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

Question

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 bee

Answer 1

I was able to show a confirmation dialog that could cancel navigation by overriding the following methods in the FormsApplicationActivity.

  // navigation back button
  public override bool OnOptionsItemSelected(IMenuItem item)
  {
     if (item.ItemId == 16908332)
     {
        var currentViewModel = (IViewModel)navigator.CurrentPage.BindingContext;
        currentViewModel.CanNavigateFromAsync().ContinueWith(t =>
        {
           if (t.Result)
           {
              navigator.PopAsync();
           }
        }, TaskScheduler.FromCurrentSynchronizationContext());
        return false;
     }
     else
     {
        return base.OnOptionsItemSelected(item);
     }
  }

  // hardware back button
  public async override void OnBackPressed()
  {
     var currentViewModel = (IViewModel)navigator.CurrentPage.BindingContext;

     // could display a confirmation dialog (ex: "Cancel changes?")
     var canNavigate = await currentViewModel.CanNavigateFromAsync();
     if (canNavigate)
     {
        base.OnBackPressed();
     }
  }

The navigator.CurrentPage is a wrapper around the INavigation service. I do not have to cancel navigation from modal pages so I am only handling the NavigationStack.

this.navigation.NavigationStack[this.navigation.NavigationStack.Count - 1];