WPF MVVM实现ListBox添加不同的控件

    public class TemplateSelectorDemo: DataTemplateSelector
    {
        public DataTemplate CalendarDataTemplate { get; set; }

        public DataTemplate EllipseDataTemplate { get; set; }

        public DataTemplate RectangleDataTemplate { get; set; }

        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            if (container is FrameworkElement && item != null)
            {
                if (item is CalendarViewModel)
                    return CalendarDataTemplate;

                if (item is EllipseViewModel)
                    return EllipseDataTemplate;

                if (item is RectangleViewModel)
                    return RectangleDataTemplate;

                else return null;
            }
            return null;
        }
    }

   public class MenuViewModel:Screen
    {
        public MenuViewModel()
        {
            CurrentViewModel =new CalendarViewModel()
            {
            };
            ViewModelsCollection.Add(CurrentViewModel);
            CurrentViewModel = new EllipseViewModel()
            {
            };
            ViewModelsCollection.Add(CurrentViewModel);
            CurrentViewModel = new RectangleViewModel()
            {
            };
            ViewModelsCollection.Add(CurrentViewModel);
        }

        private Screen _CurrentViewModel;
        public Screen CurrentViewModel
        {
            get { return _CurrentViewModel; }
            set { Set(ref _CurrentViewModel, value); }
        }

        private Screen _SelectedViewModel;
        public Screen SelectedViewModel
        {
            get { return _SelectedViewModel; }
            set { Set(ref _SelectedViewModel, value); }
        }

        private ObservableCollection<Screen> _ViewModelsCollection=new ObservableCollection<Screen>();
        public ObservableCollection<Screen> ViewModelsCollection
        {
            get { return _ViewModelsCollection; }
            set { Set(ref _ViewModelsCollection, value); }
        }

    }