JisolGame/JNFrame2/Assets/HotScripts/JNGame/Runtime/Util/SingletonScene.cs

using System;
using UnityEngine;


namespace JNGame.Runtime.Util
{
    /// <summary>
    /// Be aware this will not prevent a non singleton constructor
    ///   such as `T myT = new T();`
    /// To prevent that, add `protected T () {}` to your singleton class.
    ///
    /// As a note, this is made as MonoBehaviour because we need Coroutines.
    /// </summary>
    public class SingletonScene<T> : MonoBehaviour where T : MonoBehaviour
    {
        private static T _instance;

        private static object _lock = new object();

        public static T Instance
        {
            get
            {
                if (applicationIsQuitting)
                {
                    Debug.LogWarning("[Singleton] Instance '" + typeof(T) +
                                     "' already destroyed on application quit." +
                                     " Won't create again - returning null.");
                    return null;
                }

                lock (_lock)
                {
                    if (_instance == null)
                    {
                        try
                        {
                            _instance = (T)FindObjectOfType(typeof(T));

                            if (FindObjectsOfType(typeof(T)).Length > 1)
                            {
                                Debug.LogError("[Singleton] Something went really wrong " +
                                               " - there should never be more than 1 singleton!" +
                                               " Reopening the scene might fix it.");
                                return _instance;
                            }

                            if (_instance == null)
                            {
                                GameObject singleton = new GameObject();
                                _instance = singleton.AddComponent<T>();
                                singleton.name = "(singleton) " + typeof(T).ToString();

                                DontDestroyOnLoad(singleton);

                                Debug.Log("[Singleton] An instance of " + typeof(T) +
                                          " is needed in the scene, so '" + singleton +
                                          "' was created with DontDestroyOnLoad.");
                            }
                            else
                            {
                                Debug.Log("[Singleton] Using instance already created: " +
                                          _instance.gameObject.name);
                            }
                        }
                        catch (Exception e)
                        {
                            Debug.LogError(e.Message);
                        }
                    }

                    return _instance;
                }
            }
        }

        private static bool applicationIsQuitting = false;

        /// <summary>
        /// When Unity quits, it destroys objects in a random order.
        /// In principle, a Singleton is only destroyed when application quits.
        /// If any script calls Instance after it have been destroyed,
        ///   it will create a buggy ghost object that will stay on the Editor scene
        ///   even after stopping playing the Application. Really bad!
        /// So, this was made to be sure we're not creating that buggy ghost object.
        /// </summary>
        public void OnDestroy()
        {
            applicationIsQuitting = true;
            OnDispose();
        }
        
        protected virtual void OnDispose()
        {
        }
    }
}