Expanded Event
The Expanded event is sent when a node has been expanded by user interaction.
This event has the following syntax:
on expanded [treeviewfieldname]
The TriggerNode attribute identifies the node that was expanded.
The Expanded event occurs when a node has been expanded to show it's child nodes by clicking the small "expand" box [+] adjacent to the node. The Expanded event can be used to populate or modify the child nodes for the expanded node.
More information: