private void Update()
    {
        if (ifStart)
        {
            if (second > 0)
            {
                second = second - Time.deltaTime;
                if (second / 60 < 1)
                {
                    txtTimer.text = string.Format("00:{0:d2}", (int) second % 60);
                }
                else
                {
                    txtTimer.text = string.Format("{0:d2}:{1:d2}", (int) second / 60, (int) second % 60);
                }
            }
            else
            {
                txtTimer.text = "00:00";
                Debug.Log("计时结束");
                ifStart = false;
            }
        }
    }