问绑定到TreeView的WPF在添加时不会更新

EN

// the data type that represents a tree node
public class FurnitureTreeNode : GalaSoft.MvvmLight.ViewModelBase
{
    public string Name { get; private set; }
    public object Data { get; private set; }
    public ObservableCollection<FurnitureTreeNode> ChildNodes { get; set;}
    public Furniture(string Name, ObservableCollection<FurnitureTreeNode> ChildNodes, object Data)
    {
        this.Name = Name;
        this.ChildNodes = ChildNodes;
        this.Data = Data;
    }
}

public class FurnituresViewModel : GalaSoft.MvvmLight.ViewModelBase
{
    public ObservableCollection<FurnitureTreeNode> TopFurnitureNodes { get; set; }
    public ObservableCollection<FurnitureTreeNode> ChairNodes { get; set; }

    public FurnituresViewModel()
    {
        // initialize the top level furnitures collection
        TopFurnitureNodes = new ObservableCollection<FurnitureTreeNode>();

        // add two children
        TopFurnitureNodes.Add(new FurnitureTreeNode("Tables", null, null);
        TopFurnitureNodes.Add(new FurnitureTreeNode("Chairs", null, null);

        // add Beds only when they're ready
        BedsManager.ImportsSatisfied += AddBedsNode;
    }

    void AddBedsNode( object sender, EventArgs e )
    {
        Task.Run(() =>
        {
            TopFurnitureNodes.Add(new FurnitureTreeNode("Beds", null, null);
        }
        );
    }
}

<TreeView Name="FurnituresTreeView" ItemsSource="{Binding TopFurnitureNodes}">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Path=ChildNodes}" DataType="x:Type FurnitureTreeNode">
            <TextBlock Text="{Binding Path=Name}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>