> On Sep 21, 2016, at 10:13 AM, Doug Hill <cocoa...@breaqz.com> wrote:
> The symptoms you mention sound like a classic deadlock problem. That is, a 
> code block running on the main queue schedules another code block on the main 
> queue, but the original block never completes thus causing the app to hang or 
> the new task never runs.

Worth noting that this can only occur if both of the cross-thread dispatches 
are synchronous. As long as one of them is async, there will be no deadlock. 
This is a very good reason for preferring async dispatches! If you need to 
return a value back to the original thread, or just perform something after the 
UI update finishes, the idiom is to issue another (nested) dispatch_async at 
the end of the first block, dispatching back to the background thread. That 
nested block can then perform the response/cleanup actions.


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