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

sky/sdk/lib/rendering/object.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 'dart:sky' show Point, Size, Rect, Color, Paint, Path;
 
 import '../base/hit_test.dart';
 import '../base/node.dart';
 import '../base/scheduler.dart' as scheduler;
 
 export 'dart:sky' show Point, Size, Rect, Color, Paint, Path;
 export '../base/hit_test.dart' show HitTestTarget, HitTestEntry, HitTestResult;
 
 
 class ParentData {
   void detach() {
     detachSiblings();
   }
   void detachSiblings() { } // workaround for lack of inter-class mixins in Dart
   void merge(ParentData other) {
     // override this in subclasses to merge in data from other into this
     assert(other.runtimeType == this.runtimeType);
   }
   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 RenderObjectDisplayList extends sky.PictureRecorder {
-  RenderObjectDisplayList(double width, double height) : super(width, height);
+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);

+
   void paintChild(RenderObject child, Point position) {
     translate(position.x, position.y);
     child.paint(this);
     translate(-position.x, -position.y);
   }
 }
 
 abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   // LAYOUT
@@ skipping 164 matching lines @@
   }) { }
 
 
   // PAINTING
 
   static bool debugDoingPaint = false;
   void markNeedsPaint() {
     assert(!debugDoingPaint);
     scheduler.ensureVisualUpdate();
   }
-  void paint(RenderObjectDisplayList canvas) { }
+  void paint(RenderCanvas canvas) { }
 
 
   // EVENTS
 
   void handleEvent(sky.Event event, HitTestEntry entry) {
     // override this if you have a client, to hand it to the client
     // override this if you want to do anything with the event
   }
 
 
@@ skipping 257 matching lines @@
     int count = 1;
     ChildType child = _firstChild;
     while (child != null) {
       result += '${prefix}child ${count}: ${child.toString(prefix)}';
       count += 1;
       child = child.parentData.nextSibling;
     }
     return result;
   }
 }