Instead of using yield wait for seconds can I use a UI button for async?

Question

the code below is the code I use to load my game scene ater the intro animation plays. The way it works is that it preloads the game scene because it is big enough it takes time to load and activates it after 15 seconds.

I want to use this code in a similar manner for my main menu, but instead of preloading and waiting for seconds I want it to preload and wait for UI button.

public class IntroToForest : MonoBehaviour
{
    public float WaitToLoad = 14.0f;

    void Start()
    {
        StartCoroutine(LoadScene(WaitToLoad));
    }

    IEnumerator LoadScene(float WaitToLoad)
    {
        yield return null;
        AsyncOperation asyncOperation = SceneManager.LoadSceneAsync("Forest Map");
        asyncOperation.allowSceneActivation = false;

        while (!asyncOperation.isDone)
        {
            if (asyncOperation.progress >= 0.9f)
            {
                yield return new WaitForSeconds(WaitToLoad);
                asyncOperation.allowSceneActivation = true;
            }
            yield return null;
        }
    }
}

Side note: I incorporated DmGregory's answer into my menu controller script and got this:

public class MenuController : MonoBehaviour
{
    public string SceneName;

    AsyncOperation preload;

    void Start()
    {
        preload = SceneManager.LoadSceneAsync(SceneName);
        preload.allowSceneActivation = false;
    }
    public void Go_to_game()
    {
        if (preload.progress < 0.9f)
        {
            if (PlayerPrefs.HasKey("HasSeenIntro"))
            {
            preload.allowSceneActivation = true;
            }
            else
            {
                PlayerPrefs.SetInt("HasSeenIntro", 1);
                SceneManager.LoadScene("Intro");
            }
        }
    }
}

The problem is that the UI button will not work.

Answer 1

Rather than running this as a coroutine, ticking every frame waiting to see if the button is pressed, I'd just store the AsyncOperation and let the button give it a kick when the player finally presses it.

public class ScenePreloader : MonoBehaviour {

    public string sceneName;

    AsyncOperation preload;

    void Start() {
        preload = SceneManager.LoadSceneAsync(sceneName);
        preload.allowSceneActivation = false;
    }

    // Call this from your UI button.
    public void BeginTransition() {
        preload.allowSceneActivation = true;

        if(preload.progress < 0.9f) {
            // Here, you might want to display a loading spinner,
            // so you have coverage if the user is very fast / load is slow.
        }            
    }
}

Answer 2

Just use a boolean flag.

public class IntroToForest : MonoBehaviour
{
    public bool buttonPressed = false;

    void Start()
    {
        StartCoroutine(LoadScene("Forest Map"));
    }

    IEnumerator LoadScene(string scene)
    {
        AsyncOperation asyncOperation = SceneManager.LoadSceneAsync(scene);
        asyncOperation.allowSceneActivation = false;

        while (!asyncOperation.isDone)
        {
            if (asyncOperation.progress >= 0.9f)
            {
                if(buttonPressed)
                    asyncOperation.allowSceneActivation = true;
            }
            yield return null;
        }
    }
}

Configure your button in the Inspector to set buttonPressed to true when pressed.

Answer 3

You can use WaitWhile to define a condition for the coroutine's yield. Something like this could work, with a button configured to update the value of buttonPressed accordingly.

public class IntroToForest : MonoBehaviour
{
    public bool buttonPressed;

    void Start()
    {
        StartCoroutine(LoadScene());
    }

    IEnumerator LoadScene()
    {
        AsyncOperation asyncOperation = SceneManager.LoadSceneAsync("Forest Map");
        asyncOperation.allowSceneActivation = false;

        while(!asyncOperation.isDone)
        {
            yield return new WaitWhile(() => !buttonPressed);
            if (asyncOperation.progress >= 0.9f)
            {
                asyncOperation.allowSceneActivation = true;
            }
        }
    }
}