+(int) addA: (int) a toB: (int) b {
    sleep(3);
    return a + b;
}

dispatch_queue_t queue = dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0);

dispatch_async(queue, ^{
        int result = [self addA:3 toB:5];
        NSLog(@"result: %i", result);
});

NSLog(@"main thread");
//printed:
//main thread
//result: 8