gistfile1.m

- (void)someMethodThatDoesATimeIntensiveTaskWithCompletionBlock:(void (^)(void))block
{

	dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul), ^{
	
		while (someCondition) {
			NSErorr *error;
			// Do the time intensive task here, passing the error object along
			
			if (error) {
				dispatch_async(dispatch_get_main_queue(), ^{
				
					if (block) {
						block();
					}
					
					return;
				});
			}
		}
		
		
		dispatch_async(dispatch_get_main_queue(), ^{
		
			if (block) {
				block();
			}
		});
	});
}