Singleton.cs

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

// author: mvaganov@hotmail.com
// license: Copyfree, public domain. This is free code! Great artists, steal this code!
// latest version at: https://pastebin.com/raw/JMme8A2X
// works well with SceneCombiner: https://pastebin.com/raw/8cZ5yKSd
namespace NS {
    public class Singleton : MonoBehaviour {
        [Tooltip("If another Singleton object with this onlyAllowOne is added to the scene, it will Destroy itself after starting.\nWorks well with SceneCombiner.")]
        public string onlyAllowOne = "";
        [Tooltip("Mark true to keep this object in the scene when changing to another scene.\nThis can be a tricky design, be careful!")]
        public bool dontDestroyOnLoad = true;
        void Start () {
            bool alreadyHere = false;
            GameObject[] objs = FindObjectsOfType<GameObject> ();
            for (int i = 0; i < objs.Length && !alreadyHere; ++i) {
                Singleton[] singles = objs [i].GetComponents<Singleton> ();
                if (singles != null && singles.Length > 0) {
                    for (int d = 0; d < singles.Length; ++d) {
                        // if one of me already exists...
                        if (singles [d] != this && ((onlyAllowOne != "" && singles [d].onlyAllowOne == onlyAllowOne)
                                                || (onlyAllowOne == "" && singles [d].gameObject.name == gameObject.name))) {
                            alreadyHere = true;
                            break;
                        }
                    }
                }
            }
            if (alreadyHere) {
                Debug.Log("\'"+onlyAllowOne+"\' Singleton already exists, removing \'"+gameObject+"\'");
                gameObject.SetActive (false);
                onlyAllowOne = null;
                Destroy (gameObject);
            } else if(dontDestroyOnLoad) {
                DontDestroyOnLoad (transform.gameObject);
            }
        }

        private static Dictionary<System.Type, Component> instances = new Dictionary<System.Type, Component>();
        public static T GetComponentInstance<T>() where T : Component {
            Component instance = null;
            if(!instances.TryGetValue(typeof(T), out instance)) {
                T[] searchInstance = GameObject.FindObjectsOfType<T> ();
                if (searchInstance != null) {
                    if (searchInstance.Length > 1) {
                        throw new System.Exception ("multiple instances of " + typeof(T).Name + " found, should be singleton");
                    } else if (searchInstance.Length != 0) {
                        instance = searchInstance [0];
                    }
                }
                if(instance == null) {
                    GameObject g = new GameObject();
                    instance = g.AddComponent<T>();
                    g.name = "<" + instance.GetType().Name + ">";
                    instances [typeof(T)] = instance;
                }
            }
            return instance as T;
        }
    }
}