即使从更新调用，如何使用 StartCoroutine 使协程每次仅启动一次，如何淡入/淡出屏幕？

Question

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;

public class SceneFader : MonoBehaviour
{
    #region FIELDS
    public GameObject fadeOutUIGameobjectImage;
    public float fadeSpeed = 3f;
    //public SaveLoad saveLoad;

    private Image fadeOutUIImage;

    private void Start()
    {
        SceneManager.sceneLoaded += SceneManager_sceneLoaded;
    }

    public enum FadeDirection
    {
        In, //Alpha = 1
        Out // Alpha = 0
    }
    #endregion

    #region FADE
    public IEnumerator Fade(FadeDirection fadeDirection)
    {
        fadeOutUIGameobjectImage.SetActive(true);

        float alpha = (fadeDirection == FadeDirection.Out) ? 1 : 0;
        float fadeEndValue = (fadeDirection == FadeDirection.Out) ? 0 : 1;
        if (fadeDirection == FadeDirection.Out)
        {
            while (alpha >= fadeEndValue)
            {
                SetColorImage(ref alpha, fadeDirection);
                yield return null;
            }
            fadeOutUIGameobjectImage.SetActive(false);
        }
        else
        {
            fadeOutUIGameobjectImage.SetActive(true);
            while (alpha <= fadeEndValue)
            {
                SetColorImage(ref alpha, fadeDirection);
                yield return null;
            }
        }
    }
    #endregion

    #region HELPERS
    public IEnumerator FadeAndLoadScene(FadeDirection fadeDirection, string sceneToLoad)
    {
        yield return Fade(fadeDirection);

        SceneManager.LoadSceneAsync(sceneToLoad);
    }

    private void SceneManager_sceneLoaded(Scene arg0, LoadSceneMode arg1)
    {
        if (MenuController.LoadSceneForSavedGame == true)
        {
            var saveLoad = GameObject.Find("Save System").GetComponent<SaveLoad>();
            saveLoad.Load();
        }
    }

    private void SetColorImage(ref float alpha, FadeDirection fadeDirection)
    {
        if(fadeOutUIImage == null)
        {
            fadeOutUIImage = fadeOutUIGameobjectImage.GetComponent<Image>();
        }

        fadeOutUIImage.color = new Color(fadeOutUIImage.color.r, fadeOutUIImage.color.g, fadeOutUIImage.color.b, alpha);
        alpha += Time.deltaTime * (1.0f / fadeSpeed) * ((fadeDirection == FadeDirection.Out) ? -1 : 1);
    }
    #endregion
}

Using it in other scripts like this :

At the top :

public SceneFader sceneFader; 

Then :

StartCoroutine(sceneFader.Fade(SceneFader.FadeDirection.In));

And

StartCoroutine(sceneFader.Fade(SceneFader.FadeDirection.Out));

The problem is if I need to fade in or out at some point in the game and call it from Update in some script/s. Then it will start the Coroutine all the time over again.

I can use a flag to set it to false and true so it will start the Coroutine once but it's a bit annoying to use a flag each time in scripts. Maybe there is a way in the SceneFader script to make sure that no matter if I call the Coroutine from other script/s in Update and it will know to start the Coroutine once only? In the Update, it will call it each frame but in the SceneFader script, it will know to start the Coroutine only once when need to start it.

Still not working. I changed the SceneFader to this :

Added the flag isFading but still the fade out is interrupting the fade in. It's not that the fade in or the fade out start a lot of time in the Update they start once but the fade out start before the fade in finish.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;

public class SceneFader : MonoBehaviour
{
    #region FIELDS
    public GameObject fadeOutUIGameobjectImage;
    public float fadeSpeed = 3f;

    private Image fadeOutUIImage;
    private bool isFading = false;

    private void Start()
    {
        SceneManager.sceneLoaded += SceneManager_sceneLoaded;
    }

    public enum FadeDirection
    {
        In, //Alpha = 1
        Out // Alpha = 0
    }
    #endregion

    #region FADE
    public IEnumerator Fade(FadeDirection fadeDirection)
    {
        if(isFading)
        {
            yield return null;
        }

        isFading = true;

        fadeOutUIGameobjectImage.SetActive(true);

        float alpha = (fadeDirection == FadeDirection.Out) ? 1 : 0;
        float fadeEndValue = (fadeDirection == FadeDirection.Out) ? 0 : 1;
        if (fadeDirection == FadeDirection.Out)
        {
            while (alpha >= fadeEndValue)
            {
                SetColorImage(ref alpha, fadeDirection);
                yield return null;
            }

            fadeOutUIGameobjectImage.SetActive(false);
        }
        else
        {
            fadeOutUIGameobjectImage.SetActive(true);

            while (alpha <= fadeEndValue)
            {
                SetColorImage(ref alpha, fadeDirection);
                yield return null;
            }
        }

        isFading = false;
    }
    #endregion

    #region HELPERS

    public IEnumerator FadeAndLoadScene(FadeDirection fadeDirection, string sceneToLoad)
    {
        yield return Fade(fadeDirection);

        SceneManager.LoadSceneAsync(sceneToLoad);
    }
    private void SceneManager_sceneLoaded(Scene arg0, LoadSceneMode arg1)
    {
        if (MenuController.LoadSceneForSavedGame == true)
        {
            var saveLoad = GameObject.Find("Save System").GetComponent<SaveLoad>();
            saveLoad.Load();
        }
    }

    private void SetColorImage(ref float alpha, FadeDirection fadeDirection)
    {
        if (fadeOutUIImage == null)
        {
            fadeOutUIImage = fadeOutUIGameobjectImage.GetComponent<Image>();
        }

        fadeOutUIImage.color = new Color(fadeOutUIImage.color.r, fadeOutUIImage.color.g, fadeOutUIImage.color.b, alpha);
        alpha += Time.deltaTime * (1.0f / fadeSpeed) * ((fadeDirection == FadeDirection.Out) ? -1 : 1);
    }
    #endregion
}

In another script where I'm using the fade in/out I did something like this:

I'm starting another new Coroutine :

StartCoroutine(WaitBeforeOut());

Then inside there : I'm waiting 5 seconds before starting the fade out. This 5 seconds is enough time to let the fade in to finish and then to start the fade out :

but it's not a good solution. I don't want in any script that I'm using the fade in/out to do it this way. It's working but not so good a solution.

IEnumerator WaitBeforeOut()
    {
        yield return new WaitForSeconds(5f);

        StartCoroutine(sceneFader.Fade(SceneFader.FadeDirection.Out));
    }

Answer 1

the code for fading is incredibly simple. in pseudocode,

boolean alreadyFading = false

function doFade
   if alreadyFading == true return
   alreadyFading = true
   start coroutine _fade

coroutine _fade
   if alpha > 0.01
     alpha = alpha - 0.01
     loop
   alreadyFading = false

Yes, of course you need a local flag there, a boolean, to see if you are already animating. Note that this is true of every animation of any type (on any system!)