Rails spinoffs - May 2006 - using Singleton with Prototype

Hello all. I''m writing my own object using prototype and I 
wonder how you write and call only one instance of class. Can 
anyone share his idea of singleton?
thanks
Gregor


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On 5/22/06, Grzesiek Slusarek <gregtech-5tc4TXWwyLM@public.gmane.org>
wrote:
> Hello all. I''m writing my own object using prototype and I
> wonder how you write and call only one instance of class. Can
> anyone share his idea of singleton?
> thanks
> Gregor
>
Here is my proposal :
---8<----------------------
        <script>
        var Foo = Class.create();
        Object.extend(Foo.prototype, {
                initialize: function() {
                    this.a = 5;
                }
        });
        Object.extend(Foo, {
                instance: function() {
                    if (!this._instance) {
                        this._instance = new Foo();
                    }
                    return this._instance;
                }
        });
        var o1 = Foo.instance();
        var o2 = Foo.instance();
        alert(o1.a + '' '' + o2.a);
        o2.a = 10;
        alert(o1.a + '' '' + o2.a);

        </script>
---8<----------------------

Anyway it is not a real singleton since it is possible to call the
constructor outside from instance()
Furthermore, by hacking Foo, I do not know if I break something by
adding _instance and instance() :-\

Yours,
Nicolas Terray

I had written a non-prototype several years ago like this:

var _singletons = []; // for storing singleton instances of objects

Function.prototype.singleton = function (o) {
	if(!_singletons[this.name]) {
		_singletons[this.name] = o;
		return false;
	}
	return true;
}

Function.prototype.getSingleton = function () {
   return _singletons[this.name];
}

then you define a class like this:

function AClass() {
  if AClass.singleton(this) return AClass.getSingleton();

  this.a = a;
  /* etc */
}

That way you can call var a = new AClass() as many times as you like
and it'd return the same object instance every time. Which I think is
close to a real singleton instance of an object.

But I don't think you could do that readily with Prototype using the
Class.create() mechanism.



On 22/05/06, Nicolas Terray <nicolas.terray@gmail.com>
wrote:
> On 5/22/06, Grzesiek Slusarek <gregtech@wp.pl> wrote:
> > Hello all. I'm writing my own object using prototype and I
> > wonder how you write and call only one instance of class. Can
> > anyone share his idea of singleton?
> > thanks
> > Gregor
> >
>
> Here is my proposal :
> ---8<----------------------
>         <script>
>         var Foo = Class.create();
>         Object.extend(Foo.prototype, {
>                 initialize: function() {
>                     this.a = 5;
>                 }
>         });
>         Object.extend(Foo, {
>                 instance: function() {
>                     if (!this._instance) {
>                         this._instance = new Foo();
>                     }
>                     return this._instance;
>                 }
>         });
>         var o1 = Foo.instance();
>         var o2 = Foo.instance();
>         alert(o1.a + ' ' + o2.a);
>         o2.a = 10;
>         alert(o1.a + ' ' + o2.a);
>
>         </script>
> ---8<----------------------
>
> Anyway it is not a real singleton since it is possible to call the
> constructor outside from instance()
> Furthermore, by hacking Foo, I do not know if I break something by
> adding _instance and instance() :-\
>
> Yours,
> Nicolas Terray
> _______________________________________________
> Rails-spinoffs mailing list
> Rails-spinoffs@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails-spinoffs
>

-- 
Andrew Tetlaw
htp://tetlaw.id.au

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