淡入（Fade in）：类似打开幕布

淡出（Fade out）：类似关上幕布

方案一  使用Dotween（推荐）

using DG.Tweening;
using UnityEngine;
using UnityEngine.UI;

public class Test : MonoBehaviour
{
    public Image curtain;
    private float fadeTime = 1;
    
    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.A))
        {
            SetAlpha(true);//测试淡入
        }
        
        if (Input.GetKeyDown(KeyCode.D))
        {
            SetAlpha(false);//测试淡出
        }
    }

    /// <summary>
    /// 设置淡入淡出
    /// </summary>
    /// <param name="isFadeIn">是否是淡入</param>
    private void SetAlpha(bool isFadeIn)
    {
        curtain.DOFade(isFadeIn ? 0 : 1, fadeTime);
    }

    private void OnDisable()
    {
        curtain.DOKill();
    }
}

方案二 使用协程

using System.Collections;
using UnityEngine;
using UnityEngine.UI;

public class Test : MonoBehaviour
{
    public Image fadePlane;
    private float fadeTime = 1;
    
    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.A))
        {
            StartCoroutine(Fade(true));//测试淡入
        }
        
        if (Input.GetKeyDown(KeyCode.D))
        {
            StartCoroutine(Fade(false));//测试淡出
        }
    }
    
    IEnumerator Fade(bool isFadeIn)
    {
        var from = fadePlane.color;
        var to = new Color(from.r, from.g, from.b, isFadeIn ? 0 : 1);
        
        float speed = 1 / fadeTime;
        float percent = 0;

        while (percent < 1)
        {
            percent += Time.deltaTime * speed;
            fadePlane.color = Color.Lerp(from, to, percent);
            yield return null;
        }
    }
}

方案三  使用Animation(不推荐)