In which thread are iOS completion handler blocks called?
In practical terms it doesn't matter.
If you need your completion to run in the main thread, just dispatch it to the main thread:
[score reportScoreWithCompletionHandler:^(NSError *error) { dispatch_async(dispatch_get_main_queue(), ^{ // do your stuff here });}];
There is no specific thread for completion handlers, apple documentation says it will be a secondary thread (Definitely not the main thread). You can use DispatchQueue to access different threads in iOS.