Switch painting over to using PictureLayer

Currently we have a single PictureLayer that everyone draws into. A future
patch will teach the system to use multiple PictureLayers.

packages/flutter/lib/rendering/box.dart

@@ -8,6 +8,7 @@ import 'dart:sky' as sky;
 import 'package:sky/base/debug.dart';
 import 'package:sky/painting/box_painter.dart';
 import 'package:sky/painting/text_style.dart';
+import 'package:sky/rendering/layer.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:vector_math/vector_math.dart';
 
@@ -1703,6 +1704,8 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
     markNeedsLayout();
   }
 
+  ContainerLayer _rootLayer;
+
   // We never call layout() on this class, so this should never get
   // checked. (This class is laid out using scheduleInitialLayout().)
   bool debugDoesMeetConstraints() { assert(false); return false; }
@@ -1746,15 +1749,22 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
   void paintFrame() {
     sky.tracing.begin('RenderView.paintFrame');
     try {
-      double devicePixelRatio = sky.view.devicePixelRatio;
-      sky.PictureRecorder recorder = new sky.PictureRecorder();
-      Rect cullRect = Point.origin & (size * devicePixelRatio);
-      PaintingCanvas canvas = new PaintingCanvas(recorder, cullRect);
-      PaintingContext context = new PaintingContext(canvas);
-      canvas.drawColor(const Color(0xFF000000), sky.TransferMode.src);
-      canvas.scale(devicePixelRatio, devicePixelRatio);
+      final double devicePixelRatio = sky.view.devicePixelRatio;
+
+      // TODO(abarth): Really |_rootLayer| should be a TransformLayer that
+      // applies the devicePixelRatio.
+      Rect scaledBounds = Point.origin & (size * devicePixelRatio);
+      PaintingContext context = new PaintingContext(scaledBounds);
+      _rootLayer = new ContainerLayer(bounds: Point.origin & size);
+      _rootLayer.add(context.layer);
+      context.canvas.drawColor(const Color(0xFF000000), sky.TransferMode.src);
+      context.canvas.scale(devicePixelRatio, devicePixelRatio);
       context.paintChild(child, Point.origin);
-      sky.view.picture = recorder.endRecording();
+      context.endRecording();
+
+      // TODO(abarth): Once we have more than one PictureLayer, we should walk
+      // the layer tree to generate the final picture.
+      sky.view.picture = (_rootLayer.firstChild as PictureLayer).picture;
     } finally {
       sky.tracing.end('RenderView.paintFrame');
     }

packages/flutter/lib/rendering/layer.dart

@@ -0,0 +1,140 @@
+// 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:sky' as sky;
+import 'dart:sky' show Point, Offset, Size, Rect, Color, Paint, Path;
+
+import 'package:vector_math/vector_math.dart';
+
+class Layer {
+  Layer({ this.bounds });
+
+  Rect bounds;
+
+  ContainerLayer _parent;
+  ContainerLayer get parent => _parent;
+
+  Layer _nextSibling;
+  Layer get nextSibling => _nextSibling;
+
+  Layer _previousSibling;
+  Layer get previousSibling => _previousSibling;
+
+  void detach() {
+    if (_parent != null)
+      _parent.remove(this);
+  }
+}
+
+class PictureLayer extends Layer {
+  PictureLayer({ Rect bounds })
+    : super(bounds: bounds);
+
+  sky.Picture picture;
+}
+
+class ContainerLayer extends Layer {
+  ContainerLayer({ Rect bounds }) : super(bounds: bounds);
+
+  Layer _firstChild;
+  Layer get firstChild => _firstChild;
+
+  Layer _lastChild;
+  Layer get lastChild => _lastChild;
+
+  bool _debugUltimatePreviousSiblingOf(Layer child, { Layer equals }) {
+    while (child._previousSibling != null) {
+      assert(child._previousSibling != child);
+      child = child._previousSibling;
+    }
+    return child == equals;
+  }
+
+  bool _debugUltimateNextSiblingOf(Layer child, { Layer equals }) {
+    while (child._nextSibling != null) {
+      assert(child._nextSibling != child);
+      child = child._nextSibling;
+    }
+    return child == equals;
+  }
+
+  void add(Layer child, { Layer before }) {
+    assert(child != this);
+    assert(before != this);
+    assert(child != before);
+    assert(child != _firstChild);
+    assert(child != _lastChild);
+    assert(child._parent == null);
+    assert(child._nextSibling == null);
+    assert(child._previousSibling == null);
+    child._parent = this;
+    if (before == null) {
+      child._previousSibling = _lastChild;
+      if (_lastChild != null)
+        _lastChild._nextSibling = child;
+      _lastChild = child;
+      if (_firstChild == null)
+        _firstChild = child;
+    } else {
+      assert(_firstChild != null);
+      assert(_lastChild != null);
+      assert(_debugUltimatePreviousSiblingOf(before, equals: _firstChild));
+      assert(_debugUltimateNextSiblingOf(before, equals: _lastChild));
+      if (before._previousSibling == null) {
+        assert(before == _firstChild);
+        child._nextSibling = before;
+        before._previousSibling = child;
+        _firstChild = child;
+      } else {
+        child._previousSibling = before._previousSibling;
+        child._nextSibling = before;
+        child._previousSibling._nextSibling = child;
+        child._nextSibling._previousSibling = child;
+        assert(before._previousSibling == child);
+      }
+    }
+  }
+
+  void remove(Layer child) {
+    assert(_debugUltimatePreviousSiblingOf(child, equals: _firstChild));
+    assert(_debugUltimateNextSiblingOf(child, equals: _lastChild));
+    assert(child._parent == this);
+    if (child._previousSibling == null) {
+      assert(_firstChild == child);
+      _firstChild = child._nextSibling;
+    } else {
+      child._previousSibling._nextSibling = child._nextSibling;
+    }
+    if (child._nextSibling == null) {
+      assert(_lastChild == child);
+      _lastChild = child._previousSibling;
+    } else {
+      child._nextSibling._previousSibling = child._previousSibling;
+    }
+    child._previousSibling = null;
+    child._nextSibling = null;
+    child._parent = null;
+  }
+}
+
+class TransformLayer extends ContainerLayer {
+  TransformLayer({ this.transform, Rect bounds }) : super(bounds: bounds);
+
+  Matrix4 transform;
+}
+
+class ClipLayer extends ContainerLayer {
+  ClipLayer({ Rect bounds }) : super(bounds: bounds);
+}
+
+class ColorFilterLayer extends ContainerLayer {
+  ColorFilterLayer({
+    Rect bounds,
+    this.color,
+    this.transferMode
+  }) : super(bounds: bounds);
+
+  Color color;
+  sky.TransferMode transferMode;
+}

packages/flutter/lib/rendering/object.dart

@@ -10,6 +10,7 @@ import 'package:sky/base/debug.dart';
 import 'package:sky/base/hit_test.dart';
 import 'package:sky/base/node.dart';
 import 'package:sky/base/scheduler.dart' as scheduler;
+import 'package:sky/rendering/layer.dart';
 import 'package:vector_math/vector_math.dart';
 
 export 'dart:sky' show Point, Offset, Size, Rect, Color, Paint, Path;
@@ -32,9 +33,27 @@ class PaintingCanvas extends sky.Canvas {
 }
 
 class PaintingContext {
-  final PaintingCanvas canvas;
+  PaintingCanvas canvas;
 
-  PaintingContext(this.canvas);
+  PictureLayer get layer => _layer;
+  PictureLayer _layer;
+
+  sky.PictureRecorder _recorder;
+
+  PaintingContext(Rect bounds) {
+    _layer = new PictureLayer(bounds: bounds);
+    _recorder = new sky.PictureRecorder();
+    canvas = new PaintingCanvas(_recorder, bounds);
+  }
+
+  PaintingContext.forTesting(this.canvas);
+
+  void endRecording() {
+    canvas = null;
+    _layer.picture = _recorder.endRecording();
+    _recorder = null;
+    _layer = null;
+  }
 
   void paintChild(RenderObject child, Point point) {
     // TODO(abarth): Support compositing.
@@ -318,6 +337,12 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   bool _needsPaint = true;
   bool get needsPaint => _needsPaint;
 
+  PictureLayer _layer;
+  PictureLayer get layer {
+    assert(requiresCompositing);
+    return _layer;
+  }
+
   void markNeedsPaint() {
     assert(!debugDoingPaint);
     if (!attached) return; // Don't try painting things that aren't in the hierarchy
@@ -357,9 +382,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   void _repaint() {
     assert(!_needsLayout);
     assert(requiresCompositing);
-    sky.PictureRecorder recorder = new sky.PictureRecorder();
-    sky.Canvas canvas = new sky.Canvas(recorder, paintBounds);
-    PaintingContext context = new PaintingContext(canvas);
+    assert(_layer != null);
+    PaintingContext context = new PaintingContext(paintBounds);
+    _layer.parent.add(context.layer, before: _layer);
+    _layer.detach();
+    _layer = context._layer;
     _needsPaint = false;
     try {
       _paintWithContext(context, Offset.zero);