Singleton Factory

 

I’ve mentioned before how I dislike the Singleton pattern.One of my issues is encapsulating the single instance nature of thepattern within the class, when most often the requirement for only oneinstance is not a feature of the class but a feature of the applicationthat is using the class.

So this morning I was experimenting with various other possible waysto create singletons in Actionscript 3 and came up with a function thatcan be used to create and reuse a single instance of any class. I don’tknow how useful it will turn out to be but here it is. It’s also auseful example of a package level function.

package
{
    function Singleton( c:Class ):*
    {
        for( var i:String in instances )
        {
            if( instances[i].constructor === c )
            {
                return instances[i];
            }
        }
        var obj:* = new c();
        instances.push( obj );
        return obj;
    }
}
var instances:Array = new Array();

You can use this function to create an instance of any class (your own or an intrinsic class) as follows (for example):

import flash.geom.Point;
var a:Point = Singleton( Point );

if you request another point elsewhere in your project, for example

import flash.geom.Point;
var b:Point = Singleton( Point );

you will get the same point back again.

The one problem is the need to loop through all previously createdsingletons looking for a match before creating a new one. The moresingletons you create, the slower this will be. It’s probably finesince most applications only use a few singletons but it would be greatto get a unique identifier directly from the class and then look up thematching singleton directly. I can get the class name via the toStringmethod, but would also need the full package to ensure uniqueness. Anyideas?

Like I said, I’m not sure how useful this function will be but it’shere if anyone wants to use it. If you improve it then please let meknow.

Postscript

In the comments below, Benny suggested a way to remove the loop, andso gain a more stable lookup time, by using a Dictionary instead of anArray. His suggestion, with minor alterations by me, looks like this.

package
{
   function Singleton( c:Class ):*
   {
      return c in instances ? instances[c] : instances[c] = new c();
   }
}
import flash.utils.Dictionary;
var instances:Dictionary = new Dictionary( false );

Thanks, Benny.