OOP; listeners and scope

[SJT]

Ok, I'm going to pose a question...partly because I don't know the answer myself and I'd like it but also because I think scope is complicated question in OOP that could do with some explanation and discussion.

Here goes; let's say I create an class and then instantiate it, how does the scope of this work from instance to instance.
If this is the class, and it is placed on stage with attachMovie:

code:

---

Class = function(){

trace(this);

//the instantiated object on stage

}

Class.prototype = new Movieclip();

Object.registerClass("myClip",Class);



Class.prototype.myFunction = function(){

trace(this); 

//this here is also the instantiated object on stage 

};

---

However, this doesn't seem to be true for events.

For example:

code:

---

Class = function(){

	trace("instantiated: "+this);

	//'this' is the instantiated object on stage

	Class.prototype.me = this;

}

Class.prototype = new Movieclip();

Object.registerClass("myClip",Class);

ASBroadcaster.initialize(Class.prototype);

Class.prototype.addListener(Class.prototype);

Key.addListener(Class.prototype);

/*





*/

Class.prototype.onMyEvent = function(){

	trace("Custom Event: "+this);

	//'this' here is NOT the instantiated object on stage	

	trace("Custom Event: "+this.me);

	//we must pass a variable from the constructor to see what 'this' is on stage

};

Class.prototype.onKeyDown = function(){

	trace("Key Event: "+this);

	//'this' here is NOT the instantiated object on stage	

	trace("Key Event: "+this.me);

	//we must pass a variable from the constructor to see what 'this' is on stage

};

Class.prototype.onEnterFrame = function(){

	if ( x < 2){

		this.broadcastMessage("onMyEvent")

		trace("frame: "+this);

		x++;

	}

	//'this' here is also the instantiated object on stage

};

Class.prototype.myFunction = function(){

	trace("Custom Function: "+this); 	

	//'this' here is also the instantiated object on stage 

};

---

So, why is it that events except onEnterFrame don't have the same scope of this as other functions of an object?

[dgrigg]

Originally posted by SJT
So, why is it that events except onEnterFrame don't have the same scope of this as other functions of an object?

Part of the problem is you are defining the class as a listener, not an object, only objects can be defined as listeners (to the best of my knowledge). Second you need to put the ASBroadcaster and the two addListener lines in the class contructor, that way each time a new 'class' object is instantiated on the stage those three items get setup relative to the the new object.

Class = function(){
trace("instantiated: "+this);
//'this' is the instantiated object on stage
Class.prototype.me = this;

ASBroadcaster.initialize(this);
this.addListener(this);
Key.addListener(this);
}

I you do the above then the traces you have work properly and each reference to 'this' traces the instance name.

Hope this solves your problem.

[SJT]

Thanks, that does help explain things a lot.

The problem that then occurs though, is how to add a listener from another class you've instantiated.
It seems like a real pain to have to have to add each new object to the listener list when it's instantiated manually...that's what I was hoping adding the class would do instead.

After a bit more investigation, it does add the class to the listeners list...sort of. Unfortunately it only adds the last instance created, not every instance created...

[dgrigg]

You could try a static property class 'A', an array that contains each instance of the class 'A' instantiated objects. Then when you want to add a listener from a class 'B' loop through the array in class 'A' to get each instance.

[gSOLO_01]

Didn't look at anything else but fyi, it's AsBroadcaster... In case you're using FP7 / Flash MX 2004...

[SJT]

Thanks gSOLO_01,

I haven't upgraded yet, is AsBroadcaster now a documented object?

[dgrigg]

Originally posted by gSOLO_01
AsBroadcaster

Gotta love case sensitivity.

[gSOLO_01]

Nope, still not documented. Although they use it in their components and such, there is a new EventDispatcher Object...

[gSOLO_01]

I've seen, when wading through some of Macromedia's components, that they do this also. i.e. this.constructor = this; they then reference the constructor rather than 'this'

[SJT]

Hmm, that's interesting.

So by referencing the constructor's 'this', does that make the class prototype the scope of this, rather than the object? Or the other way round...??

[dgrigg]

Within the constructor (or the other methods) 'this' is a reference to the instantiated object. The scope of 'this' is the individual object.

[SJT]

Yeah, I've just realised I've already been doing this, but with slightly differently...oops

I've been putting this in the constructor.

code:

---

Class(){

Class.prototype._stagename = this;

}

---

Which is sort of the same thing really, just the other method is neater.

[dgrigg]

Always more than one way to skin a cat.

[ericlin]

code:

---

Class(){

        Class.prototype._stagename = this;

}

---

This function gets called each time an instance is created. So, Class.prototype._stagename will be changed and over-written each time.

If we created 3 instances, mc1, mc2, mc3, at last, Class.prototype._stagename will be mc3. Then if we do trace(mc1._stagename), the output will be mc3 not mc1;

What is "this" depends on "who" is calling the method. onEnterFrame is called by Clip itself. While the onMyEvent is called by the listener. Since we only make prototype object as listener, the listener is the prototype object not instance objects. So, "this" in onMyEvent is assumed to be prototype object.

[SJT]

Oops, Class.prototype was a typo...I've typed it too much recently I think
I did actually just use this._stagename...

Nothing to see here, move on now...