How to track active tabpage selection on form

I have to set the visibility of some fields based on which tabpage is selected on the form.

There are two tabpages arranged horizontally. I can get the IDs of the tabpages using Tab.getActivePage().id() and perform the action I want but the problem is where do I put this code.

The datasource active method didn't get triggered when changing tabpages. Tried selectionChanged() method too. Where should I put the code that can trigger it on changing tabpages. 

Thanks for any help!