How to intercept Navigation Bar Back Button Clicked in Xamarin Forms?

I have a xamarin forms page where the user can update some data in a form. I need to intercept the Navigation Bar Back Button Clicked to warn the user if some data have not been saved.How to do it?

I'm able to intercept the hardware Bar Back Button Clicked in Android using the Android.MainActivity.OnBackPressed(), but that event is raised only on hardware Bar Back Button Clicked, not on Navigation Bar Back Button Clicked.

I tried also to override Xamarin.Forms.NavigationPageOnBackButtonPressed() but it not works. Why? Any one have already solved that issue?

I also tried by overriding OnDisappear, there are two problems:

The page has already visually disappeared so the "Are you sure?" dialog appears over the previous page.
Cannot cancel the back action.
So,is it possible to intercept the navigation bar back button press?

Thanks for any help on this.