    protected function doPop($priority = null, $block = false)
    {
        // 只想取出一个 $priority 队列的数据
        if ($this->isPriority($priority)) {
            // $priority 级别的队列还未初始化。
            // $queue = $this->createQueue($priority);
            if (!$this->hasQueue($priority)) {
                return null;
            }

            return $this->queues[$priority]->lPop();
        }

        $data = null;

        foreach ($this->queues as $pri => $queue) {
            // $queue = $queue ?: $this->createQueue($pri);
            if (!$queue) {
                continue;
            }

            if (false !== ($data = $queue->lPop())) {
                break;
            }
        }

        // reset($this->queues);
        return $data;
    }