Singleton

using UnityEngine;

//  Code from:
// https://www.youtube.com/watch?v=ErJgQY5smnw&t=3s

/// <summary> If you don't already understand this class, you can't be trusted to use it. </summary>
public abstract class Singleton<T> : MonoBehaviour where T : Component {
    private static T instance;

    protected static bool DontDestroy = true;
    private static bool m_applicationIsQuitting = false;
    private void OnApplicationQuit() => m_applicationIsQuitting = true;

    /// <summary> If the instance is defined, go get it. If not, go make it, unless the application is quitting. </summary>
    public static T GetInstance() {
        //if (ApplicationLifecycle.quitting) return null;
        if (m_applicationIsQuitting) { return null; }

        if (instance == null) {
            instance = FindObjectOfType<T>();
            if (instance == null) {
                GameObject obj = new GameObject();
                obj.name = typeof(T).Name;
                instance = obj.AddComponent<T>();
            }
            // Loup for debugging
            //callingType = new StackTrace().GetFrame(1).GetMethod().DeclaringType;
        }
        return instance;
    }

    /// <summary> If the instance is defined (and application not qutting), return TRUE and out the instance.
    /// Else, return FALSE and out null. This is like GetInstance, but does NOT try to instantiate
    /// the instance if it does not exist. Mostly call this when application is quitting, or if singleton can be destroyed.</summary>
    public static bool TryGetInstance(out T inst) {
        if (m_applicationIsQuitting || instance == null) { inst = null; return false; }
        inst = instance; return true;
    }

    /* IMPORTANT!!! To use Awake in a derived class you need to do it this way
     * protected override void Awake()
     * {
     *     base.Awake();
     *     //Your code goes here
     * }
     * */
    protected virtual void Awake() {
        //UnityEngine.Debug.Assert(instance == null, "Instance of " + typeof(T) +
        //    " already exists when a singleton of it called its awake! It was called by " + callingType);

        if (instance == null) {
            instance = this as T;
            if (DontDestroy) DontDestroyOnLoad(gameObject);
        } else if (instance != this as T) {
            Destroy(gameObject);
        } else if (DontDestroy) { DontDestroyOnLoad(gameObject); }
    }
}