使用GCD验证码倒计时

 __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);