XF 2.2 Execute custom code when custom user field is saved?

[hoks]

I created a checkbox custom user field in Preferences.

Whenever a user save his preferences, I wish to check the status of the checkbox, then run some code depending on the checkbox's value.

Do I create a new event listener? If so, which event do I select?

Or do I create a class extension? if so, which class do I extend?

Sorry, still fumbling my way around add-ons.