Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

src/image/SkSurface_Raster.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Base.h"
 #include "SkImagePriv.h"
 #include "SkCanvas.h"
 #include "SkDevice.h"
 #include "SkMallocPixelRef.h"
 
 static const size_t kIgnoreRowBytesValue = (size_t)~0;
 
 class SkSurface_Raster : public SkSurface_Base {
 public:
     static bool Valid(const SkImage::Info&, size_t rb = kIgnoreRowBytesValue);
 
     SkSurface_Raster(const SkImage::Info&, void*, size_t rb);
     SkSurface_Raster(const SkImage::Info&, SkPixelRef*, size_t rb);
 
     virtual SkCanvas* onNewCanvas() SK_OVERRIDE;
     virtual SkSurface* onNewSurface(const SkImage::Info&) SK_OVERRIDE;
     virtual SkImage* onNewImageSnapshot() SK_OVERRIDE;
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
                         const SkPaint*) SK_OVERRIDE;
-    virtual void onCopyOnWrite() SK_OVERRIDE;
+    virtual void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
 
 private:
     SkBitmap    fBitmap;
     bool        fWeOwnThePixels;
 
     typedef SkSurface_Base INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
@@ skipping 78 matching lines @@
 
 void SkSurface_Raster::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y,
                               const SkPaint* paint) {
     canvas->drawBitmap(fBitmap, x, y, paint);
 }
 
 SkImage* SkSurface_Raster::onNewImageSnapshot() {
     return SkNewImageFromBitmap(fBitmap, fWeOwnThePixels);
 }
 
-void SkSurface_Raster::onCopyOnWrite() {
+void SkSurface_Raster::onCopyOnWrite(ContentChangeMode mode) {
     // are we sharing pixelrefs with the image?
-    SkASSERT(NULL !=this->getCachedImage());
+    SkASSERT(NULL != this->getCachedImage());
     if (SkBitmapImageGetPixelRef(this->getCachedImage()) == fBitmap.pixelRef()) {
         SkASSERT(fWeOwnThePixels);
-        SkBitmap prev(fBitmap);
-        prev.deepCopyTo(&fBitmap, prev.config());
+        if (kDiscard_ContentChangeMode == mode) {
+            fBitmap.setPixelRef(NULL, 0);
+            fBitmap.allocPixels();
+        } else {
+            SkBitmap prev(fBitmap);
+            prev.deepCopyTo(&fBitmap, prev.config());
+        }
         // Now fBitmap is a deep copy of itself (and therefore different from
         // what is being used by the image. Next we update the canvas to use
         // this as its backend, so we can't modify the image's pixels anymore.
         SkASSERT(NULL != this->getCachedCanvas());
         this->getCachedCanvas()->getDevice()->replaceBitmapBackendForRasterSurface(fBitmap);
     }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
@@ skipping 22 matching lines @@
 
     size_t size = (size_t)size64;
     void* pixels = sk_malloc_throw(size);
     if (NULL == pixels) {
         return NULL;
     }
 
     SkAutoTUnref<SkPixelRef> pr(SkNEW_ARGS(SkMallocPixelRef, (pixels, size, NULL, true)));
     return SkNEW_ARGS(SkSurface_Raster, (info, pr, rowBytes));
 }