Introduction to Coroutines in Unity scripts from https://www.youtube.com/watch?v=FfgI0BJ38P4. If you get value from LlamAcademy, consider becoming a Patreon supporter at https://www.patreon.com/llamacademy

FadeAlpha.cs

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

[RequireComponent(typeof(Image))]
public class FadeAlpha : MonoBehaviour
{
    public bool UseRealtimeWait = false;
    private Image Image;

    private Coroutine Coroutine;

    private void OnGUI()
    {
        if (GUI.Button(new Rect(new Vector2(25, 25), new Vector2(150, 40)), "Stop Fading"))
        {
            if (Coroutine != null)
            {
                StopCoroutine(Coroutine);
            }
        }
        if (GUI.Button(new Rect(new Vector2(25, 75), new Vector2(150, 40)), "Start Fading"))
        {
            StartFading();
        }
    }

    private void Awake()
    {
        Image = GetComponent<Image>();
    }

    void StartFading()
    {
        Coroutine = StartCoroutine(FadeOutAlpha());
    }

    IEnumerator FadeOutAlpha()
    {
        // Uncomment this line if you want to compare WaitForSeondsRealtime vs WaitForSeconds
        // Time.timeScale = 0.5f; 

        if (UseRealtimeWait)
        {
            WaitForSecondsRealtime Wait = new WaitForSecondsRealtime(1 / 30f);

            for (float alpha = 1; alpha >= 0; alpha -= 0.01f)
            {
                Color color = Image.color;

                color.a = alpha;

                Image.color = color;

                yield return Wait;
            }
        }
        else
        {
            WaitForSeconds Wait = new WaitForSeconds(1 / 30f);

            for (float alpha = 1; alpha >= 0; alpha -= 0.01f)
            {
                Color color = Image.color;

                color.a = alpha;

                Image.color = color;

                yield return Wait;
            }

        }
    }
}

FadeAlphaWithTarget.cs

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

[RequireComponent(typeof(Image))]
public class FadeAlphaWithTarget : MonoBehaviour
{
    public bool WaitUntilTargetIsInVisible;
    public Image DependencyImage;

    private Image Image;
    private void Awake()
    {
        Application.targetFrameRate = 60;
        Image = GetComponent<Image>();
    }

    void Start()
    {
        StartCoroutine(FadeAlpha());
    }

    IEnumerator FadeAlpha()
    {
        if (WaitUntilTargetIsInVisible)
        {
            yield return new WaitUntil(() => DependencyImage.color.a <= 0.01f);
        }
        else
        {
            yield return new WaitWhile(() => DependencyImage.color.a > 0);
        }

        for (float alpha = 1; alpha >= 0; alpha -= 0.01f)
        {
            Color color = Image.color;

            color.a = alpha;

            Image.color = color;

            yield return null;
        }
    }
}

FadeWaitingForFixedUpdate.cs

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

[RequireComponent(typeof(Image))]
public class FadeWaitingForFixedUpdate : MonoBehaviour
{
    private Image Image;

    private void Awake()
    {
        Image = GetComponent<Image>();
    }

    void Start()
    {
        StartCoroutine(FadeAlpha());
    }

    IEnumerator FadeAlpha()
    {
        WaitForFixedUpdate Wait = new WaitForFixedUpdate();

        for (float alpha = 1; alpha >= 0; alpha -= 0.01f)
        {
            Color color = Image.color;

            color.a = alpha;

            Image.color = color;

            Debug.Log("1. Waiting for Fixed Update");
            yield return Wait;

            Debug.Log("3. Fixed Update Completed");

        }
    }

    private void FixedUpdate()
    {
        Debug.Log("2. Fixed Update");
    }
}