Question

If I use a performSelectorOnMainThread call inside a detached thread, when does the main thread execute the request? Does it do so immediately after it finishes the current main thread operation, or is there some other type of hierarchy that determines when the performSelectorOnMainThread call executes?

Answer 1

It executes on the main thread on the next iteration of the main thread’s run loop.

Quoting Apple’s documentation:

This method queues the message on the run loop of the main thread using the default run loop modes—that is, the modes associated with the NSRunLoopCommonModes constant. As part of its normal run loop processing, the main thread dequeues the message (assuming it is running in one of the default run loop modes) and invokes the desired method.