unity协程，频繁调用时，只执行最后一次.

    public string log;
    Coroutine async;
    float threshold=5f; //时间阈值，小于该时间，则只执行后一次
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            if (async != null)
            {
                Debug.Log("停下上一个: " + log);
                StopCoroutine(async);// 如果在指定时间内，有新的操作，取消上一个协程
            }
            log = "www" + Random.Range(0, 100);
            async = StartCoroutine(DelayedAction(log));
        }
    }
    IEnumerator DelayedAction(string content)
    {
        yield return new WaitForSeconds(threshold);
        Debug.Log("success >> " + content);
        async = null;
    }