dispatch_async in NSManagedObjectContext's performBlock

Question

According to the WWDC 2012 video, "Core Data Best Practices", dispatch_sync should be used to run some kind of callback in performBlock of context, which is created as a type of NSPrivateQueueConcurrencyType.

Why is that?

Can I use dispatch_async(dispatch_get_main_queue(), 0)... to call some UI-related callbacks in private queue's context's performBlock?

Answer 1

No. NSPrivateQueueConcurrencyType manages it's own internal queue, and does not enjoy you trying to leave one of it's threads to do what you want to do (in fact, I believe it throws exceptions when this type of behavior occurs). There's a few ways of handling this, of course signalling semaphores being the more acceptable design pattern to get the truly "async feel", but as you note, dispatch_sync is usually the way to go.