Xamarin表单中的可扩展列表视图

我有下面的代码来使用Xamarin Forms显示ListView:

App.cs

public App ()
{
    MainPage = GetMainPage();
}
public static Page GetMainPage()
{
    return new NavigationPage(new DrilldownListViewByItem());
}

DrilldownListViewByItem:

public class DrilldownListViewByItem : ContentPage
{
    public DrilldownListViewByItem()
    {
        Title = "Drilldown List Using ListView";
        var listView = new ListView();
        listView.ItemsSource = new ListItem[] {
            new ListItem {Title = "First", Description="1st item"},
            new ListItem {Title = "Second", Description="2nd item"},
            new ListItem {Title = "Third", Description="3rd item"}
            };
        listView.ItemTemplate = new DataTemplate(typeof(TextCell));
        listView.ItemTemplate.SetBinding(TextCell.TextProperty, "Title");
        listView.ItemTapped += async (sender, args) =>
        {
            var item = args.Item as ListItem;
            //if (item == null) return;
            //await Navigation.PushAsync(new DetailPage(item));
            //listView.SelectedItem = null;
        };
        Content = listView;
    }
}

listtitem有以下属性:

public string Title { get; set; }
public string Description { get; set; }

我想做的是，当我在一个主要项目上添加标签时，显示子项。这些子项应该隐藏在开始，但他们应该在那里，我不想加载他们每次我在一个项目的标签。任何想法?谢谢!