Perform a block in a background thread, and call a completion block on the main thread when its done.

gistfile1.m

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
                        {
                            // Background thread stuff.
    
                            dispatch_async(dispatch_get_main_queue(), ^
                                           {
                                             // Main thread stuff.
                                           });
                        });