ios中关于发送验证码的计时

1、点击发送验证码，开始计时

-(void)startTime{

    __block int timeout=120; //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
                [self.sendAuthCode setTitle:@"发送验证码" forState:UIControlStateNormal];
                self.sendAuthCode.userInteractionEnabled = YES;
                self.sendAuthCode.titleLabel.font = [UIFont systemFontOfSize:10];
            });
        }else{

            int seconds = timeout % 120;
            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
                [self.sendAuthCode setTitle:[NSString stringWithFormat:@"%@秒后重发",strTime] forState:UIControlStateNormal];
                self.sendAuthCode.titleLabel.font = [UIFont systemFontOfSize:10];
                self.sendAuthCode.userInteractionEnabled = NO;

            });
            timeout--;

        }
    });
    dispatch_resume(_timer);

}