1 /*
2  * Copyright 2012 Google Inc.
3  *
4  * Use of this source code is governed by a BSD-style license that can be
5  * found in the LICENSE file.
6  */
7 
8 #ifndef SkWeakRefCnt_DEFINED
9 #define SkWeakRefCnt_DEFINED
10 
11 #include "SkRefCnt.h"
12 #include "../private/SkAtomics.h"
13 
14 /** \class SkWeakRefCnt
15 
16     SkWeakRefCnt is the base class for objects that may be shared by multiple
17     objects. When an existing strong owner wants to share a reference, it calls
18     ref(). When a strong owner wants to release its reference, it calls
19     unref(). When the shared object's strong reference count goes to zero as
20     the result of an unref() call, its (virtual) weak_dispose method is called.
21     It is an error for the destructor to be called explicitly (or via the
22     object going out of scope on the stack or calling delete) if
23     getRefCnt() > 1.
24 
25     In addition to strong ownership, an owner may instead obtain a weak
26     reference by calling weak_ref(). A call to weak_ref() must be balanced by a
27     call to weak_unref(). To obtain a strong reference from a weak reference,
28     call try_ref(). If try_ref() returns true, the owner's pointer is now also
29     a strong reference on which unref() must be called. Note that this does not
30     affect the original weak reference, weak_unref() must still be called. When
31     the weak reference count goes to zero, the object is deleted. While the
32     weak reference count is positive and the strong reference count is zero the
33     object still exists, but will be in the disposed state. It is up to the
34     object to define what this means.
35 
36     Note that a strong reference implicitly implies a weak reference. As a
37     result, it is allowable for the owner of a strong ref to call try_ref().
38     This will have the same effect as calling ref(), but may be more expensive.
39 
40     Example:
41 
42     SkWeakRefCnt myRef = strongRef.weak_ref();
43     ... // strongRef.unref() may or may not be called
44     if (myRef.try_ref()) {
45         ... // use myRef
46         myRef.unref();
47     } else {
48         // myRef is in the disposed state
49     }
50     myRef.weak_unref();
51 */
52 class SK_API SkWeakRefCnt : public SkRefCnt {
53 public:
54     /** Default construct, initializing the reference counts to 1.
55         The strong references collectively hold one weak reference. When the
56         strong reference count goes to zero, the collectively held weak
57         reference is released.
58     */
SkWeakRefCnt()59     SkWeakRefCnt() : SkRefCnt(), fWeakCnt(1) {}
60 
61     /** Destruct, asserting that the weak reference count is 1.
62     */
~SkWeakRefCnt()63     virtual ~SkWeakRefCnt() {
64 #ifdef SK_DEBUG
65         SkASSERT(fWeakCnt == 1);
66         fWeakCnt = 0;
67 #endif
68     }
69 
70     /** Return the weak reference count.
71     */
getWeakCnt()72     int32_t getWeakCnt() const { return fWeakCnt; }
73 
74 #ifdef SK_DEBUG
validate()75     void validate() const {
76         this->INHERITED::validate();
77         SkASSERT(fWeakCnt > 0);
78     }
79 #endif
80 
81     /** Creates a strong reference from a weak reference, if possible. The
82         caller must already be an owner. If try_ref() returns true the owner
83         is in posession of an additional strong reference. Both the original
84         reference and new reference must be properly unreferenced. If try_ref()
85         returns false, no strong reference could be created and the owner's
86         reference is in the same state as before the call.
87     */
try_ref()88     bool SK_WARN_UNUSED_RESULT try_ref() const {
89         if (sk_atomic_conditional_inc(&fRefCnt) != 0) {
90             // Acquire barrier (L/SL), if not provided above.
91             // Prevents subsequent code from happening before the increment.
92             sk_membar_acquire__after_atomic_conditional_inc();
93             return true;
94         }
95         return false;
96     }
97 
98     /** Increment the weak reference count. Must be balanced by a call to
99         weak_unref().
100     */
weak_ref()101     void weak_ref() const {
102         SkASSERT(fRefCnt > 0);
103         SkASSERT(fWeakCnt > 0);
104         sk_atomic_inc(&fWeakCnt);  // No barrier required.
105     }
106 
107     /** Decrement the weak reference count. If the weak reference count is 1
108         before the decrement, then call delete on the object. Note that if this
109         is the case, then the object needs to have been allocated via new, and
110         not on the stack.
111     */
weak_unref()112     void weak_unref() const {
113         SkASSERT(fWeakCnt > 0);
114         // Release barrier (SL/S), if not provided below.
115         if (sk_atomic_dec(&fWeakCnt) == 1) {
116             // Acquire barrier (L/SL), if not provided above.
117             // Prevents code in destructor from happening before the decrement.
118             sk_membar_acquire__after_atomic_dec();
119 #ifdef SK_DEBUG
120             // so our destructor won't complain
121             fWeakCnt = 1;
122 #endif
123             this->INHERITED::internal_dispose();
124         }
125     }
126 
127     /** Returns true if there are no strong references to the object. When this
128         is the case all future calls to try_ref() will return false.
129     */
weak_expired()130     bool weak_expired() const {
131         return fRefCnt == 0;
132     }
133 
134 protected:
135     /** Called when the strong reference count goes to zero. This allows the
136         object to free any resources it may be holding. Weak references may
137         still exist and their level of allowed access to the object is defined
138         by the object's class.
139     */
weak_dispose()140     virtual void weak_dispose() const {
141     }
142 
143 private:
144     /** Called when the strong reference count goes to zero. Calls weak_dispose
145         on the object and releases the implicit weak reference held
146         collectively by the strong references.
147     */
internal_dispose()148     void internal_dispose() const override {
149         weak_dispose();
150         weak_unref();
151     }
152 
153     /* Invariant: fWeakCnt = #weak + (fRefCnt > 0 ? 1 : 0) */
154     mutable int32_t fWeakCnt;
155 
156     typedef SkRefCnt INHERITED;
157 };
158 
159 #endif
160