Singleton Classes in Actionscript 3.0

Singleton Classes are based on the Singleton Design Pattern and have a single instance of the class throughout the application. When would one need a singleton class ? A simple example would be a database manager class which would instantiate a connection to the database at the application initialization and have methods for data manipulation of the database tables. In such a case multiple instances of this class is not required and will end up consuming memory and other issues.

How does one implement a Singleton Class ?

Recommended Way

1. Declare a private variable of the type of the same singleton class inside it.
2. Declare a private or protected constructor to prevent the initiation of the class
3. Implement a public getInstance() method which will instantiate the variable declared in step 1 if it has not been instantiated, otherwise return the variable.

Pseudocode

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 Class Singleton{
 private variable SingletonInstance type Singleton ;
 private/protected Constructor(){
 //not implemented
 }
 public function getInstance(){
 if (SingletonInstance == null) then{
 Instantiate SingletonInstance;
 }
 return SingletonInstance;
 }
 }

The next obvious question, which is also the premise of this blog, is how does one implement a Singleton class in actionscript. Well, Actionscript 3.0 does not allow private or protected constructors thus leaving a lot of scope for experimentation and the quest for the perfect implementation, as this Google Search results with the words “singleton classes + actionscript” retruns quite a few implementation variations.

The one that I used for my implementation was sort of inspired by Andrew Trice’s Singleton in AS3 example.

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package
 {
 public class Singleton
 {
 private static var singleton : Singleton 
 
public static function getInstance() : Singleton
{
if ( singleton == null )
singleton = new Singleton( arguments.callee );
return singleton;
}
//NOTE: AS3 does not allow for private or protected constructors
public function Singleton( caller : Function = null )
{
if( caller != Singleton.getInstance )
throw new Error ("Singleton is a singleton class, use getInstance() instead");
if ( Singleton.singleton != null )
throw new Error( "Only one Singleton instance should be instantiated" );
//put instantiation code here
}
}
}

Have you come across a better Singleton Implementation ? I would definitely want to learn

Update : As Jove Shi pointed out on SDN the code above would break in the folowing case :

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var f:Function = Singleton.getInstance;
 var s1:Singleton = new Singleton(f);
 var s2:Singleton = new Singleton(f);
 trace("complete: " + (s1 == s2));

The suggested workaround would be :

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private static var instance:ModelLocator;
 
public function ModelLocator(){
if(instance != null){
throw new YourError("ModelLocator singleton error!");
}
instance = this;
}
 
public static function getInstance():ModelLocator{
if(instance == null){
instance = new ModelLocator();
}
return instance;
}

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