rect.fromLTRB(inset, inset, _kEdgeSize - inset, _kEdgeSize - inset);
+    sky.RRect rrect = new sky.RRect()..setRectXY(rect, _kEdgeRadius, _kEdgeRadius);
+
+    // Outline of the empty rrect
+    paint.setStyle(sky.PaintingStyle.stroke);
+    canvas.drawRRect(rrect, paint);
+
+    // Radial gradient that changes size
+    if (_checkedAnimation.value > 0) {
+      paint.setStyle(sky.PaintingStyle.fill);
+      paint.setShader(
+        new sky.Gradient.radial(
+          new Point(_kEdgeSize / 2.0, _kEdgeSize / 2.0),
+          _kEdgeSize * (_kMidpoint - _checkedAnimation.value) * 8.0,
+          [const sky.Color(0x00000000), _kUncheckedColor],
+          [0.0, 1.0]

[Matt Perry, 2015/06/15 18:22:10]

FYI, [0, 1] is the default, so you can just leave this param off.

+        )
+      );
+      canvas.drawRRect(rrect, paint);
+    }
+
+    if (_checkedAnimation.value > _kMidpoint) {
+      double t = (_checkedAnimation.value - _kMidpoint) / (1.0 - _kMidpoint);
+
+      // Solid filled rrect
+      paint.setStyle(sky.PaintingStyle.strokeAndFill);
+      paint.color = new Color.fromARGB((t * 255).floor(),
+                                       _kCheckedColor.red,
+                                       _kCheckedColor.green,
+                                       _kCheckedColor.blue);
+      canvas.drawRRect(rrect, paint);
+
+      // White inner check
+      paint.color = const sky.Color(0xFFFFFFFF);
+      paint.setStyle(sky.PaintingStyle.stroke);
+      sky.Path path = new sky.Path();
+      sky.Point start = new sky.Point(_kEdgeSize * 0.2, _kEdgeSize * 0.5);
+      sky.Point mid = new sky.Point(_kEdgeSize * 0.4, _kEdgeSize * 0.7);
+      sky.Point end = new sky.Point(_kEdgeSize * 0.8, _kEdgeSize * 0.3);
+      Point lerp(Point p1, Point p2, double t)
+        => new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
+      sky.Point drawStart = lerp(start, mid, 1.0 - t);
+      sky.Point drawEnd = lerp(mid, end, t);
+      path.moveTo(drawStart.x, drawStart.y);
+      path.lineTo(mid.x, mid.y);
+      path.lineTo(drawEnd.x, drawEnd.y);
+      canvas.drawPath(path, paint);
+    }
+  }
+
+  double get _width => _kEdgeSize + 2.0;
+  double get _height => _kEdgeSize + 2.0;
+  EdgeDims get _margin => const EdgeDims.symmetric(horizontal: 5.0);
+
   UINode build() {
     return new EventListenerNode(
       new Container(
         margin: const EdgeDims.symmetric(horizontal: 5.0),
-        width: _kEdgeSize + 2.0,
-        height: _kEdgeSize + 2.0,
+        width: _width,
+        height: _height,
         child: new CustomPaint(
           token: _checkedAnimation.value,
-          callback: (sky.Canvas canvas, Size size) {
-            // Choose a color between grey and the theme color
-            sky.Paint paint = new sky.Paint()..strokeWidth = 2.0
-                                             ..color = _kUncheckedColor;
-
-            // The rrect contracts slightly during the animation
-            double inset = 2.0 - (_checkedAnimation.value - _kMidpoint).abs() * 2.0;
-            sky.Rect rect = new sky.Rect.fromLTRB(inset, inset, _kEdgeSize - inset, _kEdgeSize - inset);
-            sky.RRect rrect = new sky.RRect()..setRectXY(rect, _kEdgeRadius, _kEdgeRadius);
-
-            
-            // Outline of the empty rrect
-            paint.setStyle(sky.PaintingStyle.stroke);
-            canvas.drawRRect(rrect, paint);
-
-            // Radial gradient that changes size
-            if (_checkedAnimation.value > 0) {
-              paint.setStyle(sky.PaintingStyle.fill);
-              paint.setShader(
-                new sky.Gradient.radial(
-                  new Point(_kEdgeSize / 2.0, _kEdgeSize / 2.0),
-                  _kEdgeSize * (_kMidpoint - _checkedAnimation.value) * 8.0,
-                  [const sky.Color(0x00000000), _kUncheckedColor],
-                  [0.0, 1.0]
-                )
-              );
-              canvas.drawRRect(rrect, paint);
-            }
-
-            if (_checkedAnimation.value > _kMidpoint) {
-              double t = (_checkedAnimation.value - _kMidpoint) / (1.0 - _kMidpoint);
-
-              // Solid filled rrect
-              paint.setStyle(sky.PaintingStyle.strokeAndFill);
-              paint.color = new Color.fromARGB((t * 255).floor(),
-                                               _kCheckedColor.red,
-                                               _kCheckedColor.green,
-                                               _kCheckedColor.blue);
-              canvas.drawRRect(rrect, paint);
-
-              // White inner check
-              paint.color = const sky.Color(0xFFFFFFFF);
-              paint.setStyle(sky.PaintingStyle.stroke);
-              sky.Path path = new sky.Path();
-              sky.Point start = new sky.Point(_kEdgeSize * 0.2, _kEdgeSize * 0.5);
-              sky.Point mid = new sky.Point(_kEdgeSize * 0.4, _kEdgeSize * 0.7);
-              sky.Point end = new sky.Point(_kEdgeSize * 0.8, _kEdgeSize * 0.3);
-              Point lerp(Point p1, Point p2, double t)
-                => new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
-              sky.Point drawStart = lerp(start, mid, 1.0 - t);
-              sky.Point drawEnd = lerp(mid, end, t);
-              path.moveTo(drawStart.x, drawStart.y);
-              path.lineTo(mid.x, mid.y);
-              path.lineTo(drawEnd.x, drawEnd.y);
-              canvas.drawPath(path, paint);
-            }
-          }
+          callback: _customPaintCallback
         )
       ),
       onGestureTap: _handleClick
     );
   }
+}
 
+// TODO(jackson): This should change colors with the theme
+sky.Color _kSwitchOnColor = colors.Purple[500];
+const sky.Color _kSwitchOffColor = const sky.Color(0xFFFAFAFA);
+sky.Color _kTrackOnColor = new sky.Color(_kSwitchOnColor.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 = 10.0;
+const double _kTrackRadius = _kTrackHeight / 2.0;
+const double _kTrackWidth = _kSwitchWidth - (_kThumbRadius - _kTrackRadius) * 2.0;
+
+class Switch extends Checkbox {
+  // TODO(jackson): Hit-test the switch so that it can respond to both taps and swipe gestures
+
+  Switch({
+    Object key,
+    bool checked,
+    ValueChanged onChanged
+  }) : super(key: key, checked: checked, onChanged: onChanged);
+
+  double get _width => _kSwitchWidth + 2.0;
+  double get _height => _kSwitchHeight + 2.0;
+  EdgeDims get _margin => const EdgeDims.symmetric(horizontal: 5.0);
+
+  void _customPaintCallback(sky.Canvas canvas, Size size) {

[abarth-chromium, 2015/06/15 18:19:12]

You probably want to extract a base class from Checkbox and have Switch and
Checkbox both inherit from that.  Maybe name the base class Toggleable?

+    sky.Color thumbColor = checked ? _kSwitchOnColor : _kSwitchOffColor;
+    sky.Color trackColor = checked ? _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 - (_checkedAnimation.value - _kMidpoint).abs() * 2.0;
+    Point thumbPos = new Point(
+      _kTrackRadius + _checkedAnimation.value * (_kTrackWidth - _kTrackRadius * 2),
+      _kSwitchHeight / 2.0
+    );
+    canvas.drawCircle(thumbPos.x, thumbPos.y, _kThumbRadius - inset, paint);
+  }
 }