Singleton

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By AngryAnt.

Contents

Description

People have recently asked about singleton creation quite often in the IRC channel so rather than retyping a basic implementation each time, I'll have this to link to. Yay! :)

Curious, but not quite sure what a singleton is? Ask a friend: [1]

Singletons are generally handy for providing easy access to game state and control code. I've provided example implementation for a basic class type which needn't be attached to a game object in order to function and after this an implementation which works as any other component.

Members of both singleton types are accessed the same way:

MySingleton.Instance.MySingletonMember;

The non-component example

public class MySingleton
{
	private static MySingleton instance;
 
	public MySingleton () 
	{
		if (instance != null)
		{
			Debug.LogError ("Cannot have two instances of singleton. Self destruction in 3...");
			return;
		}
 
		instance = this;
	}
 
	public static MySingleton Instance
	{
		get
		{
			if (instance == null)
			{
				new MySingleton ();
			}
 
			return instance;
		}
	}
}

Component-based example

using UnityEngine;
 
public class MySingleton : MonoBehaviour
{
	private static MySingleton instance;
 
	public static MySingleton Instance
	{
		get
		{
			if (instance == null)
			{
				instance = new GameObject ("MySingleton").AddComponent<MySingleton> ();
			}
 
			return instance;
		}
	}
 
	public void OnApplicationQuit ()
	{
		instance = null;
	}
}

Score tracking singleton

The singleton in this example keeps track of the game score. Getting and setting this value is done like so:

MySingleton.Instance.Score += 5;
Debug.Log ("Score is now: " + MySingleton.Instance.Score);

And the singleton class:

public class MySingleton
{
    private static MySingleton instance;
 
    public MySingleton () 
    {
        if (instance != null)
        {
            Debug.LogError ("Cannot have two instances of singleton. Self destruction in 3...");
            return;
        }
 
        instance = this;
    }
 
    public static MySingleton Instance
    {
        get
        {
            if (instance == null)
            {
                new MySingleton ();
            }
 
            return instance;
        }
    }
 
 
 
	private int score;
 
 
 
	public int Score
	{
		get
		{
			return score;
		}
		set
		{
			score = value;
		}
	}
}

Time tracking and GameOver broadcasting singleton

This singleton works similarly to the score tracking singleton by also maintaining a list of GameObjects which register and unregister themselves with the singleton in order to receive a "GameOver" message when the tracked timer reaches zero or lower. The timer is set just like the score is in the singleton above and GameObjects register and unregister with the singleton like so:

MySingleton.Instance.Register (gameObject);
MySingleton.Instance.Unregister (gameObject);

Registering and unregistering would likely make sense to do in Awake and OnDisable of a script attached to the GameObjects needing it. And the singleton class:

using System.Collections;
using UnityEngine;
 
 
 
public class MySingleton
{
    private static MySingleton instance;
 
    public MySingleton () 
    {
        if (instance != null)
        {
            Debug.LogError ("Cannot have two instances of singleton. Self destruction in 3...");
            return;
        }
 
        instance = this;
        Init ();
    }
 
    public static MySingleton Instance
    {
        get
        {
            if (instance == null)
            {
                new MySingleton ();
            }
 
            return instance;
        }
    }
 
 
 
	private int timer;
	private ArrayList listeners;
 
 
 
	private void Init ()
	{
		listeners = new ArrayList ();
	}
 
 
 
	public int Timer
	{
		get
		{
			return timer;
		}
		set
		{
			timer = value;
			if (timer <= 0)
			{
				foreach (GameObject listener in listeners)
				{
					listener.SendMessage ("GameOver");
				}
			}
		}
	}
 
 
 
	public GameObject RegisterListener (GameObject listener)
	{
		listeners.Add (listener);
 
		return listener;
	}
 
 
 
	public bool UnregisterListener (GameObject listener)
	{
		if (!listeners.Contains (listener))
		{
			return false;
		}
 
		listeners.Remove (listener);
	}
}

Mapping static methods to singleton methods

Having to go through the Instance property for every function of the singleton gets old fast. Really the user most of the time does not need to know that he is dealing with a singleton, so this:

MySingleton.Instance.Register (gameObject);
MySingleton.Instance.Unregister (gameObject);

should for simplicity be remapped to this:

MySingleton.Register (gameObject);
MySingleton.Unregister (gameObject);

To make the above true, simply make your methods static and have *them* take the pain of going through the Instance property - like so:

using System.Collections;
using UnityEngine;
 
 
 
public class MySingleton
{
    private static MySingleton instance;
 
    public MySingleton () 
    {
        if (instance != null)
        {
            Debug.LogError ("Cannot have two instances of singleton. Self destruction in 3...");
            return;
        }
 
        instance = this;
        Init ();
    }
 
    private static MySingleton Instance
    {
        get
        {
            if (instance == null)
            {
                new MySingleton ();
            }
 
            return instance;
        }
    }
 
 
 
	private int timer;
	private ArrayList listeners;
 
 
 
	private void Init ()
	{
		listeners = new ArrayList ();
	}
 
 
 
	public static int Timer
	{
		get
		{
			return Instance.timer;
		}
		set
		{
			Instance.timer = value;
			if (value <= 0)
			{
				foreach (GameObject listener in Instance.listeners)
				{
					listener.SendMessage ("GameOver");
				}
			}
		}
	}
 
 
 
	public static GameObject RegisterListener (GameObject listener)
	{
		Instance.listeners.Add (listener);
 
		return listener;
	}
 
 
 
	public static bool UnregisterListener (GameObject listener)
	{
		if (!Instance.listeners.Contains (listener))
		{
			return false;
		}
 
		Instance.listeners.Remove (listener);
	}
}

Voilá! Less cruft in the world! Everyone rejoice!

Generic Based Singleton

By Opless

This is uber simple, relies on the readonly keyword and generics to implement a threadsafe Singleton of practically any class.

Singleton<Foo>.Instance will be Foo (your own class) and guaranteed to be a singleton. The concept was lifted from a MSDN article that I recently discovered.

        public sealed class Singleton<T> where T : class, new()
        {
                /// <summary>
                /// Singleton implementation, readonly and static ensure thread safeness.
                /// </summary>
                public static readonly T Instance = new T ();
        }

Generic Based Singleton for MonoBehaviours

By Berenger

By inheriting from that class instead of MonoBehaviour, you can use all the singleton pattern without having to code it. It will search the object or intialiaze it in Awake, depending of the call order, or create if none exist. Just put your initilizations in Init() instead of Awake().

Example :

GameMaster.cs

public class GameMaster : MonoSingleton< GameMaster >
{
    public int difficulty = 0;
    public override void Init(){ difficulty = 5; }
}

OtherClass.cs

You forgot a "using UnityEngine;" fixed. :P

using UnityEngine;
public class OtherClass: MonoBehaviour
{
    void Start(){ print( GameMaster.instance.difficulty ); } // 5
}

The code :

using UnityEngine;
public abstract class MonoSingleton<T> : MonoBehaviour where T : MonoSingleton<T>
{
    private static T m_Instance = null;
    public static T instance
    {
        get
        {
            // Instance requiered for the first time, we look for it
            if( m_Instance == null )
            {
                m_Instance = GameObject.FindObjectOfType(typeof(T)) as T;
 
                // Object not found, we create a temporary one
                if( m_Instance == null )
                {
                    Debug.LogWarning("No instance of " + typeof(T).ToString() + ", a temporary one is created.");
                    m_Instance = new GameObject("Temp Instance of " + typeof(T).ToString(), typeof(T)).GetComponent<T>();
 
                    // Problem during the creation, this should not happen
                    if( m_Instance == null )
                    {
                        Debug.LogError("Problem during the creation of " + typeof(T).ToString());
                    }
                }
                m_Instance.Init();
            }
            return m_Instance;
        }
    }
    // If no other monobehaviour request the instance in an awake function
    // executing before this one, no need to search the object.
    private void Awake()
    {
        if( m_Instance == null )
        {
            m_Instance = this as T;
            m_Instance.Init();
        }
    }
 
    // This function is called when the instance is used the first time
    // Put all the initializations you need here, as you would do in Awake
    public virtual void Init(){}
 
    // Make sure the instance isn't referenced anymore when the user quit, just in case.
    private void OnApplicationQuit()
    {
        m_Instance = null;
    }
}
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