WaitForFixedUpdate在物理帧的最后执行：

https://docs.unity3d.com/Manual/ExecutionOrder.html

 

Time.frameCount是逻辑帧自增，和物理帧无关 

https://answers.unity.com/questions/933751/timeframecount-vs-timerenderedframecount.html

 

 

  IEnumerator CountDown_WaitForFixedUpdate()
    {
        Debug.Log("1yield WaitForFixedUpdate - step  " + Time.frameCount + " fixedCount:" + fixedCount);
        yield return new WaitForFixedUpdate();
        Debug.Log("2yield WaitForFixedUpdate - step   " + Time.frameCount + " fixedCount:" + fixedCount);
        //yield return new WaitForFixedUpdate();
        //         Debug.Log("3yield WaitForFixedUpdate - step  " + Time.frameCount);
    }


    public new Coroutine StartCoroutine(IEnumerator enumerator)
    {
        return base.StartCoroutine(ToFixedUpdate(enumerator));
    }

    IEnumerator ToFixedUpdate(IEnumerator a)
    {
        while (a.MoveNext())
        {
            var cur = a.Current;
            if (cur == null)
                cur = new WaitForFixedUpdate();
            yield return cur;
        }
    }



    // Update is called once per frame
    void Update()
    {
        Debug.LogError("Update  " + Time.frameCount);
    }

    private static int fixedCount = 0;
    private void FixedUpdate()
    {
        fixedCount = fixedCount + 1;
        Debug.LogError("0 FixedUpdate  " + Time.frameCount + " fixedCount:" + fixedCount);
        Debug.LogError("1 FixedUpdate " + Time.frameCount + " fixedCount:" + fixedCount);
        this.StartCoroutine(CountDown_WaitForFixedUpdate());
        Debug.LogError("2 FixedUpdate " + Time.frameCount + " fixedCount:" + fixedCount);
    }