When I run my app in the simulator at some point the UI becomes completely unresponsive (table view doesn't scroll, buttons don't react, etc).
I know that in general this is the result of locking up the main thread (which is responsible for handling the UI) but all heavy load is happening asynchronously (as far as I can tell).
I've looked at the CPU meter and all threads seem to be at 0%. Oddly enough, if I try to profile using instruments then the UI doesn't get locked up. I also didn't notice anything odd in instruments, CPU usage seems to be normal/low and the time profiler doesn't seem to show any odd behavior.
My general question is: What are some other reasons why the UI would become unresponsive even if the CPU load is 0? What are the best ways to debug this kind of thing?
More specifically: The details of the app are probably too complex for this particular question, but just for some context:
- I'm using AFIncrementalStore + CoreData
- The UI unresponsiveness only happens when I try to fetch multiple relationships (via the incremental store delegate.
- During this time there doesn't seem to be any network activity (that I can tell). I can monitor logs on my server and nothing seems to be hitting it during that time.
- As mentioned above, when running with instruments/profiling the UI doesn't lock up.
- I'm testing this with a single view controller using a fetched results controller to pull the data.
My best guess is that there's a deadlock happening somewhere on the main thread, which is why the CPU load reports 0% but still the UI is unresponsive. I have no idea how to debug this however. This could either be something deep in CoreData land or could be something in one of the 3rd party AF-classes. I've sprinkled breakpoints all over the place and none of them get hit.
I've been scratching my head for a few hours now. Without getting into details about my particular app's implementation, I was hoping to get some general debugging tips for this sort of problem. Anything else I should try?
I'm running XCode 5.0.2, the simulator is on iOS 7.0.3 (for what it's worth).
Any tips & tricks are greatly appreciated!