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?
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.
Google got me here, so if anyone is new to this stuff as I am, and would benefit from a code example, here is one:
__block dispatch_queue_t currentQ = dispatch_get_current_queue();
[managedObjectContext performBlock:^{
//do the stuff you need to do in a background thread
dispatch_sync(currentQ, ^(){
//callback code for once the background stuff is complete
});
}];
来源：https://stackoverflow.com/questions/13468705/dispatch-async-in-nsmanagedobjectcontexts-performblock