ShaderMask needs to play nicely with compositing

This patch adds ShaderLayer and makes ShaderMask use it.

Fixes #1155

examples/widgets/card_collection.dart

@@ -393,8 +393,8 @@ class CardCollectionState extends State<CardCollection> {
 
   ui.Shader _createShader(Rect bounds) {
     return new LinearGradient(
-        begin: Point.origin,
+        begin: bounds.topLeft,
-        end: new Point(0.0, bounds.height),
+        end: bounds.bottomLeft,
         colors: <Color>[const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],
         stops: <double>[0.1, 0.35]
     )

packages/flutter/lib/src/rendering/layer.dart

@@ -392,3 +392,31 @@ class OpacityLayer extends ContainerLayer {
     settings.add('alpha: $alpha');
   }
 }
+
+/// A composited layer that applies a shader to hits children.
+class ShaderMaskLayer extends ContainerLayer {
+  ShaderMaskLayer({ Offset offset: Offset.zero, this.shader, this.maskRect, this.transferMode }) : super(offset: offset);
+
+  /// The shader to apply to the children.
+  ui.Shader shader;
+
+  /// The size of the shader.
+  Rect maskRect;
+
+  /// The tranfer mode to apply when blending the shader with the children.
+  TransferMode transferMode;
+
+  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+    Offset childOffset = offset + layerOffset;
+    builder.pushShaderMask(shader, maskRect.shift(childOffset), transferMode);
+    addChildrenToScene(builder, childOffset);
+    builder.pop();
+  }
+
+  void debugDescribeSettings(List<String> settings) {
+    super.debugDescribeSettings(settings);
+    settings.add('shader: $shader');
+    settings.add('maskRect: $maskRect');
+    settings.add('transferMode: $transferMode');
+  }
+}

packages/flutter/lib/src/rendering/object.dart

@@ -18,8 +18,6 @@ import 'node.dart';
 export 'layer.dart';
 export 'package:flutter/gestures.dart' show HitTestEntry, HitTestResult;
 
-typedef ui.Shader ShaderCallback(Rect bounds);
-
 /// Base class for data associated with a [RenderObject] by its parent.
 ///
 /// Some render objects wish to store data on their children, such as their
@@ -298,27 +296,21 @@ class PaintingContext {
     childContext._stopRecordingIfNeeded();
   }
 
-  static Paint _getPaintForShaderMask(Rect bounds,
-                                      ShaderCallback shaderCallback,
-                                      TransferMode transferMode) {
-    return new Paint()
-     ..transferMode = transferMode
-     ..shader = shaderCallback(bounds);
-  }
-
   /// Push a shader mask.
   ///
   /// This function will call painter synchronously with a painting context that
   /// will be masked with the given shader.
-  ///
+  void pushShaderMask(Offset offset, ui.Shader shader, Rect maskRect, TransferMode transferMode, PaintingContextCallback painter) {
-  /// WARNING: This function does not yet support compositing.
+    _stopRecordingIfNeeded();
-  void pushShaderMask(bool needsCompositing, Offset offset, Rect bounds, ShaderCallback shaderCallback, TransferMode transferMode, PaintingContextCallback painter) {
+    ShaderMaskLayer shaderLayer = new ShaderMaskLayer(
-    assert(!needsCompositing); // TODO(abarth): Implement compositing for shader masks.
+      shader: shader,
-    canvas.saveLayer(bounds.shift(offset), _disableAntialias);
+      maskRect: maskRect,
-    painter(this, offset);
+      transferMode: transferMode
-    Paint shaderPaint = _getPaintForShaderMask(bounds, shaderCallback, transferMode);
+    );
-    canvas.drawRect(bounds, shaderPaint);
+    _appendLayer(shaderLayer, offset);
-    canvas.restore();
+    PaintingContext childContext = new PaintingContext._(shaderLayer, _paintBounds);
+    painter(childContext, Offset.zero);
+    childContext._stopRecordingIfNeeded();
   }
 }
 

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -540,7 +540,7 @@ class RenderOpacity extends RenderProxyBox {
     assert(opacity >= 0.0 && opacity <= 1.0);
   }
 
-  bool get alwaysNeedsCompositing => _alpha != 0 && _alpha != 255;
+  bool get alwaysNeedsCompositing => child != null && (_alpha != 0 && _alpha != 255);
 
   /// The fraction to scale the child's alpha value.
   ///
@@ -580,6 +580,8 @@ class RenderOpacity extends RenderProxyBox {
   }
 }
 
+typedef ui.Shader ShaderCallback(Rect bounds);
+
 class RenderShaderMask extends RenderProxyBox {
   RenderShaderMask({ RenderBox child, ShaderCallback shaderCallback, TransferMode transferMode })
     : _shaderCallback = shaderCallback, _transferMode = transferMode, super(child);
@@ -604,9 +606,14 @@ class RenderShaderMask extends RenderProxyBox {
     markNeedsPaint();
   }
 
+  bool get alwaysNeedsCompositing => child != null;
+
   void paint(PaintingContext context, Offset offset) {
-    if (child != null)
+    if (child != null) {
-      context.pushShaderMask(needsCompositing, offset, Point.origin & size, _shaderCallback, _transferMode, super.paint);
+      assert(needsCompositing);
+      Rect rect = Point.origin & size;
+      context.pushShaderMask(offset, _shaderCallback(rect), rect, _transferMode, super.paint);
+    }
   }
 }
 

packages/flutter/test/widget/shader_mask_test.dart

@@ -11,8 +11,8 @@ import 'package:test/test.dart';
 
 ui.Shader createShader(Rect bounds) {
   return new LinearGradient(
-      begin: Point.origin,
+      begin: bounds.topLeft,
-      end: new Point(0.0, bounds.height),
+      end: bounds.bottomLeft,
       colors: <Color>[const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],
       stops: <double>[0.1, 0.35]
   )