NSManagedObjectContext saving/merging over multiple threads through notifcation center issue?

I think the first works because of the asynchronous dispatch. Without that, the notification center will pause until any particular notification completes. The problem you were getting sounds like a typical stall in which the code just pauses without error until eventually the debugger times out.

Not sure if this helps, but where you have nil for the queue try using a generic queue. Take a look at the receptionist design pattern in the apple docs.

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