Question

Does this work well as a Singleton in actionscript? i.e. are there any gotchas I should be aware of, and is it true that only one instance of AVManager would be created in this case:

Note that I do get the expected output (only one time of “instantiated first time” and numbers follow sequence):

instantiated first time! 1

FILE 1: 2

FILE 2: 3

and finally 4

Here are the files….

AV_CONFIG.as:

package {
    public class AV_CONFIG {
        public static var AVM:AVManager = new AVManager();
    }
}

AVManager.as:

package {
    import flash.events.EventDispatcher;

    public class AVManager extends EventDispatcher {
        public var someConstantData:uint = 1;

        public function AVManager() {
            trace('instantiated first time!', someConstantData);
        }

    }
}

Then:

File1.as:

package {
    import AV_CONFIG;
    import flash.display.Sprite;

    public class File1 extends Sprite {

        public function File1() {
            AV_CONFIG.AVM.someConstantData++
            trace('FILE 1:', AV_CONFIG.AVM.someConstantData);
        }

    }
}

File2.as:

package {
    import AV_CONFIG;
    import flash.display.Sprite;

    public class File2 extends Sprite {

        public function File2() {
            AV_CONFIG.AVM.someConstantData++
            trace('FILE 2:', AV_CONFIG.AVM.someConstantData);
        }

    }
}

Main.as (the DocumentClass):

package {
    import AV_CONFIG;
    import flash.display.Sprite;

    public class Main extends Sprite {

        public function Main() {
            var f1:File1 = new File1();
            var f2:File2 = new File2();
            AV_CONFIG.AVM.someConstantData++
            trace('and finally', AV_CONFIG.AVM.someConstantData);
        }

    }
}

Answer 1

Generally with singletons you want to:

1. Limit or dissolve the ability to create instances of that class.
2. Create a means of getting an instance of that class statically.

Example:

public class AvManager
{

    private static var _instance:AvManager;
    internal static var created:Boolean = false;

    public function AvManager()
    {
        if(AvManager.created)
        {
            throw new Error("Cannot created instances of AvManager, use AvManager.instance.");
        }

        AvManager.created = true;
    }

    public static function get instance():AvManager
    {
        if(_instance == null)
        {
            _instance = new AvManager();
        }

        return _instance;
    }

    public function test():void
    {
        trace("Working.");
    }

}

Where you could now use:

AvManager.instance.test(); // Working.