Animate checkboxes

sky/sdk/lib/framework/widgets/checkbox.dart

Index: sky/sdk/lib/framework/widgets/checkbox.dart
diff --git a/sky/sdk/lib/framework/widgets/checkbox.dart b/sky/sdk/lib/framework/widgets/checkbox.dart
index 1d2630c48487e9f77f3d34126202361c8a9c698e..a6b341844895a0e35081f2d48a5b260386fcbab4 100644
--- a/sky/sdk/lib/framework/widgets/checkbox.dart
+++ b/sky/sdk/lib/framework/widgets/checkbox.dart
@@ -6,22 +6,54 @@ import 'dart:sky' as sky;
 
 import 'package:sky/framework/theme2/colors.dart' as colors;
 
+import '../animation/animated_value.dart';
+import '../animation/curves.dart';
 import '../rendering/box.dart';
-import 'button_base.dart';
+import 'animated_component.dart';
 import 'wrappers.dart';
 
 typedef void ValueChanged(value);
 
-class Checkbox extends ButtonBase {
+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
+sky.Color _kCheckedColor = colors.Purple[500];
+const double _kEdgeSize = 20.0;
+const double _kEdgeRadius = 1.0;
 
-  Checkbox({ Object key, this.onChanged, this.checked }) : super(key: key);
+class Checkbox extends AnimatedComponent {
+
+  Checkbox({
+    Object key,
+    this.checked,
+    this.onChanged
+  }) : super(key: key) {
+    _checkedAnimation = new AnimatedValue(checked ? 1.0 : 0.0);
+  }
 
   bool checked;
+  AnimatedValue _checkedAnimation;
   ValueChanged onChanged;
 
   void syncFields(Checkbox source) {
-    checked = source.checked;
     onChanged = source.onChanged;
+    if (checked != source.checked) {
+      checked = source.checked;
+      double targetValue = checked ? 1.0 : 0.0;
+      double difference = (_checkedAnimation.value - targetValue).abs();
+      if (difference > 0) {
+        double duration = difference * _kCheckDuration;
+        _checkedAnimation.stop();
+        Curve curve;
+        if (targetValue > _checkedAnimation.value) {
+          curve = easeIn;
+        } else {
+          curve = easeOut;
+        }
+        _checkedAnimation.animateTo(targetValue, duration, curve: curve);
+      }
+    }
     super.syncFields(source);
   }
 
@@ -29,37 +61,68 @@ class Checkbox extends ButtonBase {
     onChanged(!checked);
   }
 
-  UINode buildContent() {
-    // TODO(jackson): This should change colors with the theme
-    sky.Color color = highlight ? colors.Purple[500] : const sky.Color(0x8A000000);
-    const double kEdgeSize = 20.0;
-    const double kEdgeRadius = 1.0;
+  UINode build() {
     return new EventListenerNode(
       new Container(
         margin: const EdgeDims.symmetric(horizontal: 5.0),
-        width: kEdgeSize + 2.0,
-        height: kEdgeSize + 2.0,
+        width: _kEdgeSize + 2.0,
+        height: _kEdgeSize + 2.0,
         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;
 
-            sky.Paint paint = new sky.Paint()..color = color
-                                             ..strokeWidth = 2.0;
+            // 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);
 
-            // Draw the outer rrect
-            paint.setStyle(checked ? sky.PaintingStyle.strokeAndFill : sky.PaintingStyle.stroke);
-            sky.Rect rect = new sky.Rect.fromLTRB(0.0, 0.0, kEdgeSize, kEdgeSize);
-            sky.RRect rrect = new sky.RRect()..setRectXY(rect, kEdgeRadius, kEdgeRadius);
+            
+            // Outline of the empty rrect
+            paint.setStyle(sky.PaintingStyle.stroke);
             canvas.drawRRect(rrect, paint);
 
-            // Draw the inner check
-            if (checked) {
-              // TODO(jackson): Use the theme color
+            // 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();
-              path.moveTo(kEdgeSize * 0.2, kEdgeSize * 0.5);
-              path.lineTo(kEdgeSize * 0.4, kEdgeSize * 0.7);
-              path.lineTo(kEdgeSize * 0.8, kEdgeSize * 0.3);
+              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);
             }
           }