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I encountered a problem with non-iOS developer while describing the flow of OperationQueue. We already know that with OperationQueue we can start as many threads as we want to execute the tasks in sync/async way.

But in practical, some of the people want proof for the OperationQueue is getting executed in the background and not with UI(Main) thread. I just want to demonstrate that when operation queue starts, it already starts its execution in the background. I already have convinced that when we try to set qos for the operationQueue that we create, it has all the parameters of global queue's qos viz: default userInitiated userInteractive background and utility. So that is already perfect example in code to prove that all the OperationQueue operations mentioned are run on global thread. Unless we declare the OperationQueue.main

Vin
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As Shadowrun said, you can add assertions for Thread.isMainThread. Also, if you ever want to add test that you are not on the main queue, you can add a precondition:

dispatchPrecondition(.notOnQueue(.main))

But it should be noted that the whole purpose of creating an operation queue is to get things off the main thread. E.g., the old Concurrency Programming Guide: Operation Queues says [emphasis added]:

Operations are designed to help you improve the level of concurrency in your application. Operations are also a good way to organize and encapsulate your application’s behavior into simple discrete chunks. Instead of running some bit of code on your application’s main thread, you can submit one or more operation objects to a queue and let the corresponding work be performed asynchronously on one or more separate threads.

This is not to say that operation queues cannot contribute to main thread responsiveness problems. You can wait from the main thread for operations added to an operation queue (which is obviously a very bad idea). Or, if you neglect to set a reasonable maxConcurrentOperationCount and have thread explosion, that can introduce all sorts of unexpected behaviors. Or you can entangle the main thread with operation queues through a misuse of semaphores or dispatch groups.

But operations on an operation queue (other than OperationQueue.main) simply do not run on the main thread. Technically, you could get yourself in trouble if you started messing with the target queue of the underlying queue, but I can’t possibly imagine that you are doing that. If you are having main thread problems, your problem undoubtedly rests elsewhere.

Rob
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