帮我看看源代码的逻辑对不对，如何修改

源代码如下：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class gfdg : MonoBehaviour {
    public float timer = 12;
   // public bool OnTimer = false;
    // Use this for initialization
    void Start () {
        
    }
    
    // Update is called once per frame
    void Update () {
        if(Input.GetKeyDown(KeyCode.H))
        {
            StartCoroutine(Countdown());
            
        }
        if(Input.GetKeyDown(KeyCode.J))
        {
            StopCoroutine(Countdown());
            
        }
        
    }
    IEnumerator Countdown()
    {
        while (true)
        {
           
                timer -= Time.deltaTime;
           
            
                yield return new WaitForSeconds(timer=12f);

            Debug.Log("This message appears after 12 seconds!");
        }
    }
}