Implement SkBaseDevice snapshot support

src/gpu/SkGpuDevice.h

Index: src/gpu/SkGpuDevice.h
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 07eb348f8de41ad66efd990e2f1cb46dbf428780..d166abc290ea184b3e48b0c98138f3117cc3f193 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -24,6 +24,7 @@ struct GrSkDrawProcs;
 class GrAccelData;
 struct GrCachedLayer;
 class GrTextContext;
+class SkSurface_Gpu;
 
 /**
  *  Subclass of SkBaseDevice, which directs all drawing to the GrGpu owned by the
@@ -50,6 +51,10 @@ public:
 
     virtual ~SkGpuDevice();
 
+    void setSurface(SkSurface_Gpu* surface) {
+        fSurface = surface;
+    }
+
     SkGpuDevice* cloneDevice(const SkSurfaceProps& props) {
         SkBaseDevice* dev = this->onCreateDevice(CreateInfo(this->imageInfo(), kPossible_TileUsage,
                                                             props.pixelGeometry()),
@@ -62,8 +67,6 @@ public:
     // set all pixels to 0
     void clearAll();
 
-    void replaceRenderTarget(bool shouldRetainContent);
-
     GrRenderTarget* accessRenderTarget() SK_OVERRIDE;
 
     SkImageInfo imageInfo() const SK_OVERRIDE {
@@ -72,6 +75,12 @@ public:
 
     const SkSurfaceProps& surfaceProps() const { return fSurfaceProps; }
 
+    void discard() SK_OVERRIDE;
+    SkImage* newImageSnapshot() SK_OVERRIDE;
+    SkImage* newImageSnapshot(const SkSurfaceProps* props, SkSurface::Budgeted);
+    int getBackendSampleCnt() const { return fRenderTarget->numSamples(); }
+    bool isBackendBudgeted() const;
+
     void drawPaint(const SkDraw&, const SkPaint& paint) SK_OVERRIDE;
     virtual void drawPoints(const SkDraw&, SkCanvas::PointMode mode, size_t count,
                             const SkPoint[], const SkPaint& paint) SK_OVERRIDE;
@@ -146,6 +155,8 @@ private:
     // remove when our clients don't rely on accessBitmap()
     SkBitmap                        fLegacyBitmap;
     bool                            fNeedClear;
+    SkSurface_Gpu*                  fSurface;
+    SkAutoTUnref<SkImage>           fSnapshot;
 
     SkGpuDevice(GrRenderTarget*, const SkSurfaceProps*, unsigned flags);
 
@@ -157,8 +168,22 @@ private:
 
     bool forceConservativeRasterClip() const SK_OVERRIDE { return true; }
 
-    // sets the render target and clip on context
-    void prepareDraw(const SkDraw&);
+    /* Prepares the device for modification and sets the clip on context.
+     * returns true if the draw can proceed. */
+    bool prepareDraw(const SkDraw&);
+
+    /* Prepares the device for modification.
+     * returns true if the draw can proceed. */
+    bool prepareWrite();
+
+    enum ModifyMode { kDrawContent_ModifyMode, kDiscard_ModifyMode };
+    /** Prepares the render target for modification.
+     * 1. Detaches the snapshot, if needed.
+     * 2. Creates new render target if snapshot refers to the current render target.
+     * 3. Does the deferred clear, if needed.
+     * returns true if the render target is functional and can be rendered to.
+     */
+    bool prepareBackendRenderTarget(ModifyMode);
 
     /**
      * Implementation for both drawBitmap and drawBitmapRect.