Adds API for retaining intermediate engine layers

lib/stub_ui/lib/src/engine/compositor/layer.dart

@@ -8,7 +8,7 @@ part of engine;
 ///
 /// A layer is the lowest-level rendering primitive. It represents an atomic
 /// painting command.
-abstract class Layer {
+abstract class Layer implements ui.EngineLayer {
   /// The layer that contains us as a child.
   ContainerLayer parent;
 
@@ -173,7 +173,7 @@ class ClipRRectLayer extends ContainerLayer {
 }
 
 /// A layer that transforms its child layers by the given transform matrix.
-class TransformLayer extends ContainerLayer {
+class TransformLayer extends ContainerLayer implements ui.OffsetEngineLayer, ui.TransformEngineLayer {
   /// The matrix with which to transform the child layers.
   final Matrix4 _transform;
 
@@ -287,7 +287,7 @@ class PictureLayer extends Layer {
 ///
 /// The shape clips its children to a given [Path], and casts a shadow based
 /// on the given elevation.
-class PhysicalShapeLayer extends ContainerLayer {
+class PhysicalShapeLayer extends ContainerLayer implements ui.PhysicalShapeEngineLayer {
   final double _elevation;
   final ui.Color _color;
   final ui.Color _shadowColor;

lib/stub_ui/lib/src/engine/compositor/layer_scene_builder.dart

@@ -4,12 +4,6 @@
 
 part of engine;
 
-class EngineLayerImpl extends ui.EngineLayer {
-  final ContainerLayer _layer;
-
-  EngineLayerImpl(this._layer);
-}
-
 class LayerScene implements ui.Scene {
   final LayerTree layerTree;
 
@@ -60,7 +54,7 @@ class LayerSceneBuilder implements ui.SceneBuilder {
   @override
   void addRetained(ui.EngineLayer retainedLayer) {
     if (currentLayer == null) return;
-    currentLayer.add((retainedLayer as EngineLayerImpl)._layer);
+    currentLayer.add(retainedLayer);
   }
 
   @override
@@ -95,80 +89,81 @@ class LayerSceneBuilder implements ui.SceneBuilder {
   }
 
   @override
-  ui.EngineLayer pushBackdropFilter(ui.ImageFilter filter,
-      {Object webOnlyPaintedBy}) {
+  ui.BackdropFilterEngineLayer pushBackdropFilter(ui.ImageFilter filter,
+      {ui.BackdropFilterEngineLayer oldLayer}) {
     throw new UnimplementedError();
   }
 
   @override
-  ui.EngineLayer pushClipPath(ui.Path path,
-      {ui.Clip clipBehavior = ui.Clip.antiAlias, Object webOnlyPaintedBy}) {
+  ui.ClipPathEngineLayer pushClipPath(ui.Path path,
+      {ui.Clip clipBehavior = ui.Clip.antiAlias, ui.ClipPathEngineLayer oldLayer}) {
     pushLayer(ClipPathLayer(path));
     return null;
   }
 
   @override
-  ui.EngineLayer pushClipRRect(ui.RRect rrect,
-      {ui.Clip clipBehavior, Object webOnlyPaintedBy}) {
+  ui.ClipRRectEngineLayer pushClipRRect(ui.RRect rrect,
+      {ui.Clip clipBehavior, ui.ClipRRectEngineLayer oldLayer}) {
     pushLayer(ClipRRectLayer(rrect));
     return null;
   }
 
   @override
-  ui.EngineLayer pushClipRect(ui.Rect rect,
-      {ui.Clip clipBehavior = ui.Clip.antiAlias, Object webOnlyPaintedBy}) {
+  ui.ClipRectEngineLayer pushClipRect(ui.Rect rect,
+      {ui.Clip clipBehavior = ui.Clip.antiAlias, ui.ClipRectEngineLayer oldLayer}) {
     pushLayer(ClipRectLayer(rect));
     return null;
   }
 
   @override
-  ui.EngineLayer pushColorFilter(ui.Color color, ui.BlendMode blendMode,
-      {Object webOnlyPaintedBy}) {
+  ui.ColorFilterEngineLayer pushColorFilter(ui.Color color, ui.BlendMode blendMode,
+      {ui.ColorFilterEngineLayer oldLayer}) {
     throw new UnimplementedError();
   }
 
   @override
-  ui.EngineLayer pushOffset(double dx, double dy, {Object webOnlyPaintedBy}) {
+  ui.OffsetEngineLayer pushOffset(double dx, double dy, {ui.OffsetEngineLayer oldLayer}) {
     final matrix = Matrix4.translationValues(dx, dy, 0.0);
     final layer = TransformLayer(matrix);
     pushLayer(layer);
-    return EngineLayerImpl(layer);
+    return layer;
   }
 
   @override
-  ui.EngineLayer pushOpacity(int alpha,
-      {Object webOnlyPaintedBy, ui.Offset offset = ui.Offset.zero}) {
+  ui.OpacityEngineLayer pushOpacity(int alpha,
+      {ui.OpacityEngineLayer oldLayer, ui.Offset offset = ui.Offset.zero}) {
     // TODO(het): Implement opacity
     pushOffset(0.0, 0.0);
     return null;
   }
 
   @override
-  ui.EngineLayer pushPhysicalShape(
+  ui.PhysicalShapeEngineLayer pushPhysicalShape(
       {ui.Path path,
       double elevation,
       ui.Color color,
       ui.Color shadowColor,
       ui.Clip clipBehavior = ui.Clip.none,
-      Object webOnlyPaintedBy}) {
+      ui.PhysicalShapeEngineLayer oldLayer}) {
     final layer =
         PhysicalShapeLayer(elevation, color, shadowColor, path, clipBehavior);
     pushLayer(layer);
-    return EngineLayerImpl(layer);
+    return layer;
   }
 
   @override
-  ui.EngineLayer pushShaderMask(
+  ui.ShaderMaskEngineLayer pushShaderMask(
       ui.Shader shader, ui.Rect maskRect, ui.BlendMode blendMode,
-      {Object webOnlyPaintedBy}) {
+      {ui.ShaderMaskEngineLayer oldLayer}) {
     throw new UnimplementedError();
   }
 
   @override
-  ui.EngineLayer pushTransform(Float64List matrix4, {Object webOnlyPaintedBy}) {
-    final matrix = Matrix4.fromList(matrix4);
-    pushLayer(TransformLayer(matrix));
-    return null;
+  ui.TransformEngineLayer pushTransform(Float64List matrix4, {ui.TransformEngineLayer oldLayer}) {
+    final Matrix4 matrix = Matrix4.fromList(matrix4);
+    final TransformLayer layer = TransformLayer(matrix);
+    pushLayer(layer);
+    return layer;
   }
 
   @override

lib/stub_ui/lib/src/engine/surface/clip.dart

@@ -56,7 +56,7 @@ mixin _DomClip on PersistedContainerSurface {
 }
 
 /// A surface that creates a rectangular clip.
-class PersistedClipRect extends PersistedContainerSurface with _DomClip {
+class PersistedClipRect extends PersistedContainerSurface with _DomClip implements ui.ClipRectEngineLayer {
   PersistedClipRect(Object paintedBy, this.rect) : super(paintedBy);
 
   final ui.Rect rect;
@@ -99,7 +99,7 @@ class PersistedClipRect extends PersistedContainerSurface with _DomClip {
 }
 
 /// A surface that creates a rounded rectangular clip.
-class PersistedClipRRect extends PersistedContainerSurface with _DomClip {
+class PersistedClipRRect extends PersistedContainerSurface with _DomClip implements ui.ClipRRectEngineLayer {
   PersistedClipRRect(Object paintedBy, this.rrect, this.clipBehavior)
       : super(paintedBy);
 
@@ -148,7 +148,7 @@ class PersistedClipRRect extends PersistedContainerSurface with _DomClip {
   }
 }
 
-class PersistedPhysicalShape extends PersistedContainerSurface with _DomClip {
+class PersistedPhysicalShape extends PersistedContainerSurface with _DomClip implements ui.PhysicalShapeEngineLayer {
   PersistedPhysicalShape(Object paintedBy, this.path, this.elevation, int color,
       int shadowColor, this.clipBehavior)
       : this.color = ui.Color(color),
@@ -313,7 +313,7 @@ class PersistedPhysicalShape extends PersistedContainerSurface with _DomClip {
 }
 
 /// A surface that clips it's children.
-class PersistedClipPath extends PersistedContainerSurface {
+class PersistedClipPath extends PersistedContainerSurface implements ui.ClipPathEngineLayer {
   PersistedClipPath(Object paintedBy, this.clipPath, this.clipBehavior)
       : super(paintedBy);
 

lib/stub_ui/lib/src/engine/surface/offset.dart

@@ -5,7 +5,7 @@
 part of engine;
 
 /// A surface that translates its children using CSS transform and translate.
-class PersistedOffset extends PersistedContainerSurface {
+class PersistedOffset extends PersistedContainerSurface implements ui.OffsetEngineLayer {
   PersistedOffset(Object paintedBy, this.dx, this.dy) : super(paintedBy);
 
   /// Horizontal displacement.

lib/stub_ui/lib/src/engine/surface/opacity.dart

@@ -5,7 +5,7 @@
 part of engine;
 
 /// A surface that makes its children transparent.
-class PersistedOpacity extends PersistedContainerSurface {
+class PersistedOpacity extends PersistedContainerSurface implements ui.OpacityEngineLayer {
   PersistedOpacity(Object paintedBy, this.alpha, this.offset)
       : super(paintedBy);
 

lib/stub_ui/lib/src/engine/surface/transform.dart

@@ -5,7 +5,7 @@
 part of engine;
 
 /// A surface that transforms its children using CSS transform.
-class PersistedTransform extends PersistedContainerSurface {
+class PersistedTransform extends PersistedContainerSurface implements ui.TransformEngineLayer {
   PersistedTransform(Object paintedBy, this.matrix4) : super(paintedBy);
 
   final Float64List matrix4;