Optionally clear PlatformCanvas instances.

skia/ext/bitmap_platform_device_mac.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_MAC_H_
 #define SKIA_EXT_BITMAP_PLATFORM_DEVICE_MAC_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
@@ skipping 10 matching lines @@
 //
 // The device owns the pixel data, when the device goes away, the pixel data
 // also becomes invalid. THIS IS DIFFERENT THAN NORMAL SKIA which uses
 // reference counting for the pixel data. In normal Skia, you could assign
 // another bitmap to this device's bitmap and everything will work properly.
 // For us, that other bitmap will become invalid as soon as the device becomes
 // invalid, which may lead to subtle bugs. Therefore, DO NOT ASSIGN THE
 // DEVICE'S PIXEL DATA TO ANOTHER BITMAP, make sure you copy instead.
 class BitmapPlatformDevice : public PlatformDevice, public SkDevice {
  public:
-  // |context| may be NULL.
+  // Creates a BitmapPlatformDevice instance. |is_opaque| should be set if the
+  // caller knows the bitmap will be completely opaque and allows some
+  // optimizations.
+  // |context| may be NULL. If |context| is NULL, then the bitmap backing store
+  // is not initialized.
   static BitmapPlatformDevice* Create(CGContextRef context,
                                       int width, int height,
                                       bool is_opaque);
 
+  // Creates a BitmapPlatformDevice instance.  If |is_opaque| is false,
+  // then the bitmap is initialzed to 0.
+  static BitmapPlatformDevice* CreateAndClear(int width, int height,
+                                              bool is_opaque);
+
   // Creates a context for |data| and calls Create.
+  // If |data| is NULL, then the bitmap backing store is not initialized.
   static BitmapPlatformDevice* CreateWithData(uint8_t* data,
                                               int width, int height,
                                               bool is_opaque);
 
   virtual ~BitmapPlatformDevice();
 
   // PlatformDevice overrides
   virtual CGContextRef GetBitmapContext() OVERRIDE;
   virtual void DrawToNativeContext(CGContextRef context, int x, int y,
                                    const CGRect* src_rect) OVERRIDE;
@@ skipping 23 matching lines @@
   // Data associated with this device, guaranteed non-null. We hold a reference
   // to this object.
   BitmapPlatformDeviceData* data_;
 
   DISALLOW_COPY_AND_ASSIGN(BitmapPlatformDevice);
 };
 
 }  // namespace skia
 
 #endif  // SKIA_EXT_BITMAP_PLATFORM_DEVICE_MAC_H_