__block int timeout = 60;

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

    dispatch_source_t source_t = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

    dispatch_source_set_timer(source_t, dispatch_walltime(NULL, 0), 1*NSEC_PER_SEC, 0);

    dispatch_source_set_event_handler(source_t, ^{

        if (timeout<=0) {

            dispatch_source_cancel(source_t);

            dispatch_async(dispatch_get_main_queue(), ^{

                NSLog(@"重新开始倒计时");

            });

        }

        else

        {

            dispatch_async(dispatch_get_main_queue(), ^{

                NSLog(@"开始倒计时剩余%.2ds",timeout%60);

            });

            timeout --;

        }

    });

    dispatch_resume(source_t);