Add SkImageGenerator Interface

src/lazy/SkDiscardablePixelRef.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 SkDiscardablePixelRef_DEFINED
+#define SkDiscardablePixelRef_DEFINED
+
+#include "SkPixelRef.h"
+#include "SkImageGenerator.h"
+#include "SkImageInfo.h"
+
+class SkDiscardableMemory;
+
+/**
+ * An interface that allows a purgable PixelRef to re-decode an image.
+ */
+
+class SkDiscardablePixelRef : public SkPixelRef {
+public:
+    /**
+     *  Takes ownership of SkImageGenerator.  If this method fails for
+     *  whatever reason, it will return false and immediatetely delete
+     *  the generator.  If it succeeds, it will modify destination
+     *  bitmap.
+     *
+     *  If Install fails or when the SkDiscardablePixelRef that is
+     *  installed into destination is destroyed, it will call
+     *  SkDELETE() on the generator.  Therefore, generator should be
+     *  allocated with SkNEW() or SkNEW_ARGS().
+     */
+    static bool Install(SkImageGenerator* generator, SkBitmap* destination);
+
+    SK_DECLARE_UNFLATTENABLE_OBJECT()
+
+protected:
+    ~SkDiscardablePixelRef();
+    virtual void* onLockPixels(SkColorTable**) SK_OVERRIDE;
+    virtual void onUnlockPixels() SK_OVERRIDE;
+    virtual bool onLockPixelsAreWritable() const SK_OVERRIDE { return false; }
+
+    virtual SkData* onRefEncodedData() SK_OVERRIDE {
+        return fGenerator->refEncodedData();
+    }
+
+private:
+    SkImageGenerator* const fGenerator;
+    const SkImageInfo fInfo;
+    const size_t fSize;  // size of memory to be allocated
+    const size_t fRowBytes;
+    // These const members should not change over the life of the
+    // PixelRef, since the SkBitmap doesn't expect them to change.
+
+    SkDiscardableMemory* fDiscardableMemory;
+
+    /* Takes ownership of SkImageGenerator. */
+    SkDiscardablePixelRef(SkImageGenerator* generator,
+                          const SkImageInfo& info,
+                          size_t size,
+                          size_t rowBytes);
+};
+#endif  // SkDiscardablePixelRef_DEFINED