addEventListener on my own extension?

Hello,

I wrote a multicast-xml-receiver extension. The received xml-data will
be handled by javascript in a webpage. So my idea was to make
something like this:
--
obj = Components.classes[cid].createInstance();
obj = obj.QueryInterface(Components.interfaces.nsIXMLMulticastSocket);
obj.join("228.5.6.7", 6789);

obj.addEventListener("dataEvent", onData, false);

....

function onData(ev) { alert(ev.getData()) } // alert the received
string

--

My question is how can i implement this?
I read that i have to implement the nsDOMEventTarget. But where do i
register the "onData" function? Can i reuse some existing code?

Thanks a lot!
Regards
Simon