Singleton

Revision as of 13:13, 28 January 2009

By AngryAnt.

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 { 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;</csharp> <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 Score { get { return score; } set { score = value; } } }</csharp>

Score tracking and GameOver broadcasting singleton

This singleton expands on 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 score reaches zero or lower. The score is set just like in the singleton above and GameObjects register and unregister with the singleton like so: <csharp>MySingleton.Instance.Register( gameObject ); MySingleton.Instance.Unregister( gameObject );</csharp> <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;
   }
   
   public static MySingleton Instance
   {
       get
       {
           if( instance == null )
           {
               new MySingleton();

instance.Init();

           }
           
           return instance;
       }
   }

private int score; private ArrayList listener;

private void Init() { listener = new ArrayList(); }

public Score { get { return score; } set { score = value; if( score <= 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>