Defines a reference of an event handler which has been registered to be notified of a certain event. The event handler can be {@link #unregister() unregistered} once no longer needed to be notified ofthe event. Once a listener has been unregistered, there is no way to register it again through this interface. That is, it should be registered as it was done previously.
Note that this interface defines the same methods as the {@code org.jtrim.event.ListenerRef} (of the JTrim library) interfacewith exactly the same contract. This is so, that later (in NetBeans 7.4) this interface can extend from the interface of JTrim.
Thread safety
Implementations of this interface are required to be safe to use by multiple threads concurrently.
Synchronization transparency
Methods of this interface are required to be
synchronization transparent.
@see NbListenerRefs
@see ListenerRef