target parameter in DispatchQueue

Question

In Swift 3, the creation of a DispatchQueue instance:

DispatchQueue(label: String,
              qos: DispatchQoS,
              attributes: DispatchQueue.Attributes,
              autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency, 
              target: DispatchQueue?)

I see the sample codes from StackOverFlow, it can be nil, .global() or .main, what's the meaning of this target parameter?

I guess .main means the queue will run on main thread, but what for .nil or .global() ?

Answer 1

There's no documentation for Swift so I dropped back to the old documentation for GCD. The closest that I've found is for the function dispatch_set_target_queue:

An object's target queue is responsible for processing the object. The target queue determines the queue on which the object's finalizer is invoked. In addition, modifying the target queue of some objects changes their behavior:

Dispatch queues:

A dispatch queue's priority is inherited from its target queue. Use the dispatch_get_global_queue function to obtain a suitable target queue of the desired priority.

If you submit a block to a serial queue, and the serial queue’s target queue is a different serial queue, that block is not invoked concurrently with blocks submitted to the target queue or to any other queue with that same target queue.

So looks like the target queue does 2 things:

1. Provide the priority for your new queue
2. Executes the finalizer (deinit) of all objects in your queue

Reading between the lines, there are some sychronization between your queue and the target queue. I don't have Xcode at the moment so I can't test.