iOS-发送短信验证码倒计时

/**
 发送手机验证码
 */
-(void)startSenderYzmMessage{
    __block int timeout = ; //倒计时时间
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, , ,queue);
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, ),1.0*NSEC_PER_SEC, ); //每秒执行
    dispatch_source_set_event_handler(_timer, ^{
        if(timeout<=){ //倒计时结束，关闭
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                //此处倒计时完毕  回复正常状态
            });
        }else{
            // int minutes = timeout / 60;
            int seconds = timeout % ;
            NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
            if ([strTime isEqualToString:@""]) {
                strTime = @"";
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                //此处倒计时状态，更新倒计时显示

            });
            timeout--;
        }
    });
    dispatch_resume(_timer);
}