This post is too old, read http://www.myflashlab.com/2010/02/21/custom-event-listener/ instead.
in my other post on http://www.emstris.com/2009/01/custom-event-listeners/ I talked about how you can create a custom event listener and I did mention how easy it was... but now that I have managed to learn a lot more about them, I can tell that creating these custom events are not only easy but also VERY easy and straight 
This is what I was trying to do (I did this in my DropDownMenu class which will write about it in my next post).
I wanted my item list class to have some listeners so when ever a user selects an item from the drop down menu, it would understand that event and sets the data and closes the menu...
Thanks to ultrashock.com I found a very straight answer for this and here I try to briefly sumersize everything quickly.
Before anything, you must know that you can dispatch events using the dispatchEvent() method only if you are extending from EventDispatcher class. Don't be scared! if you're class is already extending Sprite or MovieClip then you are also good to go with the rest because Sprite and MovieClip also extend from EventDispatcher class and considering that, you will be able to use the dispatchEvent() method with no problem.
alright, now that you you know this, take a look at the below sample code and have a few seconds thinking about it:
and below will be how you may use it:
Note that you don't have to necessarily use the dispatchEvent() method in a setter function! common, be a little creative. in the above code, the main consentration is on changing the variable "name" so as soon as you change it, the listener will be notified... you can add the line
dispatchEvent( new Event( "nameChanged" ) );
to anywhere in your code that your target change is happening and that's it.
I think it would be worthy to have a look at how I have used the above code in my itemList class:
_selectedItem = {lable:e.currentTarget.lable, detail:e.currentTarget.detail, thumb:e.currentTarget.thumb, data:e.currentTarget.data, index:e.currentTarget.index}; dispatchEvent(new Event("itemChanged"));
You see, whenever a new box is selected in the item list, the object which holds the data will be called and the new data will be inserted into it and exactly the same place we have placed our dispatchEvent() method.
Now we can listen to this change! That easy. let me give you a sample... simply like this:
myList.addEventListener("itemChanged", onItemChanged);
and "onItemChanged" will be function which can do whatever you wish that event handler to do
Yeah, me too, me too. I couldn't beleive how easy it was to create my own listener!
The examples above will surely clear you about how you can create your own listeners but in my dropdown menu class you will see it more and we will actually do something useful with it.
It's really a shame that I spend a couple of good days searching for this! but that what makes coding beautiful, don't you think so?
Regards,
Hadi
let me get you to the point quickly, Your custom ProgressEvent will NOT work! I'm not sure if it's right to say this for sure, but I tested this in different ways for a thousand times, but it simply won't work and will just send it's properties, bytesTotal and bytesLoaded always zero "0"! which makes it impossible to build any preloader using your custom listener! I also tried digging in the web but I didn't manage to find any article or anything talking about this little issue. Maybe it's a possible bug in AS3 and how dispatchEvent works? I really don't know. I hope someone can give some idea about this. but anyway, in my case, I don't have time to spend more time on this! So I decided to create a function for this reason and pass parameters of whatever I wish to load into the function directly and use the listeners directly. Oh, don't take this wrong, other custom listeners will work just fine and the problem seems to be with the ProgressEvent only. so if you are not going to use the progress event, you will still be able to create your own custom listeners in a separated class. But I personally rather create a function now as it will help me out of the headache at least. if anyone has a better experience with this, please do let me know. I'm going to get back to my Box class and clean it up with an almost stand alone function for taking care of the loading process of external files. hmm, I don't feel satisfied with having to do it with a function in every class I may need something to load... but I have to do this until I find a work around the problem. 
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