I have a combination of fields: One post_object and one user.
I need to update a DOM element of the page (just to show it in screen) whenever any of these two fields receives a new value (it has multiple values ON).
That DOM element will show a calculated value based on some COUNTs of the currently selected values in both post_object and user fields.
I have not found so far a filter to do that, nor I know where I can start.
You can have a look at my question and the answer. It’s basically the same thing. You can then retrieve the changed values with ajax, if you put the get_field() in the ajax action.