ios如何设定时常并判断操作是否超时

Question

我的写法是

NSDate *timeout = [[NSDate alloc]initWithTimeIntervalSinceNow:30];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    while ([timeout timeIntervalSinceNow] > 0) {
        NSLog(@"%f",[timeout timeIntervalSinceNow]);
        //do check stuff
        sleep(1);
    }
});