Singleton

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By AngryAnt.

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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:

<csharp>MySingleton.Instance.MySingletonMember;</csharp>

Note, however that for the component-based singleton (the second implementation), you shouldn't attempt to access the instance earlier than in Start of the calling component.

The non-component example

<csharp>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; } } }</csharp>

Component-based example

<csharp>using UnityEngine;

public class MySingleton : MonoBehaviour { private static MySingleton instance;

public void Awake() { if( instance != null ) { Debug.LogError( "Cannot have two instances of singleton. Self destruction in 3..." ); Destroy( this ); return; }

instance = this; }

public static MySingleton Instance { get { if( instance == null ) { Debug.LogError( "Attempt to access instance of MonoBehaviour singleton earlier than Start or without it being attached to a GameObject." ); }

return instance; } }

public void OnApplicationQuit() { instance = null; } }</csharp>

Score tracking singleton

The singleton in this example keeps track of the game score. Getting and setting this value is done like so: <csharp>MySingleton.Instance.Score += 5; Debug.Log( "Score is now: " + MySingleton.Instance.Score );</csharp>

And the singleton class: <csharp>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; } } }</csharp>

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: <csharp>MySingleton.Instance.Register( gameObject ); MySingleton.Instance.Unregister( gameObject );</csharp> 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: <csharp>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 listener;


private void Init() { listener = 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 ); } }</csharp>

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