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_ 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" 12 #include "base/time/time.h" 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. 74 virtual bool PostDelayedTask(const tracked_objects::Location& from_here, 75 const Closure& task, 76 base::TimeDelta delay) = 0; 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_ 154
