I have a XamDataTree that is setup for multiple selection.  When the user single left clicks without ctrl or shift pressed, the tree seems to clear the selection then select the clicked node.  I need to run specialized code immediately before this clear happens.

I have not found a SelectedNodesCollectionChanging event.  

Any advice on how to add this would be appreciated.

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I think the above is sufficient, but just to add some more detail...

I am using MVVM binding for tree nodes using the IsSelected property.  The root of the problem is that the viewmodel for a tree item can't tell the difference between "a user manually unselected just me" and "I am being unselected as part of clearing the tree".  I can manage this myself if I know it is happening, but need some sort of hook when is going on.