tim tregubov

tell me a story

Generic Singleton MonoBehavior Class

In Unity3D having a singleton class is very useful, whether for “global” state or simply for the convenience of having a static accessor so you don’t have to have lots of: FindObjectOfType(typeof(Builder)) as Builder;

So you code up a C# singleton and then realize that you actually need it to be a MonoBehavior, not just a ScriptableObject – say you want the singleton to run coroutines, or have a transform, or any other MonoBehavior feature. But monobehaviors can’t be initialized with a constructor.

So what you want is a monobehavior pseudo singleton pattern. The unity community wiki has some good info about the various options but all require boilerplate.

So I wrote a parametrized generic pseudo singleton monobehavior class. I attempted to compile all the best advice out there, so it is threadsafe, has some extra stuff for setting a parent, doesn’t implement an Awake, and initializes lazily.

The key was the C# where keyword, used to constrain the parameterizing type. So in this case both the class is a MonoBehavior and its parameterization type is a MonoBehavior.

So now this blog has its first code section:

Singleton.cs # 
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using UnityEngine;
using System.Collections;

/// <summary>
/// MONOBEHAVIOR PSEUDO SINGLETON ABSTRACT CLASS
/// usage    : best is to be attached to a gameobject but if not that is ok,
///      : this will create one on first access
/// example  : '''public sealed class MyClass : Singleton<MyClass> {'''
/// references   : http://tinyurl.com/d498g8c
///      : http://tinyurl.com/cc73a9h
///      : http://unifycommunity.com/wiki/index.php?title=Singleton
/// </summary>
public abstract class Singleton<T> : MonoBehaviour where T : MonoBehaviour
{

  private static T _instance = null;
          
  /// <summary>
  /// gets the instance of this Singleton
  /// use this for all instance calls:
  /// MyClass.Instance.MyMethod();
  /// or make your public methods static
  /// and have them use Instance
  /// </summary>
  public static T Instance {
      get {
          if (_instance == null) {
              _instance = (T)FindObjectOfType (typeof(T));
              if (_instance == null) {
                  
                  string goName = typeof(T).ToString ();           
                  
                  GameObject go = GameObject.Find (goName);
                  if (go == null) {
                      go = new GameObject ();
                      go.name = goName;
                  }
                  
                  _instance = go.AddComponent<T> ();                 
              }
          }
          return _instance;
      }
  }
  
  /// <summary>
  /// for garbage collection
  /// </summary>
  public virtual void OnApplicationQuit ()
  {
      // release reference on exit
      _instance = null;
  }
  
  // in your child class you can implement Awake()
  // and add any initialization code you want such as
  // DontDestroyOnLoad(go);
  // if you want this to persist across loads
  // or if you want to set a parent object with SetParent()
  
  /// <summary>
  /// parent this to another gameobject by string
  /// call from Awake if you so desire
  /// </summary>
  protected void SetParent (string parentGOName)
  {
      if (parentGOName != null) {
          GameObject parentGO = GameObject.Find (parentGOName);
          if (parentGO == null) {
              parentGO = new GameObject ();
              parentGO.name = parentGOName;
          }
          this.transform.parent = parentGO.transform;
      }
  }
}

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