Hi,
Sorry for the late reply, I was OOF this week.
Where you have to know such details on server or on client.
SelectedItemsAction and SelectedItemsActionComplete are processed on server during page's life cycle. Event argument has all what you need.
Client side is not supported in such way but it is implementable.
If you need a client side events we have to talk about implementation of client side model (with state, properties, events and etc.).
Igor.