public NavigationController(INavigationScreen screen)
        {
            if (screen == null) throw new ArgumentNullException("screen");
            _screen = screen;

            RegisterCommands();
        }
        public NavigationController(INavigationScreen screen)
        {
            if (screen == null)
            {
                throw new ArgumentNullException("screen");
            }
            _screen = screen;

            RegisterCommands();
        }
        private void navigationItems_SelectionChanged(object sender,
                                                      SelectionChangedEventArgs e)
        {
            INavigationScreen screen =
                (INavigationScreen)navigationItems.SelectedItem;

            if (screen != null)
            {
                screen.ExecutableObject.ExecuteAsync();
            }
        }