Unity keyboard state issue between scene loads

Question

I'm facing an issue when loading scenes in unity.

My game is composed of full screen levels where players exit to the next level when touching the edges. So, when the player touches an edge I load next level and position the player at the correct position to start traversing the new scene. For example if I'm exiting Level1 from the right side, I load Level2 and I position player on the left.

Player is a "DonDestroyOnLoad" game object.

The problem is that unity seems to only keep last key pressed between scene loads. For example if I walk to the right in level1 and reach the edge of level, second level is loaded and player keeps moving to the right because the key is still pressed. But, If I walk to the right in level1 and jump (while pressing right arrow key) and I touch the edge of the screen, level2 is loaded but the Right key is lost (the first I pressed in level one).

I have found this here at stackexchange -> Why doesn't Unity keep keydown states between scenes, and how can I change that? but it seems very hacky.

Answer 1

This appears to be a bug wherein even a keyboard polling gameObject loses track of second keypresses across scene loads. One workaround is to keep a minimal main level that keeps track of all the inputs, scores, handles basic mechanics etc, and loads and unloads scenery overlays additively, so those controllers always persist.

Here's a simplistic script that illustrates this:

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

public class KeyPressTest : MonoBehaviour
{
    float timer;
    bool nextSceneLoaded;
    public string nextScene;


    void Update()
    {
        timer += Time.deltaTime;
        if(timer > 2 && !nextSceneLoaded)
        {
            SceneManager.LoadScene(nextScene,LoadSceneMode.Additive);
            nextSceneLoaded = true;
        }
        if (Input.GetKey(KeyCode.A))
        {
            Debug.Log("A " + timer.ToString());
        }
        if (Input.GetKey(KeyCode.S))
        {
            Debug.Log("S " + timer.ToString());
        }
    }
}

Put this in a start scene and provide a second scene name. Hold down the A & S keys and you'll see they continue to both be polled after the second scene loads.

EDIT Note -- editted intro to this answer per @Notbad's clarifying comment below, as after further experimentation it does appear that even if a gameObject is marked as DontDestroyOnload it will lose the second keypress when a new scene is loaded in single mode. To illustrate this, here is the above script modified so the start scene is discarded and only the poller persists. Sure enough, after the second scene loads, only one key will still be polled...

// Doesn't Work! 
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

public class KeyPressTest : MonoBehaviour
{
    float timer;
    bool nextSceneLoaded;
    public string nextScene;

    private void Start()
    {
        DontDestroyOnLoad(gameObject);
    }


    void Update()
    {
        timer += Time.deltaTime;
        if(timer > 2 && !nextSceneLoaded)
        {
            SceneManager.LoadScene(nextScene,LoadSceneMode.Single);
            nextSceneLoaded = true;
        }
        if (Input.GetKey(KeyCode.A))
        {
            Debug.Log("A " + timer.ToString());
        }
        if (Input.GetKey(KeyCode.S))
        {
            Debug.Log("S " + timer.ToString());
        }
    }
}
```

Answer 2

Just for reference I ended doing this in my game controller:

void Start()
{
    DontDestroyOnLoad(gameObject);
}

void OnEnable()
{
    SceneManager.sceneLoaded += OnSceneLoaded;
}

void OnDisable()
{
    SceneManager.sceneLoaded -= OnSceneLoaded;
}

public void MoveToNextScene(string sceneName, ExitType exitType)
{
    _lastExitType = exitType;
    SceneManager.LoadScene(sceneName, LoadSceneMode.Additive);
}



void OnSceneLoaded(Scene scene, LoadSceneMode mode)
{
    StartCoroutine(_OnSceneLoaded(scene));
}

// Update is called once per frame
IEnumerator _OnSceneLoaded(Scene scene)
{
    var asyncOperation = SceneManager.UnloadSceneAsync(0);
    while (!asyncOperation.isDone)
        yield return asyncOperation;
    ...
}

Thanks to @Acme Nerd Games instructions. It felt a bit hacky to me but well, it at least works and I can move to other more important subjects.

The only thing that I don't like is that I could end with both screens loaded for a litle period of time and the player could notice it. But it is easy fixable with a quick fade out effect.