Modifications to DisplayList for better Slimming Paint

Source/platform/graphics/DisplayList.h

Index: Source/platform/graphics/DisplayList.h
diff --git a/Source/platform/graphics/DisplayList.h b/Source/platform/graphics/DisplayList.h
index 9d8623c1b7f656483476aae946adcf33d2378cf4..d18fea770dc930de1b10b82ace4d734842033a3f 100644
--- a/Source/platform/graphics/DisplayList.h
+++ b/Source/platform/graphics/DisplayList.h
@@ -32,15 +32,13 @@
 #define DisplayList_h
 
 #include "platform/geometry/FloatRect.h"
-
+#include "platform/geometry/IntRect.h"
+#include "platform/geometry/LayoutPoint.h"
+#include "third_party/skia/include/core/SkPicture.h"
 #include "wtf/FastAllocBase.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 
-class SkCanvas;
-class SkPicture;
-class SkPictureRecorder;
-
 namespace blink {
 
 class IntSize;
@@ -49,25 +47,49 @@ class PLATFORM_EXPORT DisplayList FINAL : public WTF::RefCounted<DisplayList> {
     WTF_MAKE_FAST_ALLOCATED;
     WTF_MAKE_NONCOPYABLE(DisplayList);
 public:
-    DisplayList(const FloatRect&);
-    ~DisplayList();
+    static PassRefPtr<DisplayList> create(const FloatRect& bounds)
+    {
+        return adoptRef(new DisplayList(bounds));
+    }
+
+    virtual ~DisplayList() { }
 
-    const FloatRect& bounds() const;
+    const FloatRect& bounds() const { return m_bounds; }
 
     // This entry point will return 0 when the DisplayList is in the
     // midst of recording (i.e., between a beginRecording/endRecording pair)
     // and if no recording has ever been completed. Otherwise it will return
     // the picture created by the last endRecording call.
-    SkPicture* picture() const;
+    SkPicture* picture() const { return m_picture.get(); }
+    void setPicture(SkPicture* picture) { m_picture = adoptRef(picture); }
+    void clearPicture() { m_picture.clear(); }
 
-    SkCanvas* beginRecording(const IntSize&, uint32_t recordFlags = 0);
-    bool isRecording() const { return m_recorder; }
-    void endRecording();
+    const SkMatrix& transform() const { return m_transform; }
+    void setTransform(const SkMatrix& transform) { m_transform = transform; }
+    void setTransformFromPaintOffset(const LayoutPoint& paintOffset)
+    {
+        SkMatrix m;
+        m.setTranslate(paintOffset.x().toFloat(), paintOffset.y().toFloat());
+        setTransform(m);
+    }
+    void clearTransform() { m_transform.reset(); }
+
+    const SkRect& clip() const { return m_clip; }
+    void setClip(const IntRect& rect) { m_clip = rect; }

[chrishtr, 2014/09/02 22:05:58]

Can you sketch the use cases for clip and transform?

I'd ideally like to see these methods added only along with code that itself
provides a use case.

[Stephen Chennney, 2014/09/03 14:19:37]

In 2 of the 3 cases I converted (LinkHighlight and RenderImage) a clip is
applied immediately to restrict drawing to the content area. 

And all of them need a transform because the paintOffset argument is ubiquitous
to paint methods and always needs to be applied.

Furthermore, Florin and I talked about the problem of balancing clips and
transforms and we both agreed that it is better to keep these out of the
SkPictures that are cached, so instead we put them on the DisplayList.

[chrishtr, 2014/09/03 17:02:24]

Setting clip and transform externally to DisplayList is consistent
with the current plans.

[chrishtr, 2014/09/03 17:03:02]

Sent that comment too soon. Meant to say: therefore ok.

+    void setClip(const FloatRect& rect) { m_clip = rect; }
+    void clearClip() { m_clip.setEmpty(); }
 
 private:
+    DisplayList(const FloatRect& bounds) : m_bounds(bounds)
+    {
+        clearTransform();
+        clearClip();
+    }
+
     FloatRect m_bounds;
+    SkMatrix m_transform;
+    SkRect m_clip; // TODO: Do we need to support other types of clips here?
     RefPtr<SkPicture> m_picture;
-    OwnPtr<SkPictureRecorder> m_recorder;
 };
 
 } // namespace blink