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 <stddef.h> 9 10 #include "base/base_export.h" 11 #include "base/callback.h" 12 #include "base/location.h" 13 #include "base/memory/ref_counted.h" 14 #include "base/time/time.h" 15 16 namespace base { 17 18 struct TaskRunnerTraits; 19 20 // A TaskRunner is an object that runs posted tasks (in the form of 21 // Closure objects). The TaskRunner interface provides a way of 22 // decoupling task posting from the mechanics of how each task will be 23 // run. TaskRunner provides very weak guarantees as to how posted 24 // tasks are run (or if they're run at all). In particular, it only 25 // guarantees: 26 // 27 // - Posting a task will not run it synchronously. That is, no 28 // Post*Task method will call task.Run() directly. 29 // 30 // - Increasing the delay can only delay when the task gets run. 31 // That is, increasing the delay may not affect when the task gets 32 // run, or it could make it run later than it normally would, but 33 // it won't make it run earlier than it normally would. 34 // 35 // TaskRunner does not guarantee the order in which posted tasks are 36 // run, whether tasks overlap, or whether they're run on a particular 37 // thread. Also it does not guarantee a memory model for shared data 38 // between tasks. (In other words, you should use your own 39 // synchronization/locking primitives if you need to share data 40 // between tasks.) 41 // 42 // Implementations of TaskRunner should be thread-safe in that all 43 // methods must be safe to call on any thread. Ownership semantics 44 // for TaskRunners are in general not clear, which is why the 45 // interface itself is RefCountedThreadSafe. 46 // 47 // Some theoretical implementations of TaskRunner: 48 // 49 // - A TaskRunner that uses a thread pool to run posted tasks. 50 // 51 // - A TaskRunner that, for each task, spawns a non-joinable thread 52 // to run that task and immediately quit. 53 // 54 // - A TaskRunner that stores the list of posted tasks and has a 55 // method Run() that runs each runnable task in random order. 56 class BASE_EXPORT TaskRunner 57 : public RefCountedThreadSafe<TaskRunner, TaskRunnerTraits> { 58 public: 59 // Posts the given task to be run. Returns true if the task may be 60 // run at some point in the future, and false if the task definitely 61 // will not be run. 62 // 63 // Equivalent to PostDelayedTask(from_here, task, 0). 64 bool PostTask(const Location& from_here, OnceClosure task); 65 66 // Like PostTask, but tries to run the posted task only after |delay_ms| 67 // has passed. Implementations should use a tick clock, rather than wall- 68 // clock time, to implement |delay|. 69 virtual bool PostDelayedTask(const Location& from_here, 70 OnceClosure task, 71 base::TimeDelta delay) = 0; 72 73 // Returns true iff tasks posted to this TaskRunner are sequenced 74 // with this call. 75 // 76 // In particular: 77 // - Returns true if this is a SequencedTaskRunner to which the 78 // current task was posted. 79 // - Returns true if this is a SequencedTaskRunner bound to the 80 // same sequence as the SequencedTaskRunner to which the current 81 // task was posted. 82 // - Returns true if this is a SingleThreadTaskRunner bound to 83 // the current thread. 84 // TODO(http://crbug.com/665062): 85 // This API doesn't make sense for parallel TaskRunners. 86 // Introduce alternate static APIs for documentation purposes of "this runs 87 // in pool X", have RunsTasksInCurrentSequence() return false for parallel 88 // TaskRunners, and ultimately move this method down to SequencedTaskRunner. 89 virtual bool RunsTasksInCurrentSequence() const = 0; 90 91 // Posts |task| on the current TaskRunner. On completion, |reply| 92 // is posted to the thread that called PostTaskAndReply(). Both 93 // |task| and |reply| are guaranteed to be deleted on the thread 94 // from which PostTaskAndReply() is invoked. This allows objects 95 // that must be deleted on the originating thread to be bound into 96 // the |task| and |reply| Closures. In particular, it can be useful 97 // to use WeakPtr<> in the |reply| Closure so that the reply 98 // operation can be canceled. See the following pseudo-code: 99 // 100 // class DataBuffer : public RefCountedThreadSafe<DataBuffer> { 101 // public: 102 // // Called to add data into a buffer. 103 // void AddData(void* buf, size_t length); 104 // ... 105 // }; 106 // 107 // 108 // class DataLoader : public SupportsWeakPtr<DataLoader> { 109 // public: 110 // void GetData() { 111 // scoped_refptr<DataBuffer> buffer = new DataBuffer(); 112 // target_thread_.task_runner()->PostTaskAndReply( 113 // FROM_HERE, 114 // base::Bind(&DataBuffer::AddData, buffer), 115 // base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer)); 116 // } 117 // 118 // private: 119 // void OnDataReceived(scoped_refptr<DataBuffer> buffer) { 120 // // Do something with buffer. 121 // } 122 // }; 123 // 124 // 125 // Things to notice: 126 // * Results of |task| are shared with |reply| by binding a shared argument 127 // (a DataBuffer instance). 128 // * The DataLoader object has no special thread safety. 129 // * The DataLoader object can be deleted while |task| is still running, 130 // and the reply will cancel itself safely because it is bound to a 131 // WeakPtr<>. 132 bool PostTaskAndReply(const Location& from_here, 133 OnceClosure task, 134 OnceClosure reply); 135 136 protected: 137 friend struct TaskRunnerTraits; 138 139 TaskRunner(); 140 virtual ~TaskRunner(); 141 142 // Called when this object should be destroyed. By default simply 143 // deletes |this|, but can be overridden to do something else, like 144 // delete on a certain thread. 145 virtual void OnDestruct() const; 146 }; 147 148 struct BASE_EXPORT TaskRunnerTraits { 149 static void Destruct(const TaskRunner* task_runner); 150 }; 151 152 } // namespace base 153 154 #endif // BASE_TASK_RUNNER_H_ 155