重新启动游戏后，GameObjects将不会加载

Question

I am making a maze game and the keys which are needed to be collected to complete it wont appear again if the game restarts, I get the following error;

MissingReferenceException: The object of type 'MazeDirectives' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.

I am just disabling the MazeKey object, not destroying it, can anyone help? Below is my code;

MazeKey.cs

using UnityEngine;
using System.Collections;

public class MazeKey : MonoBehaviour
{
    void OnTriggerEnter2D(Collider2D other)
    {

        transform.parent.SendMessage("OnKeyFound", SendMessageOptions.DontRequireReceiver);
        gameObject.SetActive(false);
    }
}

MazeDirectives.cs

MazeGoal mazeGoal;
MazeKey mazeKey;

void StartDirectives()
    {
        mazeGoal = Instantiate(mazeGoalPrefab, MazeGenerator.instance.mazeGoalPosition, Quaternion.identity) as MazeGoal;
        mazeGoal.transform.SetParent(transform);

        mazeKeyPositions = MazeGenerator.instance.GetRandomFloorPositions(keysToFind);

        for (int i = 0; i < mazeKeyPositions.Count; i++)
        {
            MazeKey mazeKey = Instantiate(mazeKeyPrefab, mazeKeyPositions[i], Quaternion.identity) as MazeKey;
            mazeKey.transform.SetParent(transform);

        }
    }

To restart the game I use the code below;

void OnTriggerEnter2D(Collider2D other)
{
    if (other.tag == "Player")
    {
        SceneManager.LoadScene(SceneManager.GetActiveScene().name);
        gameObject.SetActive(true);
    }
}

MazeGoal.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class MazeGoal : MonoBehaviour
{

public Sprite closedGoalSprite;
public Sprite openedGoalSprite;

void Start()
{

    GetComponentInChildren<SpriteRenderer>().sprite = closedGoalSprite;



}

public void OpenGoal()
{
    GetComponentInChildren<SpriteRenderer>().sprite = openedGoalSprite;
}

void OnTriggerEnter2D()
{
    transform.parent.SendMessage("OnGoalReached", SendMessageOptions.DontRequireReceiver);
}

Answer 1

Explenation

The exception you get is not talking about the MazeKey object but rather the MazeDirectives component.

Unfortunately you hit the most important information in the comments:

private void Awake() 
{ 
    MazeGenerator.OnMazeReady += StartDirectives; 
}

so OnMazeReady seems to be static and not instanced so it will not be destroyed when a new Scene is loaded but keeps intact bloating into the new Scene!

When you call

MazeGenerator.OnMazeReady += StartDirectives;

you add the call to the StartDirectives method of an instance of MazeDirectives as listener to that static event.

Now when you reload the Scene all GameObject s and thereby their instances of components are destroyed

=> so is the instance of MazeGenerator ... BUT the static event OnMazeReady is not destroyed!

so after the next Awake call you now have two listeners

• The one from the "second"/new loaded Scene
• Still the "old" one you added the first time

But since the instance of MazeDirectives you added the first listener for is destroyed when the scene is reload and a new instance generated, you get that exception

MissingReferenceException: The object of type 'MazeDirectives' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object.

when the method tries to access the transform value of the destroyed instance.

---

Solution 1a

So you should remove the listener when you destroy the instance

private void OnDestroy()
{
     MazeGenerator.OnMazeReady -= StartDirectives;
}

---

Solution 1b

or overwrite it with only exactly one listener at a time

private void Awake() 
{ 
    MazeGenerator.OnMazeReady = StartDirectives; 
}

this second aproach obviously is only useful when there is no other instance or class listening to that event. The question is how much sense does it make to use an event than? And I would than anyway remove it if not needed just to be sure

private void OnDestroy()
{
     MazeGenerator.OnMazeReady = null;
}

---

Solution 2

I would prefere this solution.

Don't make MazeGenerator.OnMazeReady static at all. Since anyway I see that you are using a Singleton pattern eg in

 MazeGenerator.instance.mazeGoalPosition

you could instead just make OnMazeReady Non-static and instead use it the same way:

private void Awake()
{
    MazeGenerator.instance.OnMazeReady += startDirectives;
}

so it will be destroyed together with that instance of MazeGenerator .

---

General note

I would always remove all listeners I ever added as soon as possible to avoid exactly the issue you have.

You could additionally remove it eg already inside of StartDirectives to make sure the method is executed only once even if the same Scene "accidentely" invoked OnMazeReady twice.

Hint: I said additionally since it is always save/possible to remove a listener even if it wasn't added before and you should allways leave the one in OnDestroy in case the StartDirectives is never called before the object is destroyed.

Answer 2

Update:

This answer is wrong at first place. The exception is complaining about accessing the MazeDirectives 's transform , not mazeGoal object. But the comments below did give some useful info. So I'm keeping this post for references.

For complete solution, see here .

---

From the line mazeGoal.transform.SetParent(transform); throws the exception:

MissingReferenceException: The object of type 'MazeDirectives' has been 
destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.

From here :

The load of a new Scene destroys all current Scene objects.

The mazeGoal has been destroyed when you called the

SceneManager.LoadScene(SceneManager.GetActiveScene().name);

to restart the game.

And from MonoBehaviour.Awake() ,

Awake is called only once during the lifetime of the script instance.

Since you only assign the mazeGoal variable inside StartDirectives function which been called in Awake , after loading the same scene again, the actual object of mazeGoal has been destroyed.

If you want to reuse the same object when loading a new scene, you can use DontDestroyOnLoad to keep the mazeGoal object.

Or you can move the StartDirectives to Start function which will be called every time the gameobject is created and reinitialize your mazeGoal .