退出队列是一个单线程操作，即使消息流本身配置为多实例时也是如此。
Dequeuing is single threaded, even if the message flow itself is configured for multiple instances.
所有新线程都被放在全局运行队列中。
这个队列允许您将任何物体进入它，为此事甚至线程。
This queue allows you to put any object into it, for that matter even Thread.
的调度队列上，并且它是完全线程安全的。
It runs entirely within its own GCD dispatch_queue, and is completely thread-safe.
然后一组线程可以从队列中取出并执行他们。
Then a set of threads can take from the queue and execute them in order.
然后一组线程可以从队列中取出并执行他们。
Then a set of threads can take from the queue and execute them in order.
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