Initialize bitmap to (0, 0, 0, 0) for CanvasPaintT<T> instances.

skia/ext/canvas_paint_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_CANVAS_PAINT_MAC_H_
 #define SKIA_EXT_CANVAS_PAINT_MAC_H_
 #pragma once
 
 #include "skia/ext/canvas_paint_common.h"
 #include "skia/ext/platform_canvas.h"
 
 #import <Cocoa/Cocoa.h>
 
 namespace skia {
 
 // A class designed to translate skia painting into a region to the current
 // graphics context.  On construction, it will set up a context for painting
 // into, and on destruction, it will commit it to the current context.
+// Note: The created context is always inialized to (0, 0, 0, 0).
 template <class T>
 class CanvasPaintT : public T {
  public:
   // This constructor assumes the result is opaque.
   explicit CanvasPaintT(NSRect dirtyRect)
       : context_(NULL),
         rectangle_(dirtyRect),
         composite_alpha_(false) {
     init(true);
   }
@@ skipping 52 matching lines @@
 
  private:
   void init(bool opaque) {
     PlatformCanvas* canvas = GetPlatformCanvas(this);
     if (!canvas->initialize(rectangle_.size.width,
                             rectangle_.size.height,
                             opaque, NULL)) {
       // Cause a deliberate crash;
       *(volatile char*) 0 = 0;
     }
+    canvas->clear(SkColorSetARGB(0, 0, 0, 0));
 
     // Need to translate so that the dirty region appears at the origin of the
     // surface.
     canvas->translate(-SkDoubleToScalar(rectangle_.origin.x),
                       -SkDoubleToScalar(rectangle_.origin.y));
 
     context_ = GetBitmapContext(GetTopDevice(*canvas));
   }
 
   CGContext* context_;
   NSRect rectangle_;
   // See description above setter.
   bool composite_alpha_;
 
   // Disallow copy and assign.
   CanvasPaintT(const CanvasPaintT&);
   CanvasPaintT& operator=(const CanvasPaintT&);
 };
 
 typedef CanvasPaintT<PlatformCanvas> PlatformCanvasPaint;
 
 }  // namespace skia
 
 
 #endif  // SKIA_EXT_CANVAS_PAINT_MAC_H_