AS3.0 Better Singletons

I’ve found a better more robust way of enforcing singletons in AS3.0

[as]
package
{
public class Singleton
{
public static var instance:Singleton;

public static function getInstance():Singleton
{
if( instance == null ) instance = new Singleton( new SingletonEnforcer() );
return instance;
}

public function Singleton( pvt:SingletonEnforcer )
{
// init class
}
}
}

internal class SingletonEnforcer{}
[/as]
Note: The class “SingletonEnforcer” is actually placed within the same .as file as the Singleton class, but placed outside the package{} parenthesis, therefore, the class “SingletonEnforcer” can only be accessed from within this .as file, so if the Singleton’s constructor is called from anywhere else, you’ll get an error.

August 16th, 2007 by in ActionScript 3.0, Flash, Tips9 comments

9 Comments so far

  1. Unformatt / August 16th, 2007 23:28

    This is nice – I guess ECMA doesn’t like private constructors? Any reason not to include the PrivateClass in the package?

  2. gabriel / August 17th, 2007 07:55

    Exactly, ECMA doesn’t allow private constructors. The PrivateClass is outside the package so that i can only be accessed by code within the current .as file (if it was in the package{} I’d assume it would be accessible elsewhere within the package.

  3. robert shearing / August 20th, 2007 01:23

    I discovered another implenentation by Daniel Hai on http://www.onflex.org/code/

    The private static instance variable is instantiated on declaration. Simple and clean:

    package
    {
    public class Singleton
    {

    private static var instance:Singleton = new Singleton();

    public function Singleton()
    {

    if( instance ) throw new Error( “Singleton and can only be accessed through Singleton.getInstance()” );
    }

    public static function getInstance():Singleton
    {
    return instance;
    }
    }
    }

  4. Gabriel Handford / October 2nd, 2007 03:52

    Shouldn’t it be:

    public static var get instance():Singleton { return getInstance(); }

    and change all the instance to _instance (to avoid the conflict)?

  5. Rob / August 24th, 2008 07:01

    The only problem with this solution is that a developer can simply pass null into the constructor:

    var s : Singleton = new Singleton(null);

    You have to explicitly check the nullity and type of the parameter:

    public function Singleton(enforcer:*=null){
    if(enforcer==null || !(enforcer is SingletonEnforcer))
    throw new Error(“FAIL!”);
    }

  6. gabriel / August 25th, 2008 13:06

    would be a stupid developer to pass null.

  7. dimpiax / August 27th, 2008 13:44

    This if pretty realisation of Singelton fo Me:

    private static var __called : Boolean;
    private static var __instance : Class;

    public static function get instance() : Class {
    Class.__called = true;
    if(Class.__instance == null) Class.__instance = new Class();

    return Class.__instance;
    }

    public function Class() {
    if(__instance) throw Error(‘This class uses Singelton pattern.’);
    else if(!__called) throw Error(‘You must access to class by Class.instance’);
    }

  8. Cotton / November 2nd, 2008 02:06

    We should keep the parameter list of construct clear.

    package
    {

    public class Singleton
    {

    private static var instance:Singleton;
    private static var isLock:Boolean = true;

    public function Singleton ()
    {
    if (isLock) throw new Error(‘ooops’) else isLock = true;
    }

    public static function getInstance ():Singleton
    {
    return instance ||= (isLock = false, new Singleton());
    }

    }

    }

  9. estetik / June 3rd, 2009 22:55

    “Can I map “this” information?” Or
    they ask, “Is there an application I can use on our network that I can
    plot this info on?”