刷新时如何记住WPF XmlDataProvider绑定Treeview的IsExpanded属性？

I have a WPF treeview that is databound to an XmlDataProvider - the XML source is used by many PCs. When the XmlDataProvider is refreshed all the TreeViewNodes collapse. I would like each PC to remember the status of the IsExpanded attribute.I have a WPF treeview that is databound to an X