title: How to intercept Navigation Bar Back Button Clicked in Xamarin Forms?
body: <p>I have a xamarin form page where a 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?</p>
  
  <p>I'm able to intercept the hardware Bar Back Button Clicked in Android using the <code>Android.MainActivity.OnBackPressed()</code>, but that event is raised only on hardware Bar Back Button Clicked, not on Navigation Bar Back Button Clicked.</p>
  
  <p>I tried also to override <code>Xamarin.Forms.NavigationPageOnBackButtonPressed()</code> but it doesn't work. Why?
  Any one have already solved that issue? </p>
  
  <p>I also tried by overriding <code>OnDisappear()</code>, there are two problems:</p>
  
  <ol>
  <li>The page has already visually disappeared so the "Are you sure?" dialog appears over the previous page.</li>
  <li>Cannot cancel the back action. </li>
  </ol>
  
  <p>So, is it possible to intercept the navigation bar back button press?</p>