C# 始终在动态填充上显示展开/折叠箭头

我有一个简单的自定义树视图组件：

  public partial class ThemesTreeView: UserControl, IThemesTreeView {
  public delegate void ThemeSelected(ThemeModel theme);
  public event ThemeSelected OnThemeSelection;

  public delegate void ThemeDoubleClick(ThemeModel theme);
  public event ThemeDoubleClick OnThemeDoubleClick;

  public ThemesTreeView() {
   InitializeComponent();
  }   


  private void UserControl_Loaded(object sender, RoutedEventArgs e) {      
   var caseCovers = GetTreeViewItem((Int32) ThemeType.CaseCover, "1", "Case Covers", "album_cover.png");
   var musics = GetTreeViewItem((Int32) ThemeType.Music, "1", "Music", "music_run.png");
   var movies = GetTreeViewItem((Int32) ThemeType.Video, "1", "Scenic Video Inserts", "video.png");   

   tvThemes.Items.Add(caseCovers);
   tvThemes.Items.Add(musics);
   tvThemes.Items.Add(movies);
  }

  private ThemesTreeViewItem GetTreeViewItem(Int32 tag, String uid, String text, String imagePath) {
   ThemesTreeViewItem item = new ThemesTreeViewItem();
   item.Uid = uid;
   item.Tag = tag;
   item.IsExpanded = false;

   // create stack panel
   StackPanel stack = new StackPanel();
   stack.Orientation = Orientation.Horizontal;

   // create Image
   Image image = new Image();
   image.Source = new BitmapImage(new Uri("pack://application:,,/images/" + imagePath));
   image.Width = 16;
   image.Height = 16;
   // Label
   Label lbl = new Label();
   lbl.Content = text;

   // Add into stack
   stack.Children.Add(image);
   stack.Children.Add(lbl);

   // assign stack to header
   item.Header = stack;
   return item;
  }
 }
}

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我跳过了一些下载和填充子节点的代码。当用户单击一个节点时，子节点被下载并添加到treeview中。但问题是在添加节点之前，没有展开/折叠箭头。如何始终显示箭头？

复制treeviewitem样式。查找treenode的一部分，并更正与具有子节点的treenode状态类似的节点样式