Unity 如何实现一个定时器

using System;
using UnityEngine;

/// 
/// 时间单位
/// 
public enum TimeUnit
{
    MILLSECOND, //毫秒
    SECOND,     //秒
    MINUTE,     //分钟
    HOUR,       //小时
}

/// 
/// 定时器
/// 
public class Timer
{
    /// 
    /// 计时结束回调事件
    /// 
    public event Action OnEnd;
    /// 
    /// 计时更新事件
    /// 
    public event Action OnUpdate;

    //定时时长
    private float duration;
    //开始计时时间
    private float beginTime;
    //已计时间
    private float elapsedTime;
    //缓存时间（用于暂停）
    private float cacheTime;
    //是否循环
    private bool loop;
    //是否暂停
    private bool isPaused;
    //是否计时完成
    private bool isFinished;
    
    /// 
    /// 构造函数
    /// 
    /// 定时时长
    /// 时间单位
    /// 是否循环
    public Timer(float duration, TimeUnit timeUnit = TimeUnit.SECOND, bool loop = false)
    {
        switch (timeUnit)
        {
            case TimeUnit.MILLSECOND:   this.duration = duration / 1000;  break;
            case TimeUnit.SECOND:       this.duration = duration;         break;
            case TimeUnit.MINUTE:       this.duration = duration * 60;    break;
            case TimeUnit.HOUR:         this.duration = duration * 3600;  break;
        }
        this.loop = loop;
        beginTime = Time.realtimeSinceStartup;
    }
    /// 
    /// 更新定时器
    /// 
    public void Update()
    {
        if(!isFinished && !isPaused)
        {
            elapsedTime = Time.realtimeSinceStartup - beginTime;
            OnUpdate?.Invoke(Mathf.Clamp(elapsedTime, 0, duration));
            if(elapsedTime >= duration)
            {
                OnEnd?.Invoke();
                if (loop)
                {
                    beginTime = Time.realtimeSinceStartup;
                }
                else
                {
                    isFinished = true;
                }
            }
        }
    }
    /// 
    /// 暂停定时器
    /// 
    public void Pause()
    {
        if (!isFinished)
        {
            isPaused = true;
            cacheTime = Time.realtimeSinceStartup;
        }
    }
    /// 
    /// 恢复定时器
    /// 
    public void Unpause()
    {
        if(!isFinished && isPaused)
        {
            beginTime += Time.realtimeSinceStartup - cacheTime;
            isPaused = false;
        }
    }
    /// 
    /// 停止定时器
    /// 
    public void Stop()
    {
        isFinished = true;
    }
}

Example:

    Timer timer;
    private void Start()
    {
        timer = new Timer(3f);
        timer.OnEnd += () => Debug.Log("计时结束 总计时长3秒");
        timer.OnUpdate += v => Debug.Log($"计时中 已计时长{v}秒");
    }

    private void Update()
    {
        timer.Update();
    }