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_forward.h" 12 #include "base/memory/ref_counted.h" 13 #include "base/time/time.h" 14 15 namespace tracked_objects { 16 class Location; 17 } // namespace tracked_objects 18 19 namespace base { 20 21 struct TaskRunnerTraits; 22 23 // A TaskRunner is an object that runs posted tasks (in the form of 24 // Closure objects). The TaskRunner interface provides a way of 25 // decoupling task posting from the mechanics of how each task will be 26 // run. TaskRunner provides very weak guarantees as to how posted 27 // tasks are run (or if they're run at all). In particular, it only 28 // guarantees: 29 // 30 // - Posting a task will not run it synchronously. That is, no 31 // Post*Task method will call task.Run() directly. 32 // 33 // - Increasing the delay can only delay when the task gets run. 34 // That is, increasing the delay may not affect when the task gets 35 // run, or it could make it run later than it normally would, but 36 // it won't make it run earlier than it normally would. 37 // 38 // TaskRunner does not guarantee the order in which posted tasks are 39 // run, whether tasks overlap, or whether they're run on a particular 40 // thread. Also it does not guarantee a memory model for shared data 41 // between tasks. (In other words, you should use your own 42 // synchronization/locking primitives if you need to share data 43 // between tasks.) 44 // 45 // Implementations of TaskRunner should be thread-safe in that all 46 // methods must be safe to call on any thread. Ownership semantics 47 // for TaskRunners are in general not clear, which is why the 48 // interface itself is RefCountedThreadSafe. 49 // 50 // Some theoretical implementations of TaskRunner: 51 // 52 // - A TaskRunner that uses a thread pool to run posted tasks. 53 // 54 // - A TaskRunner that, for each task, spawns a non-joinable thread 55 // to run that task and immediately quit. 56 // 57 // - A TaskRunner that stores the list of posted tasks and has a 58 // method Run() that runs each runnable task in random order. 59 class BASE_EXPORT TaskRunner 60 : public RefCountedThreadSafe<TaskRunner, TaskRunnerTraits> { 61 public: 62 // Posts the given task to be run. Returns true if the task may be 63 // run at some point in the future, and false if the task definitely 64 // will not be run. 65 // 66 // Equivalent to PostDelayedTask(from_here, task, 0). 67 bool PostTask(const tracked_objects::Location& from_here, 68 const Closure& task); 69 70 // Like PostTask, but tries to run the posted task only after 71 // |delay_ms| has passed. 72 // 73 // It is valid for an implementation to ignore |delay_ms|; that is, 74 // to have PostDelayedTask behave the same as PostTask. 75 virtual bool PostDelayedTask(const tracked_objects::Location& from_here, 76 const Closure& task, 77 base::TimeDelta delay) = 0; 78 79 // Returns true if the current thread is a thread on which a task 80 // may be run, and false if no task will be run on the current 81 // thread. 82 // 83 // It is valid for an implementation to always return true, or in 84 // general to use 'true' as a default value. 85 virtual bool RunsTasksOnCurrentThread() const = 0; 86 87 // Posts |task| on the current TaskRunner. On completion, |reply| 88 // is posted to the thread that called PostTaskAndReply(). Both 89 // |task| and |reply| are guaranteed to be deleted on the thread 90 // from which PostTaskAndReply() is invoked. This allows objects 91 // that must be deleted on the originating thread to be bound into 92 // the |task| and |reply| Closures. In particular, it can be useful 93 // to use WeakPtr<> in the |reply| Closure so that the reply 94 // operation can be canceled. See the following pseudo-code: 95 // 96 // class DataBuffer : public RefCountedThreadSafe<DataBuffer> { 97 // public: 98 // // Called to add data into a buffer. 99 // void AddData(void* buf, size_t length); 100 // ... 101 // }; 102 // 103 // 104 // class DataLoader : public SupportsWeakPtr<DataLoader> { 105 // public: 106 // void GetData() { 107 // scoped_refptr<DataBuffer> buffer = new DataBuffer(); 108 // target_thread_.task_runner()->PostTaskAndReply( 109 // FROM_HERE, 110 // base::Bind(&DataBuffer::AddData, buffer), 111 // base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer)); 112 // } 113 // 114 // private: 115 // void OnDataReceived(scoped_refptr<DataBuffer> buffer) { 116 // // Do something with buffer. 117 // } 118 // }; 119 // 120 // 121 // Things to notice: 122 // * Results of |task| are shared with |reply| by binding a shared argument 123 // (a DataBuffer instance). 124 // * The DataLoader object has no special thread safety. 125 // * The DataLoader object can be deleted while |task| is still running, 126 // and the reply will cancel itself safely because it is bound to a 127 // WeakPtr<>. 128 bool PostTaskAndReply(const tracked_objects::Location& from_here, 129 const Closure& task, 130 const Closure& reply); 131 132 protected: 133 friend struct TaskRunnerTraits; 134 135 // Only the Windows debug build seems to need this: see 136 // http://crbug.com/112250. 137 friend class RefCountedThreadSafe<TaskRunner, 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
