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/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 tracked_objects::Location& from_here, 65 const Closure& task); 66 67 // Like PostTask, but tries to run the posted task only after 68 // |delay_ms| has passed. 69 // 70 // It is valid for an implementation to ignore |delay_ms|; that is, 71 // to have PostDelayedTask behave the same as PostTask. 72 virtual bool PostDelayedTask(const tracked_objects::Location& from_here, 73 const Closure& task, 74 base::TimeDelta delay) = 0; 75 76 // Returns true if the current thread is a thread on which a task 77 // may be run, and false if no task will be run on the current 78 // thread. 79 // 80 // It is valid for an implementation to always return true, or in 81 // general to use 'true' as a default value. 82 virtual bool RunsTasksOnCurrentThread() const = 0; 83 84 // Posts |task| on the current TaskRunner. On completion, |reply| 85 // is posted to the thread that called PostTaskAndReply(). Both 86 // |task| and |reply| are guaranteed to be deleted on the thread 87 // from which PostTaskAndReply() is invoked. This allows objects 88 // that must be deleted on the originating thread to be bound into 89 // the |task| and |reply| Closures. In particular, it can be useful 90 // to use WeakPtr<> in the |reply| Closure so that the reply 91 // operation can be canceled. See the following pseudo-code: 92 // 93 // class DataBuffer : public RefCountedThreadSafe<DataBuffer> { 94 // public: 95 // // Called to add data into a buffer. 96 // void AddData(void* buf, size_t length); 97 // ... 98 // }; 99 // 100 // 101 // class DataLoader : public SupportsWeakPtr<DataLoader> { 102 // public: 103 // void GetData() { 104 // scoped_refptr<DataBuffer> buffer = new DataBuffer(); 105 // target_thread_.task_runner()->PostTaskAndReply( 106 // FROM_HERE, 107 // base::Bind(&DataBuffer::AddData, buffer), 108 // base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer)); 109 // } 110 // 111 // private: 112 // void OnDataReceived(scoped_refptr<DataBuffer> buffer) { 113 // // Do something with buffer. 114 // } 115 // }; 116 // 117 // 118 // Things to notice: 119 // * Results of |task| are shared with |reply| by binding a shared argument 120 // (a DataBuffer instance). 121 // * The DataLoader object has no special thread safety. 122 // * The DataLoader object can be deleted while |task| is still running, 123 // and the reply will cancel itself safely because it is bound to a 124 // WeakPtr<>. 125 bool PostTaskAndReply(const tracked_objects::Location& from_here, 126 const Closure& task, 127 const Closure& reply); 128 129 protected: 130 friend struct TaskRunnerTraits; 131 132 // Only the Windows debug build seems to need this: see 133 // http://crbug.com/112250. 134 friend class RefCountedThreadSafe<TaskRunner, TaskRunnerTraits>; 135 136 TaskRunner(); 137 virtual ~TaskRunner(); 138 139 // Called when this object should be destroyed. By default simply 140 // deletes |this|, but can be overridden to do something else, like 141 // delete on a certain thread. 142 virtual void OnDestruct() const; 143 }; 144 145 struct BASE_EXPORT TaskRunnerTraits { 146 static void Destruct(const TaskRunner* task_runner); 147 }; 148 149 } // namespace base 150 151 #endif // BASE_TASK_RUNNER_H_ 152
