NavigationService.cs

public class NavigationService : IDisposable
{
    public NavigationService()
    {
        SystemNavigationManager.GetForCurrentView().BackRequested += this.OnBackRequested;
    }

    public event EventHandler<BackRequestedEventArgs> BackRequested;

    public void SubscribeBefore(EventHandler<BackRequestedEventArgs> handler)
    {
        this.BackRequested = (EventHandler<BackRequestedEventArgs>)Delegate.Combine(handler, this.BackRequested);
    }

    public void Dispose()
    {
        SystemNavigationManager.GetForCurrentView().BackRequested -= this.OnBackRequested;
    }

    private void OnBackRequested(object sender, BackRequestedEventArgs e)
    {
        this.BackRequested?.Invoke(sender, e);

        if (!e.Handled)
        {
            Debug.WriteLine("Back handled by navigation service");
            e.Handled = true;
            this.GoBack();
        }
    }

    private void GoBack()
    {
        var rootFrame = Window.Current.Content as Frame;

        if (rootFrame?.CanGoBack == true)
        {
            rootFrame.GoBack();
        }
    }
}