onCreateDevice -> NULL now means the caller should create its own (bitmap) device

include/core/SkDevice.h

 /*
  * Copyright 2010 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkDevice_DEFINED
 #define SkDevice_DEFINED
 
@@ skipping 330 matching lines @@
 
         const SkImageInfo       fInfo;
         const TileUsage         fTileUsage;
         const SkPixelGeometry   fPixelGeometry;
         const bool              fForImageFilter;
     };
 
     /**
      *  Create a new device based on CreateInfo. If the paint is not null, then it represents a
      *  preview of how the new device will be composed with its creator device (this).
+     *

[robertphillips, 2015/04/29 14:55:54]

"device back layer" -> "device back" ?

[reed1, 2015/04/29 15:20:47]

Done.

+     *  The subclass may be handed this device back layer (drawDevice), so it must always return
+     *  a device that it knows how to draw, and that it knows how to identify if it is not of the
+     *  same subclass (since drawDevice is passed a SkBaseDevice*). If the subclass cannot fulfill
+     *  that contract (e.g. PDF cannot support some settings on the paint) it should return NULL,
+     *  and the caller may then decide to explicitly create a bitmapdevice, knowing that later
+     *  it could not call drawDevice with it (but it could call drawSprite or drawBitmap).
      */
     virtual SkBaseDevice* onCreateDevice(const CreateInfo&, const SkPaint*) {
         return NULL;
     }
 
     virtual void initForRootLayer(SkPixelGeometry geo);
 
 private:
     friend class SkCanvas;
     friend struct DeviceCM; //for setMatrixClip
@@ skipping 28 matching lines @@
     SkDeviceProperties* fLeakyProperties;   // will always exist.
 
 #ifdef SK_DEBUG
     bool        fAttachedToCanvas;
 #endif
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif