The Load-On-Demand feature of RadTreeView allows nodes to be added on the fly as
nodes are expanded.
The NodeExpand event fires when the "+" icon is clicked. This event
provides the opportunity to add nodes based on the identity of the clicked-on parent
node. The NodeExpand event is fired based on the ExpandMode
property of each node. When the ExpandMode property is set to
ServerSideCallBack, this will trigger an asynchronous callback that
fires the NodeExpand event. When a node is toggled, an asynchronous
callback is performed automatically to retrieve the data for just that node. Minimally,
you must set the ExpandMode to ServerSideCallback
and handle the NodeExpand event as shown in this example.