Polish selection controls

The touch ripple now starts at the touch location instead of being
centered on the widget.

Fixes #2652

packages/flutter/lib/src/material/checkbox.dart

@@ -112,7 +112,7 @@ class _RenderCheckbox extends RenderToggleable {
     final double offsetX = _kOffset + offset.dx;
     final double offsetY = _kOffset + offset.dy;
 
-    paintRadialReaction(canvas, offset + const Offset(kRadialReactionRadius, kRadialReactionRadius));
+    paintRadialReaction(canvas, offset, const Point(kRadialReactionRadius, kRadialReactionRadius));
 
     double t = position.value;
 

packages/flutter/lib/src/material/radio.dart

@@ -112,7 +112,7 @@ class _RenderRadio extends RenderToggleable {
   void paint(PaintingContext context, Offset offset) {
     final Canvas canvas = context.canvas;
 
-    paintRadialReaction(canvas, offset + const Offset(kRadialReactionRadius, kRadialReactionRadius));
+    paintRadialReaction(canvas, offset, const Point(kRadialReactionRadius, kRadialReactionRadius));
 
     Point center = (offset & size).center;
     Color radioColor = onChanged != null ? activeColor : inactiveColor;

packages/flutter/lib/src/material/switch.dart

@@ -135,7 +135,6 @@ class _RenderSwitch extends RenderToggleable {
          activeColor: activeColor,
          inactiveColor: inactiveColor,
          onChanged: onChanged,
-         minRadialReactionRadius: _kThumbRadius,
          size: const Size(_kSwitchWidth, _kSwitchHeight)
        ) {
     _drag = new HorizontalDragGestureRecognizer()
@@ -248,12 +247,12 @@ class _RenderSwitch extends RenderToggleable {
     RRect trackRRect = new RRect.fromRectXY(trackRect, _kTrackRadius, _kTrackRadius);
     canvas.drawRRect(trackRRect, paint);
 
-    Offset thumbOffset = new Offset(
-      offset.dx + kRadialReactionRadius + currentPosition * _trackInnerLength,
-      offset.dy + size.height / 2.0
+    Point thumbPosition = new Point(
+      kRadialReactionRadius + currentPosition * _trackInnerLength,
+      size.height / 2.0
     );
 
-    paintRadialReaction(canvas, thumbOffset);
+    paintRadialReaction(canvas, offset, thumbPosition);
 
     BoxPainter thumbPainter;
     if (_inactiveThumbDecoration == null && _activeThumbDecoration == null) {
@@ -272,10 +271,10 @@ class _RenderSwitch extends RenderToggleable {
     // The thumb contracts slightly during the animation
     double inset = 2.0 - (currentPosition - 0.5).abs() * 2.0;
     double radius = _kThumbRadius - inset;
-    Rect thumbRect = new Rect.fromLTRB(thumbOffset.dx - radius,
-                                       thumbOffset.dy - radius,
-                                       thumbOffset.dx + radius,
-                                       thumbOffset.dy + radius);
+    Rect thumbRect = new Rect.fromLTRB(thumbPosition.x + offset.dx - radius,
+                                       thumbPosition.y + offset.dy - radius,
+                                       thumbPosition.x + offset.dx + radius,
+                                       thumbPosition.y + offset.dy + radius);
     thumbPainter.paint(canvas, thumbRect);
   }
 }

packages/flutter/lib/src/material/toggleable.dart

@@ -9,6 +9,7 @@ import 'package:flutter/rendering.dart';
 import 'constants.dart';
 
 const Duration _kToggleDuration = const Duration(milliseconds: 200);
+final Tween<double> _kRadialReactionRadiusTween = new Tween<double>(begin: 0.0, end: kRadialReactionRadius);
 
 // RenderToggleable is a base class for material style toggleable controls with
 // toggle animations. It handles storing the current value, dispatching
@@ -20,8 +21,7 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
     Size size,
     Color activeColor,
     Color inactiveColor,
-    ValueChanged<bool> onChanged,
-    double minRadialReactionRadius: 0.0
+    ValueChanged<bool> onChanged
   }) : _value = value,
        _activeColor = activeColor,
        _inactiveColor = inactiveColor,
@@ -46,13 +46,10 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
      ..addStatusListener(_handlePositionStateChanged);
 
     _reactionController = new AnimationController(duration: kRadialReactionDuration);
-    _reaction = new Tween<double>(
-      begin: minRadialReactionRadius,
-      end: kRadialReactionRadius
-    ).animate(new CurvedAnimation(
+    _reaction = new CurvedAnimation(
       parent: _reactionController,
       curve: Curves.ease
-    ))..addListener(markNeedsPaint);
+    )..addListener(markNeedsPaint);
   }
 
   bool get value => _value;
@@ -118,6 +115,7 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
   Animation<double> _reaction;
 
   TapGestureRecognizer _tap;
+  Point _downPosition;
 
   void _handlePositionStateChanged(AnimationStatus status) {
     if (isInteractive) {
@@ -129,8 +127,10 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
   }
 
   void _handleTapDown(Point globalPosition) {
-    if (isInteractive)
+    if (isInteractive) {
+      _downPosition = globalToLocal(globalPosition);
       _reactionController.forward();
+    }
   }
 
   void _handleTap() {
@@ -139,11 +139,13 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
   }
 
   void _handleTapUp(Point globalPosition) {
+    _downPosition = null;
     if (isInteractive)
       _reactionController.reverse();
   }
 
   void _handleTapCancel() {
+    _downPosition = null;
     if (isInteractive)
       _reactionController.reverse();
   }
@@ -157,11 +159,13 @@ abstract class RenderToggleable extends RenderConstrainedBox implements Semantic
       _tap.addPointer(event);
   }
 
-  void paintRadialReaction(Canvas canvas, Offset offset) {
+  void paintRadialReaction(Canvas canvas, Offset offset, Point origin) {
     if (!_reaction.isDismissed) {
       // TODO(abarth): We should have a different reaction color when position is zero.
       Paint reactionPaint = new Paint()..color = activeColor.withAlpha(kRadialReactionAlpha);
-      canvas.drawCircle(offset.toPoint(), _reaction.value, reactionPaint);
+      Point center = Point.lerp(_downPosition ?? origin, origin, _reaction.value);
+      double radius = _kRadialReactionRadiusTween.evaluate(_reaction);
+      canvas.drawCircle(center + offset, radius, reactionPaint);
     }
   }