Poor Man's XBL

Summary

Poor Man's XBL is a light-weight element binding (or behavior) framework. It accepts binding documents in the XBL-2.0 format, but doesn't implement XBL templates and doesn't even maintain state in the bindings.

The XBL system relies completely on event delegation, i.e. event listeners are registered on the document rather than the bound-elements. For every event that might need handling by XBL bindings, the system analyzes the event path from event-target to the document-root and attaches bindings and calls handlers as appropriate.

This simplifies the code in two ways:

• There is no need to scan the document at load-time for elements that need bindings
• There is no need to account for document modifications as the system works out what bindings and handlers are applicable at the time of the event.

There are also disadvantages of this approach:

• No state is maintained in bindings. This may not be an issue if state is maintained on the element itself.
• The public interfaces of bindings are not available. This could be mitigated by a getBinding() method on the element (similar to hasBinding() in the XBL specification.
• xblEnteredDocument() or xblLeftDocument() methods are never called

What's it for?

Poor Man's XBL was written to demonstrate the usefulness of Element.matchesSelector() combined with event-delegation. Element.matchesSelector() was proposed for the W3C Selectors-API specification but was ruled out due to lack of use-cases. (It may be in a future version of the spec).

My opinion is that this is a huge hole in the spec and the lack of use-cases demonstrates that the domain isn't properly understood... but my arguments haven't had any impact (mostly because I spoke up too late I suppose), so I wrote this implementation to make the point.

There's some simple demos to prove that it works.

Licensing

Poor Man's XBL is not permitted for any use (apart from it's demonstration on this site).