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diff --git a/include/private/SkWeakRefCnt.h b/include/private/SkWeakRefCnt.h
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+++ b/include/private/SkWeakRefCnt.h
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+/*
+ * Copyright 2012 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkWeakRefCnt_DEFINED
+#define SkWeakRefCnt_DEFINED
+
+#include "SkRefCnt.h"
+#include "SkAtomics.h"
+
+/** \class SkWeakRefCnt
+
+    SkWeakRefCnt is the base class for objects that may be shared by multiple
+    objects. When an existing strong owner wants to share a reference, it calls
+    ref(). When a strong owner wants to release its reference, it calls
+    unref(). When the shared object's strong reference count goes to zero as
+    the result of an unref() call, its (virtual) weak_dispose method is called.
+    It is an error for the destructor to be called explicitly (or via the
+    object going out of scope on the stack or calling delete) if
+    getRefCnt() > 1.
+
+    In addition to strong ownership, an owner may instead obtain a weak
+    reference by calling weak_ref(). A call to weak_ref() must be balanced by a
+    call to weak_unref(). To obtain a strong reference from a weak reference,
+    call try_ref(). If try_ref() returns true, the owner's pointer is now also
+    a strong reference on which unref() must be called. Note that this does not
+    affect the original weak reference, weak_unref() must still be called. When
+    the weak reference count goes to zero, the object is deleted. While the
+    weak reference count is positive and the strong reference count is zero the
+    object still exists, but will be in the disposed state. It is up to the
+    object to define what this means.
+
+    Note that a strong reference implicitly implies a weak reference. As a
+    result, it is allowable for the owner of a strong ref to call try_ref().
+    This will have the same effect as calling ref(), but may be more expensive.
+
+    Example:
+
+    SkWeakRefCnt myRef = strongRef.weak_ref();
+    ... // strongRef.unref() may or may not be called
+    if (myRef.try_ref()) {
+        ... // use myRef
+        myRef.unref();
+    } else {
+        // myRef is in the disposed state
+    }
+    myRef.weak_unref();
+*/
+class SK_API SkWeakRefCnt : public SkRefCnt {
+public:
+    /** Default construct, initializing the reference counts to 1.
+        The strong references collectively hold one weak reference. When the
+        strong reference count goes to zero, the collectively held weak
+        reference is released.
+    */
+    SkWeakRefCnt() : SkRefCnt(), fWeakCnt(1) {}
+
+    /** Destruct, asserting that the weak reference count is 1.
+    */
+    virtual ~SkWeakRefCnt() {
+#ifdef SK_DEBUG
+        SkASSERT(fWeakCnt == 1);
+        fWeakCnt = 0;
+#endif
+    }
+
+    /** Return the weak reference count.
+    */
+    int32_t getWeakCnt() const { return fWeakCnt; }
+
+#ifdef SK_DEBUG
+    void validate() const {
+        this->INHERITED::validate();
+        SkASSERT(fWeakCnt > 0);
+    }
+#endif
+
+    /** Creates a strong reference from a weak reference, if possible. The
+        caller must already be an owner. If try_ref() returns true the owner
+        is in posession of an additional strong reference. Both the original
+        reference and new reference must be properly unreferenced. If try_ref()
+        returns false, no strong reference could be created and the owner's
+        reference is in the same state as before the call.
+    */
+    bool SK_WARN_UNUSED_RESULT try_ref() const {
+        if (sk_atomic_conditional_inc(&fRefCnt) != 0) {
+            // Acquire barrier (L/SL), if not provided above.
+            // Prevents subsequent code from happening before the increment.
+            sk_membar_acquire__after_atomic_conditional_inc();
+            return true;
+        }
+        return false;
+    }
+
+    /** Increment the weak reference count. Must be balanced by a call to
+        weak_unref().
+    */
+    void weak_ref() const {
+        SkASSERT(fRefCnt > 0);
+        SkASSERT(fWeakCnt > 0);
+        sk_atomic_inc(&fWeakCnt);  // No barrier required.
+    }
+
+    /** Decrement the weak reference count. If the weak reference count is 1
+        before the decrement, then call delete on the object. Note that if this
+        is the case, then the object needs to have been allocated via new, and
+        not on the stack.
+    */
+    void weak_unref() const {
+        SkASSERT(fWeakCnt > 0);
+        // Release barrier (SL/S), if not provided below.
+        if (sk_atomic_dec(&fWeakCnt) == 1) {
+            // Acquire barrier (L/SL), if not provided above.
+            // Prevents code in destructor from happening before the decrement.
+            sk_membar_acquire__after_atomic_dec();
+#ifdef SK_DEBUG
+            // so our destructor won't complain
+            fWeakCnt = 1;
+#endif
+            this->INHERITED::internal_dispose();
+        }
+    }
+
+    /** Returns true if there are no strong references to the object. When this
+        is the case all future calls to try_ref() will return false.
+    */
+    bool weak_expired() const {
+        return fRefCnt == 0;
+    }
+
+protected:
+    /** Called when the strong reference count goes to zero. This allows the
+        object to free any resources it may be holding. Weak references may
+        still exist and their level of allowed access to the object is defined
+        by the object's class.
+    */
+    virtual void weak_dispose() const {
+    }
+
+private:
+    /** Called when the strong reference count goes to zero. Calls weak_dispose
+        on the object and releases the implicit weak reference held
+        collectively by the strong references.
+    */
+    void internal_dispose() const override {
+        weak_dispose();
+        weak_unref();
+    }
+
+    /* Invariant: fWeakCnt = #weak + (fRefCnt > 0 ? 1 : 0) */
+    mutable int32_t fWeakCnt;
+
+    typedef SkRefCnt INHERITED;
+};
+
+#endif