How to add hooks in twisted.web (or twisted.web2)?

Question

How can I add a hook before and after processing a request on twisted.web (twisted.web2 is fine too)? The equivalent of webpy's:

app = web.application(urls, globals())
app.add_processor(web.loadhook(my_attach_callback))
app.add_processor(web.unloadhook(my_detach_callback))

Thanks!