How to observe tree node's property change after TransformToTree?

[Snailya]

I have a tree view which has two kind of nodes, virtual one or non-virtual one. When user select a virtual one, I want to redirect the selection to the non-virtual one related. So I need to observe the IsSelected proeprty change. However, if I do the observeration using the follow code, it will not work because the _items field is of a tree structure, it will only fire when the root node is selected.

_items.ToObservableChangeSet(t => t.Id)
            .AutoRefresh(x => x.IsSelected)
            .Filter(x => x.IsVirtual && x.IsSelected)
            .Flatten()
            .Select(x => x.Current)
            .InvokeCommand(Select);

So how to? Thanks.