Reduce the number of temporary Paint objects

sky/sdk/lib/rendering/box.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:math' as math;
 import 'dart:sky' as sky;
 
 import 'package:sky/base/debug.dart';
 import 'package:sky/painting/box_painter.dart';
 import 'package:sky/rendering/object.dart';
@@ skipping 638 matching lines @@
   }
 
   double _opacity;
   double get opacity => _opacity;
   void set opacity (double value) {
     assert(value != null);
     assert(value >= 0.0 && value <= 1.0);
     if (_opacity == value)
       return;
     _opacity = value;
+    _cachedPaint = null;
     markNeedsPaint();
   }
 
+  int get _alpha => (_opacity * 255).round();
+
+  Paint _cachedPaint;
+  Paint get _paint {
+    if (_cachedPaint == null) {
+      _cachedPaint =  new Paint()
+        ..color = new Color.fromARGB(_alpha, 0, 0, 0)
+        ..setTransferMode(sky.TransferMode.srcOver);
+    }
+    return _cachedPaint;
+  }
+
   void paint(PaintingCanvas canvas, Offset offset) {
     if (child != null) {
-      int a = (_opacity * 255).round();
+      int a = _alpha;
 
       if (a == 0)
         return;
 
       if (a == 255) {
         canvas.paintChild(child, offset.toPoint());
         return;
       }
 
-      Paint paint = new Paint()
-        ..color = new Color.fromARGB(a, 0, 0, 0)
-        ..setTransferMode(sky.TransferMode.srcOver);
-      canvas.saveLayer(null, paint);
+      canvas.saveLayer(null, _paint);
       canvas.paintChild(child, offset.toPoint());
       canvas.restore();
     }
   }
 }
 
 class RenderColorFilter extends RenderProxyBox {
   RenderColorFilter({ RenderBox child, Color color, sky.TransferMode transferMode })
     : _color = color, _transferMode = transferMode, super(child) {
   }
 
   Color _color;
   Color get color => _color;
   void set color (Color value) {
     assert(value != null);
     if (_color == value)
       return;
     _color = value;
+    _cachedPaint = null;
     markNeedsPaint();
   }
 
   sky.TransferMode _transferMode;
   sky.TransferMode get transferMode => _transferMode;
   void set transferMode (sky.TransferMode value) {
     assert(value != null);
     if (_transferMode == value)
       return;
     _transferMode = value;
+    _cachedPaint = null;
     markNeedsPaint();
   }
 
+  Paint _cachedPaint;
+  Paint get _paint {
+    if (_cachedPaint == null) {
+      _cachedPaint = new Paint()
+        ..setColorFilter(new sky.ColorFilter.mode(_color, _transferMode));
+    }
+    return _cachedPaint;
+  }
+
   void paint(PaintingCanvas canvas, Offset offset) {
     if (child != null) {
-      Paint paint = new Paint()
-        ..setColorFilter(new sky.ColorFilter.mode(_color, _transferMode));
-      canvas.saveLayer(null, paint);
+      canvas.saveLayer(offset & size, _paint);
       canvas.paintChild(child, offset.toPoint());
       canvas.restore();
     }
   }
 }
 
 class RenderClipRect extends RenderProxyBox {
   RenderClipRect({ RenderBox child }) : super(child);
 
   void paint(PaintingCanvas canvas, Offset offset) {
@@ skipping 516 matching lines @@
       return;
     bool needsScale = size.width != _image.width || size.height != _image.height;
     if (needsScale) {
       double widthScale = size.width / _image.width;
       double heightScale = size.height / _image.height;
       canvas.save();
       canvas.translate(offset.dx, offset.dy);
       canvas.scale(widthScale, heightScale);
       offset = Offset.zero;
     }
-    Paint paint = new Paint();
-    canvas.drawImage(_image, offset.toPoint(), paint);
+    canvas.drawImage(_image, offset.toPoint(), new Paint());
     if (needsScale)
       canvas.restore();
   }
 
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}dimensions: ${requestedSize}\n';
 }
 
 class RenderDecoratedBox extends RenderProxyBox {
 
   RenderDecoratedBox({
@@ skipping 304 matching lines @@
 
   void defaultPaint(PaintingCanvas canvas, Offset offset) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position + offset);
       child = child.parentData.nextSibling;
     }
   }
 }