akalin@chromium.org | 062f968 | 2012-02-15 10:43:19 +0900 | [diff] [blame] | 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. |
| 4 | |
| 5 | #ifndef BASE_TASK_RUNNER_H_ |
| 6 | #define BASE_TASK_RUNNER_H_ |
akalin@chromium.org | 062f968 | 2012-02-15 10:43:19 +0900 | [diff] [blame] | 7 | |
| 8 | #include "base/base_export.h" |
| 9 | #include "base/basictypes.h" |
| 10 | #include "base/callback_forward.h" |
| 11 | #include "base/memory/ref_counted.h" |
avi@chromium.org | b45ec93 | 2013-06-29 00:14:18 +0900 | [diff] [blame] | 12 | #include "base/time/time.h" |
akalin@chromium.org | 062f968 | 2012-02-15 10:43:19 +0900 | [diff] [blame] | 13 | |
| 14 | namespace tracked_objects { |
| 15 | class Location; |
| 16 | } // namespace tracked_objects |
| 17 | |
| 18 | namespace base { |
| 19 | |
| 20 | struct TaskRunnerTraits; |
| 21 | |
| 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: |
| 28 | // |
| 29 | // - Posting a task will not run it synchronously. That is, no |
| 30 | // Post*Task method will call task.Run() directly. |
| 31 | // |
| 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. |
| 36 | // |
| 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.) |
| 43 | // |
| 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. |
| 48 | // |
| 49 | // Some theoretical implementations of TaskRunner: |
| 50 | // |
| 51 | // - A TaskRunner that uses a thread pool to run posted tasks. |
| 52 | // |
| 53 | // - A TaskRunner that, for each task, spawns a non-joinable thread |
| 54 | // to run that task and immediately quit. |
| 55 | // |
| 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. |
| 64 | // |
| 65 | // Equivalent to PostDelayedTask(from_here, task, 0). |
| 66 | bool PostTask(const tracked_objects::Location& from_here, |
| 67 | const Closure& task); |
| 68 | |
| 69 | // Like PostTask, but tries to run the posted task only after |
| 70 | // |delay_ms| has passed. |
| 71 | // |
| 72 | // It is valid for an implementation to ignore |delay_ms|; that is, |
| 73 | // to have PostDelayedTask behave the same as PostTask. |
tedvessenes@gmail.com | 01b5a4d | 2012-02-26 17:17:37 +0900 | [diff] [blame] | 74 | virtual bool PostDelayedTask(const tracked_objects::Location& from_here, |
| 75 | const Closure& task, |
| 76 | base::TimeDelta delay) = 0; |
akalin@chromium.org | 062f968 | 2012-02-15 10:43:19 +0900 | [diff] [blame] | 77 | |
| 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. |
| 81 | // |
| 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; |
| 85 | |
| 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: |
| 94 | // |
| 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 | // }; |
| 101 | // |
| 102 | // |
| 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 | // } |
| 112 | // |
| 113 | // private: |
| 114 | // void OnDataReceived(scoped_refptr<DataBuffer> buffer) { |
| 115 | // // Do something with buffer. |
| 116 | // } |
| 117 | // }; |
| 118 | // |
| 119 | // |
| 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); |
| 130 | |
| 131 | protected: |
| 132 | friend struct TaskRunnerTraits; |
| 133 | |
| 134 | // Only the Windows debug build seems to need this: see |
| 135 | // http://crbug.com/112250. |
| 136 | friend class RefCountedThreadSafe<TaskRunner, TaskRunnerTraits>; |
| 137 | |
| 138 | TaskRunner(); |
| 139 | virtual ~TaskRunner(); |
| 140 | |
| 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; |
| 145 | }; |
| 146 | |
| 147 | struct BASE_EXPORT TaskRunnerTraits { |
| 148 | static void Destruct(const TaskRunner* task_runner); |
| 149 | }; |
| 150 | |
| 151 | } // namespace base |
| 152 | |
| 153 | #endif // BASE_TASK_RUNNER_H_ |