How to implement a change notifier

[agares29]

I would like to know how to implement a Change Notifier to change to another Nav Bar Option when user clicks on a button.

I tried changing the

_controller.index

For my

provider.currentIndex

Everything works fine when the user clicks on the options on the Nav Bar, but it does not work when I use the button, it just changes the icon selected on the Nav Bar bit the screen does not change

Does anybody know how to make that work?