Hi Matthew,
Thank you for using Syncfusion products.
We would like to inform that Treeview has NodeExpanding, NodeExpanded, NodeCollapsing, NodeCollapsed events in order to get a notification when Treeview nodes expanded/collapsed. You can refer our User Guide documentation for events by the following link,
We hope this helps. Please let us know if you have any further requirements.
Regards,
Gnana Priya N