google base之IncomingTaskQueue

如同名称描述的那样，这个类就是个taskqueue，也就是任务队列，添加任务到队列，然后由MessageLoop去执行task,比较关心的函数如下：

bool IncomingTaskQueue::AddToIncomingQueue(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay,
    bool nestable) {
  AutoLock locked(incoming_queue_lock_);
  PendingTask pending_task(
      from_here, task, CalculateDelayedRuntime(delay), nestable);
  return PostPendingTask(&pending_task);
}

将closeure封装到了 PendingTask 这个里边值得注意的是 from_here 这个其实就是个位置信息，记录了当前代码所在文件，代码所在的行，这个主要是为了追踪，不得不说google在这方面还是考虑得非常周全的。下面看PostPendingTask(&pending_task);这个函数;

bool IncomingTaskQueue::PostPendingTask(PendingTask* pending_task) {
  // Warning: Don't try to short-circuit, and handle this thread's tasks more
  // directly, as it could starve handling of foreign threads.  Put every task
  // into this queue.

  // This should only be called while the lock is taken.
  incoming_queue_lock_.AssertAcquired();

  if (!message_loop_) {
    pending_task->task.Reset();
    return false;
  }

  // Initialize the sequence number. The sequence number is used for delayed
  // tasks (to faciliate FIFO sorting when two tasks have the same
  // delayed_run_time value) and for identifying the task in about:tracing.
  pending_task->sequence_num = next_sequence_num_++;

  message_loop_->task_annotator()->DidQueueTask("MessageLoop::PostTask",
                                                *pending_task);

  bool was_empty = incoming_queue_.empty();
  incoming_queue_.push(*pending_task);
  pending_task->task.Reset();

  // Wake up the pump.
  message_loop_->ScheduleWork(was_empty);

  return true;
}

很显然，将task放到队列，然后调用 message_loop_->ScheduleWork(was_empty);  也就是执行计划任务，代码如下

void MessageLoop::ScheduleWork(bool was_empty) {
  if (was_empty || AlwaysNotifyPump(type_))
    pump_->ScheduleWork();
}

又转交给了pump

void MessagePumpDefault::ScheduleWork() {
  // Since this can be called on any thread, we need to ensure that our Run
  // loop wakes up.
  event_.Signal();
}

这个也简单，就是对事件置信，为什么要这么做呢，这个要看pump的代码才知道，因为在没有任务的时候会一直在event上面等侍，当置信了以后又开始循环值行任务。