| // Copyright (c) 2012 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef BASE_TASK_RUNNER_H_ |
| #define BASE_TASK_RUNNER_H_ |
| |
| #include "base/base_export.h" |
| #include "base/basictypes.h" |
| #include "base/callback_forward.h" |
| #include "base/memory/ref_counted.h" |
| #include "base/time/time.h" |
| |
| namespace tracked_objects { |
| class Location; |
| } // namespace tracked_objects |
| |
| namespace base { |
| |
| struct TaskRunnerTraits; |
| |
| // A TaskRunner is an object that runs posted tasks (in the form of |
| // Closure objects). The TaskRunner interface provides a way of |
| // decoupling task posting from the mechanics of how each task will be |
| // run. TaskRunner provides very weak guarantees as to how posted |
| // tasks are run (or if they're run at all). In particular, it only |
| // guarantees: |
| // |
| // - Posting a task will not run it synchronously. That is, no |
| // Post*Task method will call task.Run() directly. |
| // |
| // - Increasing the delay can only delay when the task gets run. |
| // That is, increasing the delay may not affect when the task gets |
| // run, or it could make it run later than it normally would, but |
| // it won't make it run earlier than it normally would. |
| // |
| // TaskRunner does not guarantee the order in which posted tasks are |
| // run, whether tasks overlap, or whether they're run on a particular |
| // thread. Also it does not guarantee a memory model for shared data |
| // between tasks. (In other words, you should use your own |
| // synchronization/locking primitives if you need to share data |
| // between tasks.) |
| // |
| // Implementations of TaskRunner should be thread-safe in that all |
| // methods must be safe to call on any thread. Ownership semantics |
| // for TaskRunners are in general not clear, which is why the |
| // interface itself is RefCountedThreadSafe. |
| // |
| // Some theoretical implementations of TaskRunner: |
| // |
| // - A TaskRunner that uses a thread pool to run posted tasks. |
| // |
| // - A TaskRunner that, for each task, spawns a non-joinable thread |
| // to run that task and immediately quit. |
| // |
| // - A TaskRunner that stores the list of posted tasks and has a |
| // method Run() that runs each runnable task in random order. |
| class BASE_EXPORT TaskRunner |
| : public RefCountedThreadSafe<TaskRunner, TaskRunnerTraits> { |
| public: |
| // Posts the given task to be run. Returns true if the task may be |
| // run at some point in the future, and false if the task definitely |
| // will not be run. |
| // |
| // Equivalent to PostDelayedTask(from_here, task, 0). |
| bool PostTask(const tracked_objects::Location& from_here, |
| const Closure& task); |
| |
| // Like PostTask, but tries to run the posted task only after |
| // |delay_ms| has passed. |
| // |
| // It is valid for an implementation to ignore |delay_ms|; that is, |
| // to have PostDelayedTask behave the same as PostTask. |
| virtual bool PostDelayedTask(const tracked_objects::Location& from_here, |
| const Closure& task, |
| base::TimeDelta delay) = 0; |
| |
| // Returns true if the current thread is a thread on which a task |
| // may be run, and false if no task will be run on the current |
| // thread. |
| // |
| // It is valid for an implementation to always return true, or in |
| // general to use 'true' as a default value. |
| virtual bool RunsTasksOnCurrentThread() const = 0; |
| |
| // Posts |task| on the current TaskRunner. On completion, |reply| |
| // is posted to the thread that called PostTaskAndReply(). Both |
| // |task| and |reply| are guaranteed to be deleted on the thread |
| // from which PostTaskAndReply() is invoked. This allows objects |
| // that must be deleted on the originating thread to be bound into |
| // the |task| and |reply| Closures. In particular, it can be useful |
| // to use WeakPtr<> in the |reply| Closure so that the reply |
| // operation can be canceled. See the following pseudo-code: |
| // |
| // class DataBuffer : public RefCountedThreadSafe<DataBuffer> { |
| // public: |
| // // Called to add data into a buffer. |
| // void AddData(void* buf, size_t length); |
| // ... |
| // }; |
| // |
| // |
| // class DataLoader : public SupportsWeakPtr<DataLoader> { |
| // public: |
| // void GetData() { |
| // scoped_refptr<DataBuffer> buffer = new DataBuffer(); |
| // target_thread_.message_loop_proxy()->PostTaskAndReply( |
| // FROM_HERE, |
| // base::Bind(&DataBuffer::AddData, buffer), |
| // base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer)); |
| // } |
| // |
| // private: |
| // void OnDataReceived(scoped_refptr<DataBuffer> buffer) { |
| // // Do something with buffer. |
| // } |
| // }; |
| // |
| // |
| // Things to notice: |
| // * Results of |task| are shared with |reply| by binding a shared argument |
| // (a DataBuffer instance). |
| // * The DataLoader object has no special thread safety. |
| // * The DataLoader object can be deleted while |task| is still running, |
| // and the reply will cancel itself safely because it is bound to a |
| // WeakPtr<>. |
| bool PostTaskAndReply(const tracked_objects::Location& from_here, |
| const Closure& task, |
| const Closure& reply); |
| |
| protected: |
| friend struct TaskRunnerTraits; |
| |
| // Only the Windows debug build seems to need this: see |
| // http://crbug.com/112250. |
| friend class RefCountedThreadSafe<TaskRunner, TaskRunnerTraits>; |
| |
| TaskRunner(); |
| virtual ~TaskRunner(); |
| |
| // Called when this object should be destroyed. By default simply |
| // deletes |this|, but can be overridden to do something else, like |
| // delete on a certain thread. |
| virtual void OnDestruct() const; |
| }; |
| |
| struct BASE_EXPORT TaskRunnerTraits { |
| static void Destruct(const TaskRunner* task_runner); |
| }; |
| |
| } // namespace base |
| |
| #endif // BASE_TASK_RUNNER_H_ |