/WebRTC任务队列学习笔记

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)
      : impl_(task_queue.release()) {}

  ~TaskQueue() {
      impl_->Delete();
  }

  // Used for DCHECKing the current queue.
  bool IsCurrent() const {
      impl_->IsCurrent();
  }

  // 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) {
      return impl_->PostTask(std::move(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) {
      return impl_->PostDelayedTask(std::move(task), 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_;

  RTC_DISALLOW_COPY_AND_ASSIGN(TaskQueue);
};

void RtpTransportControllerSend::OnSentPacket(
    const rtc::SentPacket& sent_packet) {
  task_queue_.PostTask([this, sent_packet]() {
    RTC_DCHECK_RUN_ON(&task_queue_);
    absl::optional<SentPacket> packet_msg =
        transport_feedback_adapter_.ProcessSentPacket(sent_packet);
    pacer()->UpdateOutstandingData(
        transport_feedback_adapter_.GetOutstandingData());
    if (packet_msg && controller_)
      PostUpdates(controller_->OnSentPacket(*packet_msg));
  });
}

void RtpTransportControllerSend::OnReceivedPacket(
    const ReceivedPacket& packet_msg) {
  task_queue_.PostTask([this, packet_msg]() {
    RTC_DCHECK_RUN_ON(&task_queue_);
    if (controller_)
      PostUpdates(controller_->OnReceivedPacket(packet_msg));
  });
}

class RTC_LOCKABLE RTC_EXPORT TaskQueueBase {
 public:
  // Starts destruction of the task queue.
  // On return ensures no task are running and no new tasks are able to start
  // on the task queue.
  // Responsible for deallocation. Deallocation may happen syncrhoniously during
  // Delete or asynchronously after Delete returns.
  // Code not running on the TaskQueue should not make any assumption when
  // TaskQueue is deallocated and thus should not call any methods after Delete.
  // Code running on the TaskQueue should not call Delete, but can assume
  // TaskQueue still exists and may call other methods, e.g. PostTask.
  virtual void Delete() = 0;

  // Schedules a task to execute. Tasks are executed in FIFO order.
  // If |task->Run()| returns true, task is deleted on the task queue
  // before next QueuedTask starts executing.
  // When a TaskQueue is deleted, pending tasks will not be executed but they
  // will be deleted. The deletion of tasks may happen synchronously on the
  // TaskQueue or it may happen asynchronously after TaskQueue is deleted.
  // This may vary from one implementation to the next so assumptions about
  // lifetimes of pending tasks should not be made.
  virtual void PostTask(std::unique_ptr<QueuedTask> task) = 0;

  // 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.
  virtual void PostDelayedTask(std::unique_ptr<QueuedTask> task,
                               uint32_t milliseconds) = 0;

  // Returns the task queue that is running the current thread.
  // Returns nullptr if this thread is not associated with any task queue.
  static TaskQueueBase* Current() {
      return current;
  }
  bool IsCurrent() const { return Current() == this; }

 protected:
  class CurrentTaskQueueSetter {
   public:
    ABSL_CONST_INIT thread_local TaskQueueBase* current = nullptr;

    explicit CurrentTaskQueueSetter(TaskQueueBase* task_queue) : previous_(current) {
        current = task_queue;
    }

    ~CurrentTaskQueueSetter() {
        current = previous_;
    }

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

   private:
    TaskQueueBase* const previous_;
  };

  // Users of the TaskQueue should call Delete instead of directly deleting
  // this object.
  virtual ~TaskQueueBase() = default;
};