detect if the scaledimagecache returns a purged bitmap

src/core/SkScaledImageCache.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkScaledImageCache_DEFINED
 #define SkScaledImageCache_DEFINED
 
@@ skipping 48 matching lines @@
     static ID* AddAndLockMip(const SkBitmap& original, const SkMipMap* mipMap);
 
     static void Unlock(ID*);
 
     static size_t GetBytesUsed();
     static size_t GetByteLimit();
     static size_t SetByteLimit(size_t newLimit);
 
     static SkBitmap::Allocator* GetAllocator();
 
+    /**
+     *  Call SkDebugf() with diagnostic information about the state of the cache
+     */
+    static void Dump();
+
     ///////////////////////////////////////////////////////////////////////////
 
     /**
      *  Construct the cache to call DiscardableFactory when it
      *  allocates memory for the pixels. In this mode, the cache has
      *  not explicit budget, and so methods like getBytesUsed() and
      *  getByteLimit() will return 0, and setByteLimit will ignore its argument
      *  and return 0.
      */
     SkScaledImageCache(DiscardableFactory);
@@ skipping 65 matching lines @@
 
     /**
      *  Set the maximum number of bytes available to this cache. If the current
      *  cache exceeds this new value, it will be purged to try to fit within
      *  this new limit.
      */
     size_t setByteLimit(size_t newLimit);
 
     SkBitmap::Allocator* allocator() const { return fAllocator; };
 
+    /**
+     *  Call SkDebugf() with diagnostic information about the state of the cache
+     */
+    void dump() const;
+
 public:
     struct Rec;
     struct Key;
 private:
     Rec*    fHead;
     Rec*    fTail;
 
     class Hash;
     Hash*   fHash;
 
     DiscardableFactory  fDiscardableFactory;
     // the allocator is NULL or one that matches discardables
     SkBitmap::Allocator* fAllocator;
 
     size_t  fBytesUsed;
     size_t  fByteLimit;
     int     fCount;
 
     Rec* findAndLock(uint32_t generationID, SkScalar sx, SkScalar sy,
                      const SkIRect& bounds);
     Rec* findAndLock(const Key& key);
     ID* addAndLock(Rec* rec);
 
+    void purgeRec(Rec*);
     void purgeAsNeeded();
 
     // linklist management
     void moveToHead(Rec*);
     void addToHead(Rec*);
     void detach(Rec*);
 
     void init();    // called by constructors
 
 #ifdef SK_DEBUG
     void validate() const;
 #else
     void validate() const {}
 #endif
 };
 #endif