on
Adds the listener to the event queue, returns a token that the caller can use to off itself. Calls to on are stored in the order they arrive, which means that during event propagation, the listeners will be executed in the order they were added.
Return
token An integer representing the id token of the listener. Objects that invoke this method should cache the token, which is used as a reference to remove a Listener
Parameters
eventType
A String representing the Event to be listened for
listener
An abstract class of EventListener, that contains a method to process during an Event's firing