子线程延迟执行

1.默认为主线程，dispatch_get_main_queue()修改后以下

//子线程延迟执行
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //须要执行的操做
        });