Unity: How to switch scenes and when returning to the original scene, return player to the place they switched their scene

Question

I have two scenes: Main Street & Building Scene When the player is Main Street, if the player's trigger box touches the building and the player presses "q", the scene would switch to the Building Scene. I want it so that when the player exits the Building Scene and returns to the Main Street Scene, the player is back to the position they entered the Building Scene that they entered from. Apologies in advance if this doesn't make sense.

sceneSwitchingScript:

    public int buildingToLoad;
    public Text InputText;
    public movement player;
    public Vector3 playerPrevPos;

    void OnTriggerEnter2D(Collider2D col){
        if(col.CompareTag("Player")){
            InputText.text = ("[Q] to enter");
            if(Input.GetKeyDown("q")){
                if (gameObject.tag == "EntryPoint"){
                    playerPrevPos = new Vector3(player.transform.position.x, player.transform.position.y, player.transform.position.z);
                }
                //Debug.Log(gameObject.tag);
                Application.LoadLevel(buildingToLoad);  
            }
            
        }
    }

    void OnTriggerStay2D(Collider2D col){
        if(col.CompareTag("Player")){
            if(Input.GetKeyDown("q")){
                //spawn = new Vector3(player.transform.position.x, player.transform.position.y, player.transform.position.z)
                Application.LoadLevel(buildingToLoad);
                if (gameObject.tag == "EntryPoint"){
                    playerPrevPos = new Vector3(player.transform.position.x, player.transform.position.y, player.transform.position.z);
                }
            }
        }
    }

    void OnTriggerExit2D(Collider2D col){
        if(col.CompareTag("Player")){
            InputText.text = ("");
        }
    }

Setting the player's position when they exit the building

public switchScene ss;
void OnTriggerStay2D(Collider2D col){
        if(Input.GetKeyDown("q")){
            if(col.gameObject.CompareTag("ExitPoint")){
                transform.position = ss.playerPrevPos;
            }       
        }
    }

However, these two scrips do not work and I'm not sure if this is related but when I make the player do the switch scene thing in-game, this error pops up:

NullReferenceException: Object reference not set to an instance of an object movement.OnTriggerStay2D (UnityEngine.Collider2D col)

This error message mentions the error on this line:

transform.position = ss.playerPrevPos;

Answer 1

With Unity, the traditional loading of a scene was fairly destructive. There was the concept of setting a GameObject as safe by setting it as DontDestroyOnLoad, which, in a way, removed the object from the scene altogether, which protected it from scene loads.

Unity finally properly implemented multiscene editing and loading. It's essentially DDOL, but done properly.

Now you can actually have multiple scenes loaded at the same time. Now, what that allows you to do is to have a "Manager" scene that handles all of the objects that are common between scenes, and only load (and unload) the specific objects required for that individual scene.

You'd use it like this:

SceneManager.LoadScene("YourScene", LoadSceneMode.Additive);

That would load "YourScene" in ADDITION to the currently loaded scene. Likewise, removing a scene is:

SceneManager.UnloadScene("YourScene");

Now, if you have a Manager scene, you can include in your manager scene a script that holds data for each individual scene. As a hacky example, you might have:

public Vector3 InsideSceneLastPosition { get; set; }

Which you then assign to before loading your outside scene. When you load your inside scene again, you can read InsideSceneLastPosition again to reposition your character.

Here's the link to the LoadSceneAsync page at Unity.

There's more to it than that, for instance, you have to listen for the SceneManager.sceneLoaded event to know when you've actually loaded the next scene, so that you can reposition your GameObejcts. You can find information about that here.

You can see multiscene editing my simply dragging multiple scenes from the Project window into you Hierarchy window. All the scenes listed will additively work together. But you'll have to be careful though, as there's another "gotcha" in that you cant reference objects from one scene to another. You can call scripts cross scene, but you won't be able to drag a game object from one scene, into the object field of a component on a different scene. Don't worry, you'll get the hang on it =)