Get notified from avahi

After looking for a zero-conf browser valid implementation I realised that inside my linux machine (ubuntu) I already have avahi and typing: avahi-browse -alr I get a complete list of all devices notified by their bonjour conf. **Question:** how to get events from it? I mean, how can I get notification (execute a script) when a bonjour device is connecting on my network? How to detect when it is switched off?