Port Toggleable to the new animation system

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/widgets/theme.dart';
 
 import 'basic.dart';
 import 'toggleable.dart';
@@ skipping 30 matching lines @@
 
   void customPaintCallback(sky.Canvas canvas, Size size) {
     ThemeData themeData = Theme.of(this);
     Color uncheckedColor = themeData.brightness == ThemeBrightness.light ? _kLightUncheckedColor : _kDarkUncheckedColor;
 
     // Choose a color between grey and the theme color
     sky.Paint paint = new sky.Paint()..strokeWidth = 2.0
                                      ..color = uncheckedColor;
 
     // The rrect contracts slightly during the animation
-    double inset = 2.0 - (toggleAnimation.value - _kMidpoint).abs() * 2.0;
+    double inset = 2.0 - (position.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 (toggleAnimation.value > 0) {
+    if (position.value > 0) {
       paint.setStyle(sky.PaintingStyle.fill);
       paint.setShader(
         new sky.Gradient.radial(
           new Point(_kEdgeSize / 2.0, _kEdgeSize / 2.0),
-          _kEdgeSize * (_kMidpoint - toggleAnimation.value) * 8.0,
+          _kEdgeSize * (_kMidpoint - position.value) * 8.0,
           [const sky.Color(0x00000000), uncheckedColor]
         )
       );
       canvas.drawRRect(rrect, paint);
     }
 
-    if (toggleAnimation.value > _kMidpoint) {
-      double t = (toggleAnimation.value - _kMidpoint) / (1.0 - _kMidpoint);
+    if (position.value > _kMidpoint) {
+      double t = (position.value - _kMidpoint) / (1.0 - _kMidpoint);
 
       // Solid filled rrect
       paint.setStyle(sky.PaintingStyle.strokeAndFill);
       paint.color = new Color.fromARGB((t * 255).floor(),
                                        themeData.accentColor.red,
                                        themeData.accentColor.green,
                                        themeData.accentColor.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);
     }
   }
 }