Helper class used by the observation manager classes.

The JCR observation framework defines a notification mechanism where an `EventListener` is registered with the observation manager to receive certain events during the lifetime of the registration. For the remote case, where the repository and the application run in different Java VMs on possibly different hosts, there are issues related to the observation framework.

The listener mechanism is a call-back mechanism where the server calls code in the client application. The client application code most probably hooks into other parts of that application. Therefore it is not practically feasible to just require the client listener to be serializable to be sent to the server for several reasons:

- The RMI server cannot call any method on the RMI client. To support such call-back situations, the client side application would have to register another server, which the server side would have to call.
- When trying to "transfer" the listener to the server side, the listener class would have to be available to the server side - either locally in the class path or through RMI class loading mechanisms.

  To circumvent these issues and still be able to register event listeners, support for observation events is implemented in a manner similar to the Java Management Extensions Remote API 1.0 (JSR 160) as laid out in Chapter 2.4, Adding Remote Listeners:

  The ObservationManager interface is not implemented in the RMI layer like other interfaces, which just forward calls to the API to the corresponding remote object by means of the RMI framework. Instead the client-side ObservationManager manages its own list of registered event listeners. Each listener registered with an ObservationManager is assigned a unique identifier.

  The unique identifier along with the filter configuration (event type, path, depth flag, uuid list, node type list, local flag) is sent to the server-side remote observation manager. This latter instantiates a proxy event listener representing the client side event listener contains the unique identifier as a ilnk to the client side event listener. The proxy event listener is the registered to the repository's ObservationManager with the configuration received from the client side.

  When an event arrives at the event listener proxy, the proxy creates a new RemoteEvent instance, which contains the client-side event listener identifier along with the Event objects from the EventIterator. This RemoteEvent instance is added to a server-side queue, which may be retrieved from the client-side.

  The client-side ObservationManager has a helper class ClientEventPoll, which works in the background asking the server for the RemoteEvents from the event queue. Each such event is then dispatched to the client-side event listener by calling the EventListener.onEvent() method.