`[NSThread isMainThread]` always returns YES

Question

This code

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
      NSLog(@"Main Thread? %d", [NSThread isMainThread]);
});

shows that I'm in the main thread. Even doing this:

queue = dispatch_queue_create("nonMainQueue", NULL);

still reports that I'm in the main queue. This is, it seems, because I'm using dispatch sync.

Does this mean that my code is the same as not using dispatch_sync at all? Also: what's the point of dispatch_sync if it does nothing at all, then?

Answer 1

Because queues are not threads, in order to check if you are on the main 'queue', you must use different code, something similar to this:

if (dispatch_get_current_queue() == dispatch_get_main_queue()) {
   NSLog(@"On Main Thread!");
}

Just note that dispatch_get_current_queue is deprecated, and is subject to be completely removed in a later iOS/Mac OS version.