on
Subscribes the given event observer to receive the events of the given
eventClass. The order in which notifications will be delivered to the registered listeners is not specified. The method does nothing if the given observer has been already registered.
Return
a Subscription reference with which the subscribers can unsubscribe from the Observable. If the given observer is already registered, returns a reference to already created Subscription.
Parameters
event
the event class
observer
the event observer to subscribe
<E>
the event type