Tree building observer/listener

Hi all,

Apologies, I know this has been answered somewhere before and I cant 
find it.

I am tidying up some tree templates, and I want to observe the 
template's build progress, specifically a callback on completion.

XULPlanet shows the nsIXulTreeBuilder implementing 5 methods, the 
Component Viewer shows it really implementing more like 30.

The two obvious methods are addObserver() and addListener() which would 
you use to 'observe' a tree rebuild and why?

I think I am finding it a bit confusing because the CV shows 
nsIXULTreeBuilder  inheriting from nsIXULTemplateBuilder, nsITreeView, 
and nsIRDFObserver, and I'm not sure whether I should implement a tree 
or template observer.

Cheers,

N