Is DispatchQueue using RunLoop?

Question

Every Thread has its own RunLoop, how DispatchQueue interact with them? Is DispatchQueue using RunLoop to dispatch task to Thread or do it by another way?

Answer 1

Any thread can have a run loop, but, nowadays, in practice, only the main thread does.

When you create a thread manually, it will not have a run loop. When you call RunLoop.current, the name suggests that it is grabbing the thread’s run loop, suggesting that it always will have one. But in reality, when you call current, it will return the run loop if one is already there, and if not, it creates a RunLoop for you. As the docs say:

If a run loop does not yet exist for the thread, one is created and returned.

And if you do create a run loop, you have to spin on it yourself (as shown here; and that example is over-simplified). But we don’t do that very often anymore. GCD has rendered it largely obsolete.

At a high level, GCD has pools of worker threads, one pool per quality of service (QoS). When you dispatch something via GCD to any queue (other than targeting the main queue), it grabs an available worker thread of the appropriate QoS, performs the task, and when done, marks the worker thread as available for future dispatched tasks. No run loop is needed (or desired) for these worker threads.