task_runner.h - mozsearch

// 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 <stddef.h>

#include "base/base_export.h"

#include "base/callback.h"

#include "base/location.h"

#include "base/memory/ref_counted.h"

#include "base/time/time.h"

namespace base {

struct TaskRunnerTraits;

// A TaskRunner is an object that runs posted tasks (in the form of

// OnceClosure 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 Location& from_here, OnceClosure task);

  // Like PostTask, but tries to run the posted task only after |delay_ms|

  // has passed. Implementations should use a tick clock, rather than wall-

  // clock time, to implement |delay|.

  virtual bool PostDelayedTask(const Location& from_here,

                               OnceClosure task,

                               base::TimeDelta delay) = 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| OnceClosures.  In particular, it can be useful

  // to use WeakPtr<> in the |reply| OnceClosure 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_.task_runner()->PostTaskAndReply(

  //          FROM_HERE,

  //          base::BindOnce(&DataBuffer::AddData, buffer),

  //          base::BindOnce(&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 Location& from_here,

                        OnceClosure task,

                        OnceClosure reply);

 protected:

  friend struct 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_