dispatch_semaphore_t 的使用

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(second * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_semaphore_signal(semaphore);
    });
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);