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 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 doesn't work. Why? Any one have already solved that issue?

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

1. The page has already visually disappeared so the "Are you sure?" dialog appears over the previous page.
2. Cannot cancel the back action.

So, is it possible to intercept the navigation bar back button press?

Solution 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];

Solution 2

The easiest, as @JordanMazurke also somewhat mentions, since the event for the back button cannot be handled currently (other than the physical back button for Android), is to either:

1. NavigationPage.ShowHasBackButton(this, false)
2. Pushing a Modal instead of a Page

Then afterwards, you can add an ActionbarItem from where you can handle the Event.

I personally spoke to the iOS team from Xamarin concerning this matter, and they basically told me we shouldn't expect support for handling the Event for the BackButtonPressed in the NavigationBar. The reason being, that on iOS, it's bad practice for the users to receive a message when Back is pressed.

Solution 3

Best way I have found is adding my own NavigationRenderer to intercept the navigation methods and a simple Interface

[assembly: ExportRenderer(typeof(NavigationPage), typeof(CustomerMobile.Droid.NavigationPageRenderer))]

public class NavigationPageRenderer : Xamarin.Forms.Platform.Android.AppCompat.NavigationPageRenderer
{
    public Activity context;

    public NavigationPageRenderer(Context context)
        : base(context)
    {}

    protected override Task<bool> OnPushAsync(Page page, bool animated) {...}

    protected override Task<bool> OnPopToRootAsync(Page page, bool animated){...}

    protected override Task<bool> OnPopViewAsync(Page page, bool animated)
    {
        // if the page implements my interface then first check the page 
        //itself is not already handling a redirection ( Handling Navigation) 
        //if don't then let the handler to check whether to process 
        // Navitation or not . 
        if (page is INavigationHandler handler && !handler.HandlingNavigation
             && handler.HandlePopAsync(page, animated))
            return Task.FromResult(false);
        
        return base.OnPopViewAsync(page, animated);
    }
}

Then my INavigationHandler interface would look like this

public interface INavigationHandler
{
    public bool HandlingNavigation { get; }
    public bool HandlePopAsync(Xamarin.Forms.Page view, bool animated);
    public bool HandlePopToRootAsync(Xamarin.Forms.Page view, bool animated);
    public bool HandlePuchAsync(Xamarin.Forms.Page view, bool animated);
}

Finally in any ContentView, in this example when trying to navigate back I'm just collapsing a menu and preventing a navigation back.

public partial class MenusList : INavigationHandler
{
    public bool HandlingNavigation { get; private set; }

    public bool HandlePopAsync(Page view, bool animated)
    {
        HandlingNavigation = true;
        try 
        {
            if (Menu.Expanded)
            {
                Menu.Collapse();
                return true;
            }
            else return false;
        }
        finally
        {
            HandlingNavigation = false;
        }
    }
}