Optionally clear PlatformCanvas instances.

skia/ext/bitmap_platform_device_android.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKIA_EXT_BITMAP_PLATFORM_DEVICE_ANDROID_H_
 #define SKIA_EXT_BITMAP_PLATFORM_DEVICE_ANDROID_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/compiler_specific.h"
 #include "skia/ext/platform_device.h"
 
 namespace skia {
 
 // -----------------------------------------------------------------------------
 // For now we just use SkBitmap for SkDevice
 //
 // This is all quite ok for test_shell. In the future we will want to use
 // shared memory between the renderer and the main process at least. In this
 // case we'll probably create the buffer from a precreated region of memory.
 // -----------------------------------------------------------------------------
 class BitmapPlatformDevice : public PlatformDevice, public SkDevice {
  public:
+  // Construct a BitmapPlatformDevice. |is_opaque| should be set if the caller
+  // knows the bitmap will be completely opaque and allows some optimizations.
+  // The bitmap is not initialized.
   static BitmapPlatformDevice* Create(int width, int height, bool is_opaque);
 
-  // This doesn't take ownership of |data|
+  // Construct a BitmapPlatformDevice, as above.
+  // If |is_opaque| is false, the bitmap is initialized to 0.
+  static BitmapPlatformDevice* CreateAndClear(int width, int height,
+                                              bool is_opaque);
+
+  // This doesn't take ownership of |data|. If |data| is null, the bitmap
+  // is not initialized to 0.
   static BitmapPlatformDevice* Create(int width, int height, bool is_opaque,
                                       uint8_t* data);
 
   // Create a BitmapPlatformDevice from an already constructed bitmap;
   // you should probably be using Create(). This may become private later if
   // we ever have to share state between some native drawing UI and Skia, like
   // the Windows and Mac versions of this class do.
   explicit BitmapPlatformDevice(const SkBitmap& other);
   virtual ~BitmapPlatformDevice();
 
   virtual PlatformSurface BeginPlatformPaint() OVERRIDE;
   virtual void DrawToNativeContext(PlatformSurface surface, int x, int y,
                                    const PlatformRect* src_rect) OVERRIDE;
 
  protected:
   virtual SkDevice* onCreateCompatibleDevice(SkBitmap::Config, int width,
                                              int height, bool isOpaque,
                                              Usage usage) OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(BitmapPlatformDevice);
 };
 
 }  // namespace skia
 
 #endif  // SKIA_EXT_BITMAP_PLATFORM_DEVICE_ANDROID_H_