The Tor Browser / file comparison

comparison: gfx/skia/trunk/include/core/SkWeakRefCnt.h

gfx/skia/trunk/include/core/SkWeakRefCnt.h

branch
TOR_BUG_3246
changeset 7
129ffea94266
equal deleted inserted replaced
-1:000000000000 0:dac6075e9781
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 "SkThread.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 SK_DECLARE_INST_COUNT(SkWeakRefCnt)
55
56 /** Default construct, initializing the reference counts to 1.
57 The strong references collectively hold one weak reference. When the
58 strong reference count goes to zero, the collectively held weak
59 reference is released.
60 */
61 SkWeakRefCnt() : SkRefCnt(), fWeakCnt(1) {}
62
63 /** Destruct, asserting that the weak reference count is 1.
64 */
65 virtual ~SkWeakRefCnt() {
66 #ifdef SK_DEBUG
67 SkASSERT(fWeakCnt == 1);
68 fWeakCnt = 0;
69 #endif
70 }
71
72 /** Return the weak reference count.
73 */
74 int32_t getWeakCnt() const { return fWeakCnt; }
75
76 #ifdef SK_DEBUG
77 void validate() const {
78 this->INHERITED::validate();
79 SkASSERT(fWeakCnt > 0);
80 }
81 #endif
82
83 /** Creates a strong reference from a weak reference, if possible. The
84 caller must already be an owner. If try_ref() returns true the owner
85 is in posession of an additional strong reference. Both the original
86 reference and new reference must be properly unreferenced. If try_ref()
87 returns false, no strong reference could be created and the owner's
88 reference is in the same state as before the call.
89 */
90 bool SK_WARN_UNUSED_RESULT try_ref() const {
91 if (sk_atomic_conditional_inc(&fRefCnt) != 0) {
92 // Acquire barrier (L/SL), if not provided above.
93 // Prevents subsequent code from happening before the increment.
94 sk_membar_acquire__after_atomic_conditional_inc();
95 return true;
96 }
97 return false;
98 }
99
100 /** Increment the weak reference count. Must be balanced by a call to
101 weak_unref().
102 */
103 void weak_ref() const {
104 SkASSERT(fRefCnt > 0);
105 SkASSERT(fWeakCnt > 0);
106 sk_atomic_inc(&fWeakCnt); // No barrier required.
107 }
108
109 /** Decrement the weak reference count. If the weak reference count is 1
110 before the decrement, then call delete on the object. Note that if this
111 is the case, then the object needs to have been allocated via new, and
112 not on the stack.
113 */
114 void weak_unref() const {
115 SkASSERT(fWeakCnt > 0);
116 // Release barrier (SL/S), if not provided below.
117 if (sk_atomic_dec(&fWeakCnt) == 1) {
118 // Acquire barrier (L/SL), if not provided above.
119 // Prevents code in destructor from happening before the decrement.
120 sk_membar_acquire__after_atomic_dec();
121 #ifdef SK_DEBUG
122 // so our destructor won't complain
123 fWeakCnt = 1;
124 #endif
125 this->INHERITED::internal_dispose();
126 }
127 }
128
129 /** Returns true if there are no strong references to the object. When this
130 is the case all future calls to try_ref() will return false.
131 */
132 bool weak_expired() const {
133 return fRefCnt == 0;
134 }
135
136 protected:
137 /** Called when the strong reference count goes to zero. This allows the
138 object to free any resources it may be holding. Weak references may
139 still exist and their level of allowed access to the object is defined
140 by the object's class.
141 */
142 virtual void weak_dispose() const {
143 }
144
145 private:
146 /** Called when the strong reference count goes to zero. Calls weak_dispose
147 on the object and releases the implicit weak reference held
148 collectively by the strong references.
149 */
150 virtual void internal_dispose() const SK_OVERRIDE {
151 weak_dispose();
152 weak_unref();
153 }
154
155 /* Invariant: fWeakCnt = #weak + (fRefCnt > 0 ? 1 : 0) */
156 mutable int32_t fWeakCnt;
157
158 typedef SkRefCnt INHERITED;
159 };
160
161 #endif

Mercurial Repository: The Tor Browser

Europalab SCM Repository Server

mercurial