Add GrProgramElement base class for GrEffect with deferred exec ref.

include/gpu/GrProgramElementRef.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrProgramElementRef_DEFINED
+#define GrProgramElementRef_DEFINED
+
+#include "SkRefCnt.h"
+#include "GrTypes.h"
+
+/**
+ * Helper for owning a GrProgramElement subclass and being able to convert a ref to pending
+ * execution. It is like an SkAutoTUnref for program elements whose execution can be deferred. Once
+ * in the pending execution state it is illegal to change the object that is owned by the
+ * GrProgramElementRef. Its destructor will either unref the GrProgramElement or signal that
+ * the pending execution has completed, depending on whether convertToPendingExec() was called.
+ */
+template <typename T> class GrProgramElementRef : SkNoncopyable {
+public:
+    GrProgramElementRef() : fOwnPendingExec(false), fObj(NULL) {};
+
+    // Adopts a ref from the caller.
+    explicit GrProgramElementRef(T* obj) : fOwnPendingExec(false), fObj(obj)  {}
+
+    // Adopts a ref from the caller. Do not call after convertToPendingExec.
+    void reset(T* obj) {
+        SkASSERT(!fOwnPendingExec);
+        SkSafeUnref(fObj);
+        fObj = obj;
+    }
+
+    void convertToPendingExec() {
+        SkASSERT(!fOwnPendingExec);
+        fObj->convertRefToPendingExecution();
+        fOwnPendingExec = true;
+    }
+
+    T* get() const { return fObj; }
+    operator T*() { return fObj; }
+
+    /** If T is const, the type returned from operator-> will also be const. */
+    typedef typename SkTConstType<typename SkAutoTUnref<T>::BlockRef<T>,
+                                  SkTIsConst<T>::value>::type BlockRefType;
+
+    /**
+     * GrProgramElementRef assumes ownership of the ref and manages converting the ref to a
+     * pending execution. As a result, it is an error for the user to ref or unref through
+     * GrProgramElementRef. Therefore operator-> returns BlockRef<T>*.
+     */
+    BlockRefType *operator->() const {
+        return static_cast<BlockRefType*>(fObj);
+    }
+
+    ~GrProgramElementRef() {
+        if (NULL != fObj) {
+            if (fOwnPendingExec) {
+                fObj->completedExecution();
+            } else {
+                fObj->unref();
+            }
+        }
+    }
+
+private:
+    bool fOwnPendingExec;
+    T*   fObj;
+
+    typedef SkNoncopyable INHERITED;
+};
+#endif