Decouple Canvas from DisplayList and map Picture and PictureRecorder more directly to their Skia co…

sky/sdk/lib/rendering/object.dart

Index: sky/sdk/lib/rendering/object.dart
diff --git a/sky/sdk/lib/rendering/object.dart b/sky/sdk/lib/rendering/object.dart
index 75edf32f4e3cf35634087c08f7a112f0563eeeff..4e9b64b17e975f332b2f7c1a9c711e5432dad7c9 100644
--- a/sky/sdk/lib/rendering/object.dart
+++ b/sky/sdk/lib/rendering/object.dart
@@ -26,10 +26,18 @@ class ParentData {
   String toString() => '<none>';
 }
 
-const kLayoutDirections = 4;
+class RenderPictureRecorder extends sky.PictureRecorder {
+  RenderCanvas beginRecording(double width, double height) {
+    sky.Canvas originalCanvas = super.beginRecording(width, height);
+    RenderCanvas realCanvas = new RenderCanvas(originalCanvas);
+    setCanvas(realCanvas);

[abarth-chromium, 2015/06/20 00:50:29]

I see, you're using this to solve the dangling SkCanvas* problem. 
Unfortunately, we can't use this approach to solve this problem because the API
we expose from C++ to Dart needs to be memory safe.  The way you've set things
up, we need to rely on Dart code using the C++ API with a particular idiom in
order to maintain C++ memory safety.

+    return realCanvas;
+  }
+}
+
+class RenderCanvas extends sky.Canvas {
+  RenderCanvas(sky.Canvas canvas) : super(canvas);

[Hixie, 2015/06/20 00:44:52]

We probably want to avoid making two instances of the Canvas every time we
create a canvas.

If you can create a canvas by passing it a canvas, why can't you create a canvas
by passing it a PictureRecorder?

[abarth-chromium, 2015/06/20 00:50:29]

I don't understand why we can't just pass a sky.PictureRecorder to super and
have the C++ Canvas class call |beginRecording| recording on the C++
PictureRecorder to get the SkCanvas*.  Specifically:

RenderCanvas(sky.PictureRecorder recorder, double width, double height) :
super(recorder, width, height);

And on line 31:

return new RenderCanvas(this, width, height);

 
-class RenderObjectDisplayList extends sky.PictureRecorder {
-  RenderObjectDisplayList(double width, double height) : super(width, height);
   void paintChild(RenderObject child, Point position) {
     translate(position.x, position.y);
     child.paint(this);
@@ -214,7 +222,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     assert(!debugDoingPaint);
     scheduler.ensureVisualUpdate();
   }
-  void paint(RenderObjectDisplayList canvas) { }
+  void paint(RenderCanvas canvas) { }
 
 
   // EVENTS