Need an event to capture when public ip has changed

[Yolanda Robla]

I am writing a charm that in some config files need to set the public ip of the instance. When it's first deployed, the charm is setting that as the private ip.
Then i can assign a public floating ip, and i should need that an event is triggered, so i could refresh config files with the new IP. Currently i don't have any event to achieve that.
The only workaround is to setup a config var with that public ip, and set that manually. But should be great to automatically detect these IP changes to automate the process.