XWiki promotes the idea that it allows easy collaborative web application development. Such an application might want to add an entry in the menu, or a [ del.icio.us] link to the document footer.
1. If we require users to write a java class, compile it, package it in a jar and distribute it, then we lose the collaborative/online/easy parts.
2. If we list a number of interfaces that can be extended, then we would probably have forgotten the document footer. So we limit the extension points drastically. On the other hand, if we do list all the extension points, then we'll have minor releases with something like "added an EP for this and that", and we'll have dozens of interfaces.
3. I still dream of the wiki that supports importable xar-lets (well, named differently, as we agreed .xar is not a good extension), which can contribute anything, from simple pages to complete skins, interface elements, new classes/templates/sheets. Interface elements seem hard to add if we require java classes, right?


I have developed some Firefox extensions in the past, so I am a bit biased when I insist on taking a XUL-like approach. Here are some ideas (see also http://jira.xwiki.org/jira/browse/XWIKI-649 ):

1. Right now we are putting Interface Elements (IE for short, also stands for Interface Extension) manually in the current skin. The only thing aggregated are the panels. Each of these elements is put in a <div>, maybe with an id and class. In XUL, each element having an id is a possible extension point (EP for short). It would be hard to use divs and ids, that's why we should replace these with something like #startelement("id", "class") and #endelement("id" "class"). These macros create extension points, and they aggregate any content that wants to be included in here. (as a bonus, we can keep a stack of open elements, and when we close an element (with an id) we can automatically close the missing #endelement-s)
2. An IE that wants to extend something can specify an id or/and class name, a position (before, after, at the begin, at the end), and some ordering hints. And the content, of course. What's more, an IE can add new EPs. In the wiki/velocity world, this is best accomplished using XObjects.
2a. If we want to push things to an advanced level, we can make the selection not just by an id or classname, but something like (basic) XPath or like the css (basic) selectors.
3. For java, we can create an interface equivalent to the EP XWiki class. This has the advantages that:
- it's only one interface, similar in Java and the wiki
- it can extend anything
4. #startelement and #endelement are implemented in java, by one or more components (at least one for java IE and one for wiki IE). In order to speed things up, we can make a cache of the extension points. When the platform start, it should search for the available extensions, and already prepare the list of elements to be added for each element. When startelement is called, don't search for anything, just flush the prepared list. We register a listener, so when a page is saved, check if it contains an IE object and update the affected list of extensions.
5. We should document the extension points present in the skins. First of all, we should identify a core list of EPs that should exist in all skins, with the same id/class. Each skin can add new EPs, which must be documented, and we should warn (outside) developers that only core EPs are guaranteed to exist in all the skins.
6. Each xarlet/plugin should list in the documentation the EPs affected and the EPs added. We can have extensions (xarlets) that mainly add a new EP, which other xarlets can use (like greasemonkey does for firefox).