Clarify GraphicsContext::beginLayer()/endLayer() have unexpected behaviors.

Source/platform/graphics/GraphicsContext.h

Index: Source/platform/graphics/GraphicsContext.h
diff --git a/Source/platform/graphics/GraphicsContext.h b/Source/platform/graphics/GraphicsContext.h
index 42189353bdfe69e13ce2c54ddf4266a8c248cf2f..adbba90477ea2bd52829fede55cf85628622fe66 100644
--- a/Source/platform/graphics/GraphicsContext.h
+++ b/Source/platform/graphics/GraphicsContext.h
@@ -98,9 +98,6 @@ public:
     void disableDestructionChecks() { m_disableDestructionChecks = true; }
 #endif
 
-    void saveLayer(const SkRect* bounds, const SkPaint*);
-    void restoreLayer();
-
     bool hasStroke() const { return strokeStyle() != NoStroke && strokeThickness() > 0; }
 
     float strokeThickness() const { return immutableState()->strokeData().thickness(); }
@@ -326,6 +323,8 @@ public:
     void drawLineForDocumentMarker(const FloatPoint&, float width, DocumentMarkerLineStyle);
 
     void beginTransparencyLayer(float opacity, const FloatRect* = 0);
+    // Apply CompositeOperator when the layer is composited on the backdrop (i.e. endLayer()).
+    // Don't change the current CompositeOperator state.
     void beginLayer(float opacity, CompositeOperator, const FloatRect* = 0, ColorFilter = ColorFilterNone, ImageFilter* = 0);
     void endLayer();
 
@@ -436,6 +435,9 @@ private:
     static void draw2xMarker(SkBitmap*, int);
 #endif
 
+    void saveLayer(const SkRect* bounds, const SkPaint*);
+    void restoreLayer();

[dshwang, 2014/10/16 15:58:20]

go to private:

[Justin Novosad, 2014/10/16 17:51:58]

Good idea

+
     // Helpers for drawing a focus ring (drawFocusRing)
     float prepareFocusRingPaint(SkPaint&, const Color&, int width) const;
     void drawFocusRingPath(const SkPath&, const Color&, int width);
@@ -492,9 +494,14 @@ private:
 
 #if ENABLE(ASSERT)
     unsigned m_annotationCount;
-    unsigned m_layerCount;
     bool m_disableDestructionChecks;
 #endif
+    // Keep the top of the clip stack of each layer. It's needed because beginLayer()/endLayer() has the different contract
+    // from SkCanvas::saveLayer()/restore(). When endLayer() calls SkCanvas::restore(), the clip state is restored before beginLayer().
+    // This member is used to re-apply all clip operations after beginLayer(). Important to note that SkCanvas::saveLayer()
+    // may perform an additional clip operation, which prevent endLayer() from getting the stack pointer by itself.
+    Vector<unsigned> m_clipStackPointersOfLayerStack;
+
     // Tracks the region painted opaque via the GraphicsContext.
     RegionTracker m_trackedRegion;