This event will only occur after at
least one client has connected to the server and then closes its
connection or is disconnected. This event will not occur immediately
after the server has started using the Start method, and will not
occur when the server is stopped using the Stop method. Your
application should implement an OnStart event handler for when
the server first starts, and an OnStop event handler for when
the server is stopped.
If one or more new client connections are accepted after this event
occurs, the event will be generated again when those clients disconnect
and the active client count drops to zero. Therefore it is to be expected
that this event will occur multiple times over the lifetime of the
server as it continues to listen for connections.