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

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 OnPushAsync(Page page, bool animated) {...}

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

    protected override Task 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;
        }
    }
}