About events and listeners : what's the point?

Question

Hello,
I worked on Swing and Struts by the past so I have a little experience about MVC pattern, web application and event driven developpement.
Despite that, I don't see the point using event listeners in JSF. Indeed, event-listeners code will be processed to the next server call (action generaly) which means event-listeners code could be deported in the action code.
According my researchs, the 2 major differences between action method and event listeners methods are the followiongs :
1) event listener codes can be processed before validation and model updating : in which cases this can be useful? I don't see.
2) event listener codes have access to the component which fired the event : in which cases this can be useful? I don't see.
If theses differences don't give new useful possibilities comparing to method actions so I don't underdstand the utility using events and listeners. I guess there are new possibilities but as i'm a beginner in JSF, i need help to take advantages of event driven developpement.
What I would wish is very short and simple examples about what we can do with actionEvent and changeEvent that we can't do in other way (and in particular in an action code).
Thanks you for reading me.
P.S : forgive my spelling errors :)
Corwin
Edited by: Corwin_31 on May 20, 2009 6:26 AM (spelling errors)

Answer 1

Corwin_31 wrote:
What I would wish is very short and simple examples about what we can do with actionEvent You could set custom component attributes and make use of it in action method.
You can find here an example: [http://balusc.blogspot.com/2006/06/using-datatables.html#SortingDatatable].
and changeEvent that we can't do in other way (and in particular in an action code).You could use it to populate/change other components depending on the changed value.
You can find here an example: [http://balusc.blogspot.com/2007/10/populate-child-menus.html].
I must however admit, that with the Web 2.0 nowadays (Ajaxical powers) those server side events are a bit superfluous as you could easily let Ajaxical stuff take it over. That's also why there exist Ajaxical JSF component libraries which make use of those events more "effectively" (i.e. no hard submit to server required), such as RichFaces and the upcoming JSF 2.0.