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 // 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_