Unity_Utils/Singleton.cs

using System;
using UnityEngine;

namespace Core
{
    public abstract class Singleton<T> : MonoBehaviour where T : Singleton<T>
    {
        private static T _instance;

        public static T Instance
        {
            get
            {
                if (!HasInstance)
                {
                    Debug.LogWarning(
                        $"Trying to access a singleton of type {typeof(T).Name} that is not in the scene. " +
                        "If you are trying to see if the Instance exists, you should use the HasInstance static property instead.");
                }

                return _instance;
            }
        }

        private bool _destroyedByScript;

        public static bool HasInstance => _instance != null;

        protected virtual void Awake()
        {
            _destroyedByScript = false;

            if (HasInstance)
            {
                Debug.Log($"Two or more instances of a singleton of type {typeof(T).Name} were found in the scene. " +
                          "The new instance of the singleton trying to register will be removed.");

                DestroyInstance();
                return;
            }

            try
            {
                _instance = (T) Convert.ChangeType(this, typeof(T));
            }
            catch (InvalidCastException)
            {
                Debug.Assert(false, "Singleton's T type should be the derived class.");
            }
        }

        private void DestroyInstance()
        {
            _destroyedByScript = true;
            if (gameObject.GetNumberOfComponents() == 1)
            {
                Destroy(gameObject);
            }
            else
            {
                Destroy(this);
            }
        }

    protected virtual void OnDestroy()
        {
            if (_instance == this)
            {
                _instance = null;
            }
            else if (!_destroyedByScript)
            {
                Debug.LogWarning($"Instance of {typeof(T).Name} deleted, but was not the singleton instance.");
            }
        }
    }
}