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[chromium-blink-merge.git] / base / task_runner.h
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1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef BASE_TASK_RUNNER_H_
6 #define BASE_TASK_RUNNER_H_
8 #include "base/base_export.h"
9 #include "base/basictypes.h"
10 #include "base/callback_forward.h"
11 #include "base/memory/ref_counted.h"
12 #include "base/time/time.h"
14 namespace tracked_objects {
15 class Location;
16 } // namespace tracked_objects
18 namespace base {
20 struct TaskRunnerTraits;
22 // A TaskRunner is an object that runs posted tasks (in the form of
23 // Closure objects). The TaskRunner interface provides a way of
24 // decoupling task posting from the mechanics of how each task will be
25 // run. TaskRunner provides very weak guarantees as to how posted
26 // tasks are run (or if they're run at all). In particular, it only
27 // guarantees:
29 // - Posting a task will not run it synchronously. That is, no
30 // Post*Task method will call task.Run() directly.
32 // - Increasing the delay can only delay when the task gets run.
33 // That is, increasing the delay may not affect when the task gets
34 // run, or it could make it run later than it normally would, but
35 // it won't make it run earlier than it normally would.
37 // TaskRunner does not guarantee the order in which posted tasks are
38 // run, whether tasks overlap, or whether they're run on a particular
39 // thread. Also it does not guarantee a memory model for shared data
40 // between tasks. (In other words, you should use your own
41 // synchronization/locking primitives if you need to share data
42 // between tasks.)
44 // Implementations of TaskRunner should be thread-safe in that all
45 // methods must be safe to call on any thread. Ownership semantics
46 // for TaskRunners are in general not clear, which is why the
47 // interface itself is RefCountedThreadSafe.
49 // Some theoretical implementations of TaskRunner:
51 // - A TaskRunner that uses a thread pool to run posted tasks.
53 // - A TaskRunner that, for each task, spawns a non-joinable thread
54 // to run that task and immediately quit.
56 // - A TaskRunner that stores the list of posted tasks and has a
57 // method Run() that runs each runnable task in random order.
58 class BASE_EXPORT TaskRunner
59 : public RefCountedThreadSafe<TaskRunner, TaskRunnerTraits> {
60 public:
61 // Posts the given task to be run. Returns true if the task may be
62 // run at some point in the future, and false if the task definitely
63 // will not be run.
65 // Equivalent to PostDelayedTask(from_here, task, 0).
66 bool PostTask(const tracked_objects::Location& from_here,
67 const Closure& task);
69 // Like PostTask, but tries to run the posted task only after
70 // |delay_ms| has passed.
72 // It is valid for an implementation to ignore |delay_ms|; that is,
73 // to have PostDelayedTask behave the same as PostTask.
74 virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
75 const Closure& task,
76 base::TimeDelta delay) = 0;
78 // Returns true if the current thread is a thread on which a task
79 // may be run, and false if no task will be run on the current
80 // thread.
82 // It is valid for an implementation to always return true, or in
83 // general to use 'true' as a default value.
84 virtual bool RunsTasksOnCurrentThread() const = 0;
86 // Posts |task| on the current TaskRunner. On completion, |reply|
87 // is posted to the thread that called PostTaskAndReply(). Both
88 // |task| and |reply| are guaranteed to be deleted on the thread
89 // from which PostTaskAndReply() is invoked. This allows objects
90 // that must be deleted on the originating thread to be bound into
91 // the |task| and |reply| Closures. In particular, it can be useful
92 // to use WeakPtr<> in the |reply| Closure so that the reply
93 // operation can be canceled. See the following pseudo-code:
95 // class DataBuffer : public RefCountedThreadSafe<DataBuffer> {
96 // public:
97 // // Called to add data into a buffer.
98 // void AddData(void* buf, size_t length);
99 // ...
100 // };
103 // class DataLoader : public SupportsWeakPtr<DataLoader> {
104 // public:
105 // void GetData() {
106 // scoped_refptr<DataBuffer> buffer = new DataBuffer();
107 // target_thread_.message_loop_proxy()->PostTaskAndReply(
108 // FROM_HERE,
109 // base::Bind(&DataBuffer::AddData, buffer),
110 // base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer));
111 // }
113 // private:
114 // void OnDataReceived(scoped_refptr<DataBuffer> buffer) {
115 // // Do something with buffer.
116 // }
117 // };
120 // Things to notice:
121 // * Results of |task| are shared with |reply| by binding a shared argument
122 // (a DataBuffer instance).
123 // * The DataLoader object has no special thread safety.
124 // * The DataLoader object can be deleted while |task| is still running,
125 // and the reply will cancel itself safely because it is bound to a
126 // WeakPtr<>.
127 bool PostTaskAndReply(const tracked_objects::Location& from_here,
128 const Closure& task,
129 const Closure& reply);
131 protected:
132 friend struct TaskRunnerTraits;
134 // Only the Windows debug build seems to need this: see
135 // http://crbug.com/112250.
136 friend class RefCountedThreadSafe<TaskRunner, TaskRunnerTraits>;
138 TaskRunner();
139 virtual ~TaskRunner();
141 // Called when this object should be destroyed. By default simply
142 // deletes |this|, but can be overridden to do something else, like
143 // delete on a certain thread.
144 virtual void OnDestruct() const;
147 struct BASE_EXPORT TaskRunnerTraits {
148 static void Destruct(const TaskRunner* task_runner);
151 } // namespace base
153 #endif // BASE_TASK_RUNNER_H_