倒计时很好用的

    __block int timeout=60; //倒计时时间
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
    dispatch_source_set_event_handler(_timer, ^{
        if(timeout<=0){ //倒计时结束，关闭
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                [l_timeButton setTitle:@"发送验证码" forState:UIControlStateNormal];
                [l_timeButton addTarget:self action:@selector(RepeatStartTime) forControlEvents:UIControlEventTouchUpInside];
                l_timeButton.userInteractionEnabled = YES;
            });
        }else{
            if (timeout==60)
            {
                NSString * strTime = [NSString stringWithFormat:@"%d",timeout];
                [l_timeButton setTitle:[NSString stringWithFormat:@"重新发送(%@)",strTime] forState:UIControlStateNormal];
            }
            else{
            //            int minutes = timeout / 60;
                seconds = timeout % 60;
                NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
                dispatch_async(dispatch_get_main_queue(), ^{
                    //设置界面的按钮显示 根据自己需求设置
                    NSLog(@"____%@",strTime);
                    [l_timeButton setTitle:[NSString stringWithFormat:@"重新发送(%@)",strTime] forState:UIControlStateNormal];
                    l_timeButton.userInteractionEnabled = NO;
                });
            }