WPF Caliburn Micro TreeView HierarchicalDataTemplate节点扩展事件

I have looked high and low and smashed head here too long. Working with MVVM, Caliburn Micro, WPF, XAML, and trying to late bind the data to the nodes. I have databound the first level or two, and am wanting to watch for "Expanded" events on the nodes so I could check for my little late fetch child node and perhaps do fetch data and grow the tree. I have tried many things, I think I am set now that I should be able to essentially do.. I have looked high and low and smashed head her