Calling function from another script attatched to another object not working

Question

I am trying to call a function in another c# script, that is attatched to another gameObject in my scene. I am creating an instance of the script LevelChanger in the Grounded one like this:

LevelChanger levelChanger;

Then, in the Awake() function:

GameObject gameObject = new GameObject("LevelChanger");
levelChanger = gameObject.AddComponent<LevelChanger>();

then calling like this in an IEnumerator:

levelChanger.FadeOut(true); // line 178

Then this is the LevelChanger class:

using System.Collections;
using UnityEngine;

public class LevelChanger : MonoBehaviour
{
    public Animator animator;

    public void FadeOut(bool fadeIn)
    {
        animator.SetBool("Fade", true); // line 10

        if (fadeIn) StartCoroutine(FadeInAsWell());
    }

    IEnumerator FadeInAsWell()
    {
        yield return new WaitForSeconds(0.9f);

        animator.SetBool("Fade", false);
    }
}

I've been trying to solve this problem for quite a while now, I checked everywhere (here, here, here, here and on other sites). I saw that because my LevelChanger script is attatched to a gameobject, so it is MonoBehaviour, it is not possible to create an instance of the class from another script, like this:

LevelChanger levelChanger = new LevelChanger();

or either "normally" like I was doing at first like this:

LevelChanger levelChanger;

Then just calling its functions like

levelChanger.FadeOut(true);

Most of the time I was getting a NullReferenceException at runtime (at line 176), now directly in the LevelChanger script (at line 10).

At this moment, I truly have no idea how to fix this: does anybody know how? (I'm still a beginner).

Thanks in advance!

Answer 1

Quick and dirty because little has been shared about the code and scene in question.

In Awake, get a reference to the gameObject with the already existing LevelChanger on it:

GameObject levelChangerGO = GameObject.Find("LevelChanger");
levelChanger = levelChangerGO.GetComponent<LevelChanger>();

then calling like this in an IEnumerator:

levelChanger.FadeOut(true); // line 178

Answer 2

Based on the conversation we've had in comments, I'll write an answer here to be more thorough.

So, let's recap. What you want to do is call a function from LevelChanger. But, you are unsure of how to get a reference to LevelChanger. This is the core problem every programmer faces: How do i get that stuff over there, and what's the best way to do?

Based on what you've said, your prefab exists in the scene, so you want to grab a reference to it.

One simple, lazy way is to call FindObjectOfType<LevelChanger>. This will search through the entire scene for components of that type, and will return your LevelChanger instance. I don't recommend this because it's pretty lazy and inefficient. This is only needed if one or both of your objects exist at run-time but not at edit-time

Another way is to treat LevelChanger like a singleton.

add a public static LevelChanger Instance; field to your LevelChanger. Then, on Awake() set Instance to this. ie Instance = this;

public static LevelChanger Instance;
public void Awake()
{
    Instance = this;
}

Then, in any other script, you can call LevelChanger.FadeOut(false);

This is a similarly lazy way, but not quite inefficient. It does make your code a little harder to follow and there are a lot of devs that have problems with static instances like this. Just so you know.

One other way is to create an instance of the prefab using GameObject.Instantiate() and calling the function after getting a reference to the component. This is a bit more involved, but it might be a cleaner way for you.

There are two ways to do this, so lets do the cleanest way. In your project, create a folder called Resources if you don't already have one. Resources is a name unity will specifically look for when calling Resources.Load(). Drag your prefab object into that folder to create a new prefab. Call it LevelChanger

//Spawn the prefab gameobject
GameObject gameObject = GameObject.Instantiate(Resources.Load("LevelChanger")) as GameObject;
//Get a reference to its component LevelChanger
LevelChanger levelChanger = gameObject.GetComponent<LevelChanger>();
//Call the function
levelChanger.FadeOut(false);

After your prefab finishes the fade, you can have it destroy itself via Destroy(gameObject);

Finally, since you've indicated both scripts exist at edit time, you can simply add an inspector reference. This is the easiest way, but don't go crazy with inspector references. It makes the code difficult to follow.

public LevelChanger levelChanger;

Answer 3

I went through a problem similar to that when I was trying to show Ads using Unity Ads. I solved it by first importing Unity.VisualScripting and then calling GetOrAddComponent<>() function.

using Unity.VisualScripting; \*first import VisualScripting*\
using UnityEngine;
using UnityEngine.SceneManagement;

public class GameTitleBehaviour : MonoBehaviour
{
    public void LoadLevel(string levelName)
    {
        UnityAdController unityAdController = gameObject.GetOrAddComponent<UnityAdController>(); \*Then calling GetOrAddComponent<>()*\

        \*can now use UnityAdController functions*\
        if (unityAdController.GetIsShowAds())
        {
            unityAdController.ShowAd();
        }

        SceneManager.LoadScene(levelName);
    }
}

In your case it would be:

First import VisualScripting

using Unity.VisualScripting;

Then call GetOrAddComponent<>()

LevelChanger levelChanger = gameObject.GetOrAddComponent<LevelChanger>();

Now you can use LevelChanger's functions

levelChanger.FadeOut(true);