rtc_base/task_queue.h

/*
 *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#ifndef RTC_BASE_TASK_QUEUE_H_
#define RTC_BASE_TASK_QUEUE_H_

#include <stdint.h>

#include <memory>
#include <utility>

#include "absl/memory/memory.h"
#include "api/task_queue/queued_task.h"
#include "api/task_queue/task_queue_base.h"
#include "api/task_queue/task_queue_factory.h"
#include "rtc_base/system/rtc_export.h"
#include "rtc_base/task_utils/to_queued_task.h"
#include "rtc_base/thread_annotations.h"

namespace rtc {
// Implements a task queue that asynchronously executes tasks in a way that
// guarantees that they're executed in FIFO order and that tasks never overlap.
// Tasks may always execute on the same worker thread and they may not.
// To DCHECK that tasks are executing on a known task queue, use IsCurrent().
//
// Here are some usage examples:
//
//   1) Asynchronously running a lambda:
//
//     class MyClass {
//       ...
//       TaskQueue queue_("MyQueue");
//     };
//
//     void MyClass::StartWork() {
//       queue_.PostTask([]() { Work(); });
//     ...
//
//   2) Posting a custom task on a timer.  The task posts itself again after
//      every running:
//
//     class TimerTask : public QueuedTask {
//      public:
//       TimerTask() {}
//      private:
//       bool Run() override {
//         ++count_;
//         TaskQueueBase::Current()->PostDelayedTask(
//             absl::WrapUnique(this), 1000);
//         // Ownership has been transferred to the next occurance,
//         // so return false to prevent from being deleted now.
//         return false;
//       }
//       int count_ = 0;
//     };
//     ...
//     queue_.PostDelayedTask(std::make_unique<TimerTask>(), 1000);
//
// For more examples, see task_queue_unittests.cc.
//
// A note on destruction:
//
// When a TaskQueue is deleted, pending tasks will not be executed but they will
// be deleted.  The deletion of tasks may happen asynchronously after the
// TaskQueue itself has been deleted or it may happen synchronously while the
// TaskQueue instance is being deleted.  This may vary from one OS to the next
// so assumptions about lifetimes of pending tasks should not be made.
class RTC_LOCKABLE RTC_EXPORT TaskQueue {
 public:
  // TaskQueue priority levels. On some platforms these will map to thread
  // priorities, on others such as Mac and iOS, GCD queue priorities.
  using Priority = ::webrtc::TaskQueueFactory::Priority;

  explicit TaskQueue(std::unique_ptr<webrtc::TaskQueueBase,
                                     webrtc::TaskQueueDeleter> task_queue);
  ~TaskQueue();

  TaskQueue(const TaskQueue&) = delete;
  TaskQueue& operator=(const TaskQueue&) = delete;

  // Used for DCHECKing the current queue.
  bool IsCurrent() const;

  // Returns non-owning pointer to the task queue implementation.
  webrtc::TaskQueueBase* Get() { return impl_; }

  // TODO(tommi): For better debuggability, implement RTC_FROM_HERE.

  // Ownership of the task is passed to PostTask.
  void PostTask(std::unique_ptr<webrtc::QueuedTask> task);

  // Schedules a task to execute a specified number of milliseconds from when
  // the call is made. The precision should be considered as "best effort"
  // and in some cases, such as on Windows when all high precision timers have
  // been used up, can be off by as much as 15 millseconds (although 8 would be
  // more likely). This can be mitigated by limiting the use of delayed tasks.
  void PostDelayedTask(std::unique_ptr<webrtc::QueuedTask> task,
                       uint32_t milliseconds);

  // std::enable_if is used here to make sure that calls to PostTask() with
  // std::unique_ptr<SomeClassDerivedFromQueuedTask> would not end up being
  // caught by this template.
  template <class Closure,
            typename std::enable_if<!std::is_convertible<
                Closure,
                std::unique_ptr<webrtc::QueuedTask>>::value>::type* = nullptr>
  void PostTask(Closure&& closure) {
    PostTask(webrtc::ToQueuedTask(std::forward<Closure>(closure)));
  }

  // See documentation above for performance expectations.
  template <class Closure,
            typename std::enable_if<!std::is_convertible<
                Closure,
                std::unique_ptr<webrtc::QueuedTask>>::value>::type* = nullptr>
  void PostDelayedTask(Closure&& closure, uint32_t milliseconds) {
    PostDelayedTask(webrtc::ToQueuedTask(std::forward<Closure>(closure)),
                    milliseconds);
  }

 private:
  webrtc::TaskQueueBase* const impl_;
};

}  // namespace rtc

#endif  // RTC_BASE_TASK_QUEUE_H_