Improvements for Layers logic

- Introduce some new Layer classes.
- Introduce paintChildWith* methods.
- Convert paint() methods to use paintChildWith* where appropriate.
- Fix paintBounds logic in Layer world.
- Introduce Layer.replaceWith(), so that it's clearer what's going on.
- Make RenderObjects have a ContainerLayer, not a PictureLayer.
- Introduce a PaintingContext.replacingLayer() constructor to highlight
  where we are creating a layer just to replace an older one.
- Rename some layer-related methods and fields for clarity:
   requiresCompositing -> hasLayer
   hasCompositedDescendant -> needsCompositing
   updateCompositing -> updateCompositingBits
   _needsCompositingUpdate -> _needsCompositingBitsUpdate
   markNeedsCompositingUpdate -> markNeedsCompositingBitsUpdate
- After updating compositing bits, if we find that the bit changed, we
  now call markNeedsPaint().
- Reorder markNeedsPaint() logic for clarity.
- Make flushPaint() start at the deepest node.
- Make _compositeChild() avoid repainting children with hasLayer that
  aren't dirty, instead it just reuses their existing layer.
- Made RenderView reuse the RenderObject layer instead of having its own.
- Made RenderView have hasLayer set to true.
- Add various asserts and comments.

packages/flutter/lib/rendering/layer.dart

@@ -26,15 +26,40 @@ abstract class Layer {
     if (_parent != null)
       _parent.remove(this);
   }
+  void replaceWith(Layer newLayer) {
+    assert(_parent != null);
+    assert(newLayer._parent == null);
+    assert(newLayer._nextSibling == null);
+    assert(newLayer._previousSibling == null);
+    newLayer._nextSibling = _nextSibling;
+    if (_nextSibling != null)
+      newLayer._nextSibling._previousSibling = newLayer;
+    newLayer._previousSibling = _previousSibling;
+    if (_previousSibling != null)
+      newLayer._previousSibling._nextSibling = newLayer;
+    newLayer._parent = _parent;
+    if (_parent._firstChild == this)
+      _parent._firstChild = newLayer;
+    if (_parent._lastChild == this)
+      _parent._lastChild = newLayer;
+    _nextSibling = null;
+    _previousSibling = null;
+    _parent = null;
+  }
 
+  // The paint() methods are temporary. Eventually, Layers won't have
+  // a paint() method, the entire Layer hierarchy will be handed over
+  // to the C++ side for processing. Until we implement that, though,
+  // we instead have the layers paint themselves into a canvas at
+  // paint time.
   void paint(sky.Canvas canvas);
 }
 
 class PictureLayer extends Layer {
-  PictureLayer({ Offset offset: Offset.zero, this.size })
+  PictureLayer({ Offset offset: Offset.zero, this.paintBounds })
     : super(offset: offset);
 
-  Size size;
+  Rect paintBounds;
   sky.Picture picture;
 
   bool _debugPaintLayerBorder(sky.Canvas canvas) {
@@ -43,12 +68,13 @@ class PictureLayer extends Layer {
         ..color = debugPaintLayerBordersColor
         ..strokeWidth = 2.0
         ..setStyle(sky.PaintingStyle.stroke);
-      canvas.drawRect(Point.origin & size, border);
+      canvas.drawRect(paintBounds, border);
     }
     return true;
   }
 
   void paint(sky.Canvas canvas) {
+    assert(picture != null);
     canvas.translate(offset.dx, offset.dy);
     canvas.drawPicture(picture);
     assert(_debugPaintLayerBorder(canvas));
@@ -59,17 +85,11 @@ class PictureLayer extends Layer {
 class ContainerLayer extends Layer {
   ContainerLayer({ Offset offset: Offset.zero }) : super(offset: offset);
 
-  void paint(sky.Canvas canvas) {
-    Layer child = firstChild;
-    while (child != null) {
-      child.paint(canvas);
-      child = child.nextSibling;
-    }
-  }
-
+  // TODO(ianh): hide firstChild since nobody uses it
   Layer _firstChild;
   Layer get firstChild => _firstChild;
 
+  // TODO(ianh): remove _lastChild since nobody uses it
   Layer _lastChild;
   Layer get lastChild => _lastChild;
 
@@ -89,6 +109,7 @@ class ContainerLayer extends Layer {
     return child == equals;
   }
 
+  // TODO(ianh): Remove 'before' and rename the function to 'append' since nobody uses 'before'
   void add(Layer child, { Layer before }) {
     assert(child != this);
     assert(before != this);
@@ -126,10 +147,11 @@ class ContainerLayer extends Layer {
     }
   }
 
+  // TODO(ianh): Hide this function since only detach() uses it
   void remove(Layer child) {
+    assert(child._parent == this);
     assert(_debugUltimatePreviousSiblingOf(child, equals: _firstChild));
     assert(_debugUltimateNextSiblingOf(child, equals: _lastChild));
-    assert(child._parent == this);
     if (child._previousSibling == null) {
       assert(_firstChild == child);
       _firstChild = child._nextSibling;
@@ -146,6 +168,69 @@ class ContainerLayer extends Layer {
     child._nextSibling = null;
     child._parent = null;
   }
+
+  void paint(sky.Canvas canvas) {
+    canvas.translate(offset.dx, offset.dy);
+    paintChildren(canvas);
+    canvas.translate(-offset.dx, -offset.dy);
+  }
+
+  void paintChildren(sky.Canvas canvas) {
+    Layer child = firstChild;
+    while (child != null) {
+      child.paint(canvas);
+      child = child.nextSibling;
+    }
+  }
+}
+
+class ClipRectLayer extends ContainerLayer {
+  ClipRectLayer({ Offset offset: Offset.zero, this.clipRect }) : super(offset: offset);
+
+  // clipRect is _not_ affected by given offset
+  Rect clipRect;
+
+  void paint(sky.Canvas canvas) {
+    canvas.save();
+    canvas.clipRect(clipRect);
+    canvas.translate(offset.dx, offset.dy);
+    paintChildren(canvas);
+    canvas.restore();
+  }
+}
+
+final Paint _disableAntialias = new Paint()..isAntiAlias = false;
+
+class ClipRRectLayer extends ContainerLayer {
+  ClipRRectLayer({ Offset offset: Offset.zero, this.bounds, this.clipRRect }) : super(offset: offset);
+
+  // bounds and clipRRect are _not_ affected by given offset
+  Rect bounds;
+  sky.RRect clipRRect;
+
+  void paint(sky.Canvas canvas) {
+    canvas.saveLayer(bounds, _disableAntialias);
+    canvas.clipRRect(clipRRect);
+    canvas.translate(offset.dx, offset.dy);
+    paintChildren(canvas);
+    canvas.restore();
+  }
+}
+
+class ClipPathLayer extends ContainerLayer {
+  ClipPathLayer({ Offset offset: Offset.zero, this.bounds, this.clipPath }) : super(offset: offset);
+
+  // bounds and clipPath are _not_ affected by given offset
+  Rect bounds;
+  Path clipPath;
+
+  void paint(sky.Canvas canvas) {
+    canvas.saveLayer(bounds, _disableAntialias);
+    canvas.clipPath(clipPath);
+    canvas.translate(offset.dx, offset.dy);
+    paintChildren(canvas);
+    canvas.restore();
+  }
 }
 
 class TransformLayer extends ContainerLayer {
@@ -157,21 +242,22 @@ class TransformLayer extends ContainerLayer {
     canvas.save();
     canvas.translate(offset.dx, offset.dy);
     canvas.concat(transform.storage);
-    super.paint(canvas);
+    paintChildren(canvas);
     canvas.restore();
   }
 }
 
-class ClipLayer extends ContainerLayer {
-  ClipLayer({ Offset offset: Offset.zero, this.clipRect }) : super(offset: offset);
+class PaintLayer extends ContainerLayer {
+  PaintLayer({ Offset offset: Offset.zero, this.bounds, this.paintSettings }) : super(offset: offset);
 
-  Rect clipRect;
+  // bounds is _not_ affected by given offset
+  Rect bounds;
+  Paint paintSettings; // TODO(ianh): rename this to 'paint' once paint() is gone
 
   void paint(sky.Canvas canvas) {
-    canvas.save();
+    canvas.saveLayer(bounds, paintSettings);
     canvas.translate(offset.dx, offset.dy);
-    canvas.clipRect(clipRect);
-    super.paint(canvas);
+    paintChildren(canvas);
     canvas.restore();
   }
 }
@@ -192,10 +278,9 @@ class ColorFilterLayer extends ContainerLayer {
     Paint paint = new Paint()
       ..color = color
       ..setTransferMode(transferMode);
-
     canvas.saveLayer(offset & size, paint);
     canvas.translate(offset.dx, offset.dy);
-    super.paint(canvas);
+    paintChildren(canvas);
     canvas.restore();
   }
 }

packages/flutter/lib/rendering/object.dart

@@ -41,94 +41,211 @@ class PaintingContext {
   // Don't keep a reference to the PaintingContext.canvas, since it
   // can change dynamically after any call to this object's methods.
 
-  PaintingContext(Offset offest, Size size) {
-    _startRecording(offest, size);
+  PaintingContext.withOffset(Offset offset, Rect paintBounds) {
+    _containerLayer = new ContainerLayer(offset: offset);
+    _startRecording(paintBounds);
   }
 
-  PaintingCanvas _canvas;
-  PaintingCanvas get canvas => _canvas;
-
-  PictureLayer _layer;
-  PictureLayer get layer => _layer;
+  PaintingContext.withLayer(ContainerLayer containerLayer, Rect paintBounds) {
+    _containerLayer = containerLayer;
+    _startRecording(paintBounds);
+  }
 
-  sky.PictureRecorder _recorder;
+  factory PaintingContext.replacingLayer(ContainerLayer oldLayer, Rect paintBounds) {
+    PaintingContext newContext = new PaintingContext.withOffset(oldLayer.offset, paintBounds);
+    if (oldLayer.parent != null)
+      oldLayer.replaceWith(newContext._containerLayer);
+    return newContext;
+  }
 
   PaintingContext.forTesting(this._canvas);
 
-  void _startRecording(Offset offset, Size size) {
-    assert(_layer == null);
+  ContainerLayer _containerLayer;
+  ContainerLayer get containerLayer => _containerLayer;
+
+  PictureLayer _currentLayer;
+  sky.PictureRecorder _recorder;
+  PaintingCanvas _canvas;
+  PaintingCanvas get canvas => _canvas; // Paint on this.
+
+  void _startRecording(Rect paintBounds) {
+    assert(_currentLayer == null);
     assert(_recorder == null);
     assert(_canvas == null);
-    _layer = new PictureLayer(offset: offset, size: size);
+    _currentLayer = new PictureLayer(paintBounds: paintBounds);
     _recorder = new sky.PictureRecorder();
-    _canvas = new PaintingCanvas(_recorder, Point.origin & size);
+    _canvas = new PaintingCanvas(_recorder, paintBounds);
+    _containerLayer.add(_currentLayer);
   }
 
   void endRecording() {
-    assert(_layer != null);
+    assert(_currentLayer != null);
     assert(_recorder != null);
     assert(_canvas != null);
-    _layer.picture = _recorder.endRecording();
-    _layer = null;
+    _currentLayer.picture = _recorder.endRecording();
+    _currentLayer = null;
     _recorder = null;
     _canvas = null;
   }
 
-  bool debugCanPaintChild() {
+  bool debugCanPaintChild(RenderObject child) {
     // You need to use layers if you are applying transforms, clips,
     // or similar, to a child. To do so, use the paintChildWith*()
     // methods below.
     // (commented out for now because we haven't ported everything yet)
-    // assert(canvas.getSaveCount() == 1);
+    assert(canvas.getSaveCount() == 1 || !child.needsCompositing);
     return true;
   }
 
-  void paintChild(RenderObject child, Point point) {
-    assert(debugCanPaintChild());
-    final Offset offset = point.toOffset();
-    if (!child.requiresCompositing) {
-      child._paintWithContext(this, offset);
+  void paintChild(RenderObject child, Point childPosition) {
+    assert(debugCanPaintChild(child));
+    final Offset childOffset = childPosition.toOffset();
+    if (!child.hasLayer) {
+      insertChild(child, childOffset);
     } else {
-      _compositeChild(child, offset, layer.parent, layer.nextSibling);
+      compositeChild(child, childOffset: childOffset, parentLayer: _containerLayer);
     }
   }
 
-  void paintChildWithClip(RenderObject child, Point point, Rect clipRect) {
-    assert(debugCanPaintChild());
-    final Offset offset = point.toOffset();
-    if (!child.hasCompositedDescendant) {
-      // If none of the descendants require compositing, then we don't
-      // need to use a new layer here, because at no point will any of
-      // the children introduce a new layer of their own.
+  // Below we have various variants of the paintChild() method, which
+  // do additional work, such as clipping or transforming, at the same
+  // time as painting the children.
+
+  // If none of the descendants require compositing, then these don't
+  // need to use a new layer, because at no point will any of the
+  // children introduce a new layer of their own. In that case, we
+  // just use regular canvas commands to do the work.
+
+  // If at least one of the descendants requires compositing, though,
+  // we introduce a new layer to do the work, so that when the
+  // children are split into a new layer, the work (e.g. clip) is not
+  // lost, as it would if we didn't introduce a new layer.
+
+  static final Paint _disableAntialias = new Paint()..isAntiAlias = false;
+
+  void paintChildWithClipRect(RenderObject child, Point childPosition, Rect clipRect) {
+    // clipRect is in the parent's coordinate space
+    assert(debugCanPaintChild(child));
+    final Offset childOffset = childPosition.toOffset();
+    if (!child.needsCompositing) {
       canvas.save();
-      canvas.clipRect(clipRect.shift(offset));
-      child._paintWithContext(this, offset);
+      canvas.clipRect(clipRect);
+      insertChild(child, childOffset);
       canvas.restore();
     } else {
-      // At least one of the descendants requires compositing. We
-      // therefore introduce a new layer to do the clipping, so that
-      // when the children are split into a new layer, the clip is not
-      // lost, as it would if we didn't introduce a new layer.
-      ClipLayer clip = new ClipLayer(offset: offset, clipRect: clipRect);
-      layer.parent.add(clip, before: layer.nextSibling);
-      _compositeChild(child, Offset.zero, clip, null);
+      ClipRectLayer clipLayer = new ClipRectLayer(offset: childOffset, clipRect: clipRect);
+      _containerLayer.add(clipLayer);
+      compositeChild(child, parentLayer: clipLayer);
+    }
+  }
+
+  void paintChildWithClipRRect(RenderObject child, Point childPosition, Rect bounds, sky.RRect clipRRect) {
+    // clipRRect is in the parent's coordinate space
+    assert(debugCanPaintChild(child));
+    final Offset childOffset = childPosition.toOffset();
+    if (!child.needsCompositing) {
+      canvas.saveLayer(bounds, _disableAntialias);
+      canvas.clipRRect(clipRRect);
+      insertChild(child, childOffset);
+      canvas.restore();
+    } else {
+      ClipRRectLayer clipLayer = new ClipRRectLayer(offset: childOffset, bounds: bounds, clipRRect: clipRRect);
+      _containerLayer.add(clipLayer);
+      compositeChild(child, parentLayer: clipLayer);
+    }
+  }
+
+  void paintChildWithClipPath(RenderObject child, Point childPosition, Rect bounds, Path clipPath) {
+    // bounds and clipPath are in the parent's coordinate space
+    assert(debugCanPaintChild(child));
+    final Offset childOffset = childPosition.toOffset();
+    if (!child.needsCompositing) {
+      canvas.saveLayer(bounds, _disableAntialias);
+      canvas.clipPath(clipPath);
+      canvas.translate(childOffset.dx, childOffset.dy);
+      insertChild(child, Offset.zero);
+      canvas.restore();
+    } else {
+      ClipPathLayer clipLayer = new ClipPathLayer(offset: childOffset, bounds: bounds, clipPath: clipPath);
+      _containerLayer.add(clipLayer);
+      compositeChild(child, parentLayer: clipLayer);
+    }
+  }
+
+  void paintChildWithTransform(RenderObject child, Point childPosition, Matrix4 transform) {
+    assert(debugCanPaintChild(child));
+    final Offset childOffset = childPosition.toOffset();
+    if (!child.needsCompositing) {
+      canvas.save();
+      canvas.translate(childOffset.dx, childOffset.dy);
+      canvas.concat(transform.storage);
+      insertChild(child, Offset.zero);
+      canvas.restore();
+    } else {
+      TransformLayer transformLayer = new TransformLayer(offset: childOffset, transform: transform);
+      _containerLayer.add(transformLayer);
+      compositeChild(child, parentLayer: transformLayer);
+    }
+  }
+
+  void paintChildWithPaint(RenderObject child, Point childPosition, Rect bounds, Paint paint) {
+    assert(debugCanPaintChild(child));
+    final Offset childOffset = childPosition.toOffset();
+    if (!child.needsCompositing) {
+      canvas.saveLayer(bounds, paint);
+      canvas.translate(childOffset.dx, childOffset.dy);
+      insertChild(child, Offset.zero);
+      canvas.restore();
+    } else {
+      PaintLayer paintLayer = new PaintLayer(offset: childOffset, bounds: bounds, paintSettings: paint);
+      _containerLayer.add(paintLayer);
+      compositeChild(child, parentLayer: paintLayer);
+    }
   }
+
+  // do not call directly
+  void insertChild(RenderObject child, Offset offset) {
+    child._paintWithContext(this, offset);
   }
 
-  void _compositeChild(RenderObject child, Offset offset, ContainerLayer parentLayer, Layer nextSibling) {
-    final PictureLayer originalLayer = _layer;
+  // do not call directly
+  void compositeChild(RenderObject child, { Offset childOffset: Offset.zero, ContainerLayer parentLayer }) {
+    // This ends the current layer and starts a new layer for the
+    // remainder of our rendering. It also creates a new layer for the
+    // child, and inserts that layer into the given parentLayer, which
+    // must either be our current layer's parent layer, or at least
+    // must have our current layer's parent layer as an ancestor.
+    final PictureLayer originalLayer = _currentLayer;
+    assert(() {
+      assert(parentLayer != null);
+      assert(originalLayer != null);
+      assert(originalLayer.parent != null);
+      ContainerLayer ancestor = parentLayer;
+      while (ancestor != null && ancestor != originalLayer.parent)
+        ancestor = ancestor.parent;
+      assert(ancestor == originalLayer.parent);
+      assert(originalLayer.parent == _containerLayer);
+      return true;
+    });
+
+    // End our current layer.
     endRecording();
 
-    Rect childBounds = child.paintBounds;
-    Offset childOffset = childBounds.topLeft.toOffset();
-    PaintingContext context = new PaintingContext(offset + childOffset, childBounds.size);
-    parentLayer.add(context.layer, before: nextSibling);
-    child._layer = context.layer;
-    child._paintWithContext(context, -childOffset);
+    // Create a layer for our child, and paint the child into it.
+    if (child.needsPaint || !child.hasLayer) {
+      PaintingContext newContext = new PaintingContext.withOffset(childOffset, child.paintBounds);
+      child._layer = newContext.containerLayer;
+      child._paintWithContext(newContext, Offset.zero);
+      newContext.endRecording();
+    } else {
+      assert(child._layer != null);
+      child._layer.detach();
+      child._layer.offset = childOffset;
+    }
+    parentLayer.add(child._layer);
 
-    _startRecording(originalLayer.offset, originalLayer.size);
-    originalLayer.parent.add(layer, before: context.layer.nextSibling);
-    context.endRecording();
+    // Start a new layer for anything that remains of our own paint.
+    _startRecording(originalLayer.paintBounds);
   }
 
 }
@@ -163,7 +280,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     setupParentData(child);
     super.adoptChild(child);
     markNeedsLayout();
-    markNeedsCompositingUpdate();
+    markNeedsCompositingBitsUpdate();
   }
   void dropChild(RenderObject child) { // only for use by subclasses
     assert(debugCanPerformMutations);
@@ -173,7 +290,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     child.parentData.detach();
     super.dropChild(child);
     markNeedsLayout();
-    markNeedsCompositingUpdate();
+    markNeedsCompositingBitsUpdate();
   }
 
   // Override in subclasses with children and call the visitor for each child.
@@ -254,7 +371,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   }
   void scheduleInitialLayout() {
     assert(attached);
-    assert(parent == null);
+    assert(parent is! RenderObject);
     assert(_relayoutSubtreeRoot == null);
     _relayoutSubtreeRoot = this;
     assert(() {
@@ -412,71 +529,90 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   static List<RenderObject> _nodesNeedingPaint = new List<RenderObject>();
 
-  PictureLayer _layer;
-  PictureLayer get layer {
-    assert(requiresCompositing);
-    return _layer;
-  }
+  // Override this in subclasses to indicate that instances of your
+  // class need to have their own Layer. For example, videos.
+  bool get hasLayer => false;
 
-  bool get requiresCompositing => false;
-
-  bool _hasCompositedDescendant = false;
-  bool get hasCompositedDescendant {
-    assert(!_needsCompositingUpdate);
-    return _hasCompositedDescendant;
+  ContainerLayer _layer;
+  ContainerLayer get layer {
+    assert(hasLayer);
+    assert(!_needsPaint);
+    return _layer;
   }
 
-  bool _needsCompositingUpdate = false;
-  void markNeedsCompositingUpdate() {
-    if (_needsCompositingUpdate)
+  // When the subtree is mutated, we need to recompute our
+  // "needsCompositing" bit, and our ancestors need to do the
+  // same (in case ours changed). adoptChild() and dropChild() thus
+  // call markNeedsCompositingBitsUpdate().
+  bool _needsCompositingBitsUpdate = true;
+  void markNeedsCompositingBitsUpdate() {
+    if (_needsCompositingBitsUpdate)
       return;
-    _needsCompositingUpdate = true;
-    final AbstractNode parent = this.parent;
+    _needsCompositingBitsUpdate = true;
+    final AbstractNode parent = this.parent; // TODO(ianh): remove the once the analyzer is cleverer
     if (parent is RenderObject)
-      parent.markNeedsCompositingUpdate();
+      parent.markNeedsCompositingBitsUpdate();
   }
-
-  void updateCompositing() {
-    if (!_needsCompositingUpdate)
+  bool _needsCompositing = false;
+  bool get needsCompositing {
+    // needsCompositing is true if either we have a layer or one of our descendants has a layer
+    assert(!_needsCompositingBitsUpdate); // make sure we don't use this bit when it is dirty
+    return _needsCompositing;
+  }
+  void updateCompositingBits() {
+    if (!_needsCompositingBitsUpdate)
       return;
+    bool didHaveCompositedDescendant = _needsCompositing;
     visitChildren((RenderObject child) {
-      child.updateCompositing();
-      if (child.hasCompositedDescendant)
-        _hasCompositedDescendant = true;
+      child.updateCompositingBits();
+      if (child.needsCompositing)
+        _needsCompositing = true;
     });
-    if (requiresCompositing)
-      _hasCompositedDescendant = true;
-    _needsCompositingUpdate = false;
+    if (hasLayer)
+      _needsCompositing = true;
+    if (didHaveCompositedDescendant != _needsCompositing)
+      markNeedsPaint();
+    _needsCompositingBitsUpdate = false;
   }
 
   bool _needsPaint = true;
   bool get needsPaint => _needsPaint;
-
   void markNeedsPaint() {
     assert(!debugDoingPaint);
     if (!attached) return; // Don't try painting things that aren't in the hierarchy
     if (_needsPaint) return;
-    if (requiresCompositing) {
+    if (hasLayer) {
+      // If we always have our own layer, then we can just repaint
+      // ourselves without involving any other nodes.
+      assert(_layer != null);
       _needsPaint = true;
       _nodesNeedingPaint.add(this);
       scheduler.ensureVisualUpdate();
-    } else if (parent is! RenderObject) {
-      // we're the root of the render tree (probably a RenderView)
+    } else if (parent is RenderObject) {
+      // We don't have our own layer; one of our ancestors will take
+      // care of updating the layer we're in and when they do that
+      // we'll get our paint() method called.
+      assert(_layer == null);
+      (parent as RenderObject).markNeedsPaint(); // TODO(ianh): remove the cast once the analyzer is cleverer
+    } else {
+      // If we're the root of the render tree (probably a RenderView),
+      // then we have to paint ourselves, since nobody else can paint
+      // us. We don't add ourselves to _nodesNeedingPaint in this
+      // case, because the root is always told to paint regardless.
       _needsPaint = true;
       scheduler.ensureVisualUpdate();
-    } else {
-      (parent as RenderObject).markNeedsPaint(); // TODO(ianh): remove the cast once the analyzer is cleverer
     }
   }
-
   static void flushPaint() {
     sky.tracing.begin('RenderObject.flushPaint');
     _debugDoingPaint = true;
     try {
       List<RenderObject> dirtyNodes = _nodesNeedingPaint;
       _nodesNeedingPaint = new List<RenderObject>();
-      for (RenderObject node in dirtyNodes..sort((a, b) => a.depth - b.depth)) {
-        if (node._needsPaint && node.attached)
+      // Sort the dirty nodes in reverse order (deepest first).
+      for (RenderObject node in dirtyNodes..sort((a, b) => b.depth - a.depth)) {
+        assert(node._needsPaint);
+        if (node.attached)
           node._repaint();
       };
       assert(_nodesNeedingPaint.length == 0);
@@ -485,33 +621,36 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       sky.tracing.end('RenderObject.flushPaint');
     }
   }
-
+  void initialPaint(ContainerLayer rootLayer, Size size) {
+    assert(attached);
+    assert(parent is! RenderObject);
+    assert(!_debugDoingPaint);
+    assert(hasLayer);
+    PaintingContext newContext = new PaintingContext.withLayer(rootLayer, Point.origin & size);
+    _paintLayer(newContext);
+  }
   void _repaint() {
-    assert(!_needsLayout);
-    assert(requiresCompositing);
+    assert(hasLayer);
     assert(_layer != null);
-    // TODO(abarth): Using _layer.offset isn't correct if the topLeft of our
-    // paint bounds has changed since our last repaint.
-    PaintingContext context = new PaintingContext(_layer.offset, paintBounds.size);
-    _layer.parent.add(context.layer, before: _layer);
-    _layer.detach();
-    _layer = context._layer;
-    _needsPaint = false;
+    PaintingContext newContext = new PaintingContext.replacingLayer(_layer, paintBounds);
+    _paintLayer(newContext);
+  }
+  void _paintLayer(PaintingContext context) {
+    _layer = context._containerLayer;
     try {
       _paintWithContext(context, Offset.zero);
+      context.endRecording();
     } catch (e) {
-      print('Exception raised during _repaint:\n${e}\nContext:\n${this}');
+      print('Exception raised during _paintLayer:\n${e}\nContext:\n${this}');
       if (inDebugBuild)
         rethrow;
       return;
     }
-    assert(!_needsLayout); // check that the paint() method didn't mark us dirty again
-    assert(!_needsPaint); // check that the paint() method didn't mark us dirty again
   }
-
   void _paintWithContext(PaintingContext context, Offset offset) {
-    _needsPaint = false;
     assert(!_debugDoingThisPaint);
+    assert(!_needsLayout);
+    assert(!_needsCompositingBitsUpdate);
     RenderObject debugLastActivePaint;
     assert(() {
       _debugDoingThisPaint = true;
@@ -522,9 +661,13 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
         context.canvas.save();
         context.canvas.clipRect(paintBounds.shift(offset));
       }
+      assert(!hasLayer || _layer != null);
       return true;
     });
+    _needsPaint = false;
     paint(context, offset);
+    assert(!_needsLayout); // check that the paint() method didn't mark us dirty again
+    assert(!_needsPaint); // check that the paint() method didn't mark us dirty again
     assert(() {
       if (debugPaintBoundsEnabled)
         context.canvas.restore();
@@ -532,7 +675,6 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       _debugDoingThisPaint = false;
       return true;
     });
-    assert(!_needsPaint);
   }
 
   Rect get paintBounds;

packages/flutter/lib/rendering/proxy_box.dart

@@ -295,18 +295,12 @@ class RenderOpacity extends RenderProxyBox {
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
       int a = _alpha;
-
       if (a == 0)
         return;
-
-      if (a == 255) {
+      if (a == 255)
         context.paintChild(child, offset.toPoint());
-        return;
-      }
-
-      context.canvas.saveLayer(null, _paint); // TODO(abarth): layerize
-      context.paintChild(child, offset.toPoint());
-      context.canvas.restore();
+      else
+        context.paintChildWithPaint(child, offset.toPoint(), null, _paint);
     }
   }
 }
@@ -349,11 +343,8 @@ class RenderColorFilter extends RenderProxyBox {
   }
 
   void paint(PaintingContext context, Offset offset) {
-    if (child != null) {
-      context.canvas.saveLayer(offset & size, _paint); // TODO(abarth): layerize
-      context.paintChild(child, offset.toPoint());
-      context.canvas.restore();
-    }
+    if (child != null)
+      context.paintChildWithPaint(child, offset.toPoint(), offset & size, _paint);
   }
 }
 
@@ -362,7 +353,7 @@ class RenderClipRect extends RenderProxyBox {
 
   void paint(PaintingContext context, Offset offset) {
     if (child != null)
-      context.paintChildWithClip(child, offset.toPoint(), Offset.zero & size);
+      context.paintChildWithClipRect(child, offset.toPoint(), offset & size);
   }
 }
 
@@ -393,16 +384,11 @@ class RenderClipRRect extends RenderProxyBox {
     markNeedsPaint();
   }
 
-  final Paint _paint = new Paint()..isAntiAlias = false;
-
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
       Rect rect = offset & size;
-      context.canvas.saveLayer(rect, _paint); // TODO(abarth): layerize
       sky.RRect rrect = new sky.RRect()..setRectXY(rect, xRadius, yRadius);
-      context.canvas.clipRRect(rrect);
-      context.paintChild(child, offset.toPoint());
-      context.canvas.restore();
+      context.paintChildWithClipRRect(child, offset.toPoint(), rect, rrect);
     }
   }
 }
@@ -410,8 +396,6 @@ class RenderClipRRect extends RenderProxyBox {
 class RenderClipOval extends RenderProxyBox {
   RenderClipOval({ RenderBox child }) : super(child);
 
-  final Paint _paint = new Paint()..isAntiAlias = false;
-
   Rect _cachedRect;
   Path _cachedPath;
 
@@ -426,10 +410,7 @@ class RenderClipOval extends RenderProxyBox {
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
       Rect rect = offset & size;
-      context.canvas.saveLayer(rect, _paint); // TODO(abarth): layerize
-      context.canvas.clipPath(_getPath(rect));
-      context.paintChild(child, offset.toPoint());
-      context.canvas.restore();
+      context.paintChildWithClipPath(child, offset.toPoint(), rect, _getPath(rect));
     }
   }
 }
@@ -561,11 +542,8 @@ class RenderTransform extends RenderProxyBox {
   }
 
   void paint(PaintingContext context, Offset offset) {
-    context.canvas.save();
-    context.canvas.translate(offset.dx, offset.dy);
-    context.canvas.concat(_transform.storage);
-    super.paint(context, Offset.zero);
-    context.canvas.restore();
+    if (child != null)
+      context.paintChildWithTransform(child, offset.toPoint(), _transform);
   }
 
   void applyPaintTransform(Matrix4 transform) {

packages/flutter/lib/rendering/sky_binding.dart

@@ -72,7 +72,7 @@ class SkyBinding {
   }
   void beginFrame(double timeStamp) {
     RenderObject.flushLayout();
-    _renderView.updateCompositing();
+    _renderView.updateCompositingBits();
     RenderObject.flushPaint();
     _renderView.paintFrame();
     _renderView.compositeFrame();

packages/flutter/lib/rendering/view.dart

@@ -42,8 +42,6 @@ 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; }
@@ -79,6 +77,8 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
     return true;
   }
 
+  bool get hasLayer => true;
+
   void paint(PaintingContext context, Offset offset) {
     if (child != null)
       context.paintChild(child, offset.toPoint());
@@ -88,12 +88,9 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
     sky.tracing.begin('RenderView.paintFrame');
     try {
       final double devicePixelRatio = sky.view.devicePixelRatio;
-      Matrix4 transform = new Matrix4.diagonal3Values(devicePixelRatio, devicePixelRatio, 1.0);
-      _rootLayer = new TransformLayer(transform: transform);
-      PaintingContext context = new PaintingContext(Offset.zero, size);
-      _rootLayer.add(context.layer);
-      context.paintChild(child, Point.origin);
-      context.endRecording();
+      Matrix4 logicalToDeviceZoom = new Matrix4.diagonal3Values(devicePixelRatio, devicePixelRatio, 1.0);
+      ContainerLayer rootLayer = new TransformLayer(transform: logicalToDeviceZoom);
+      initialPaint(rootLayer, size);
     } finally {
       sky.tracing.end('RenderView.paintFrame');
     }
@@ -102,9 +99,12 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
   void compositeFrame() {
     sky.tracing.begin('RenderView.compositeFrame');
     try {
+      // Eventually we will want to pass the entire layer tree to the C++ side.
+      // For now, however, we take the layer tree and paint it into a Canvas,
+      // which we then hand to the C++ side.
       sky.PictureRecorder recorder = new sky.PictureRecorder();
       sky.Canvas canvas = new sky.Canvas(recorder, Point.origin & (size * sky.view.devicePixelRatio));
-      _rootLayer.paint(canvas);
+      layer.paint(canvas);
       sky.view.picture = recorder.endRecording();
     } finally {
       sky.tracing.end('RenderView.compositeFrame');

packages/flutter/lib/rendering/viewport.dart

@@ -122,13 +122,10 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
       Offset roundedScrollOffset = _scrollOffsetRoundedToIntegerDevicePixels;
       bool _needsClip = offset < Offset.zero ||
                         !(offset & size).contains(((offset - roundedScrollOffset) & child.size).bottomRight);
-      if (_needsClip) {
-        context.canvas.save();
-        context.canvas.clipRect(offset & size);
-      }
-      context.paintChild(child, (offset - roundedScrollOffset).toPoint());
       if (_needsClip)
-        context.canvas.restore();
+        context.paintChildWithClipRect(child, (offset - roundedScrollOffset).toPoint(), offset & size);
+      else
+        context.paintChild(child, (offset - roundedScrollOffset).toPoint());
     }
   }