unity3d singleton

singleton.cs

using UnityEngine;
 
/// <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 Singleton<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)
    {
     _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);
     }
    }
 
    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;
 }
}

//public class Manager : Singleton<Manager> {
// protected Manager () {} // guarantee this will be always a singleton only - can't use the constructor!
 
 //public string myGlobalVar = "whatever";
//}