skia: Added SkDiscardableMemory interface.

src/core/SkDiscardableMemory.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 SkDiscardableMemory_DEFINED
+#define SkDiscardableMemory_DEFINED
+
+#include "SkTypes.h"
+
+/**
+ *  Interface for discardable memory. Implementation is provided by the
+ *  embedder.
+ */
+class SkDiscardableMemory {
+
+public:
+    /**
+     *  Factory method that creates, initializes and locks an SkDiscardableMemory
+     *  object. If either of these steps fails, a NULL pointer will be returned.
+     */
+    static SkDiscardableMemory* Create(size_t bytes);
+
+    /** Must not be called while locked.
+     */
+    virtual ~SkDiscardableMemory() {}
+
+    /**
+     * Locks the memory, prevent it from being discarded. Once locked. you may
+     * obtain a pointer to that memory using the data() method.
+     *
+     * lock() may return false, indicating that the underlying memory was
+     * discarded and that the lock failed.
+     *
+     * Nested calls to lock are not allowed.
+     */
+    virtual bool lock() = 0;
+
+    /**
+     * Returns the current pointer for the discardable memory. This call is ONLY
+     * valid when the discardable memory object is locked.
+     */
+    virtual void* data() = 0;
+
+    /**
+     * Unlock the memory so that it can be purged by the system. Must be called
+     * after every successful lock call.
+     */
+    virtual void unlock() = 0;
+};
+
+#endif