Title: Event Package
Cayenne includes a full-featured events mechanism. It allows creation of local and distributed event queues. It is very powerful and generic; it is not tied to Cayenne persistence features in any way and can be used in any application. The most important features are:
- Event senders do not have to deal with event dispatching details - listener handling and dispatches are done via instances of EventManager.
- Any types of listeners can be registered. Listeners do not have to implement a specific interface.
- No explicit unregistering of listeners is required. Listeners are cleaned up when they go out of scope, or when the event subject goes out of scope.
- Supports local and remote dispatches. Remote dispatches can be done via arbitrary transport by implementing transport-specific EventBridge. There are two existing EventBridge implementations - using JMS and using JavaGroups.
- Dispatches can be done both synchronously (sender waits till all listeners finish) or asyncronously (sender posts an event to the queue, and returns without waiting for the processing to complete).
- In a similar manner individual listeners can register as blocking or non-blocking.
 | The most common error when using EventSubject is not storing a reference to it. This may result in a premature cleanup of a given subject, and automatic unregistering of all listeners. Good practice is to make subject a "public static final" variable of the event sender class if a subject is common for all instances of senders; or make it an ivar of a sender instance if subject is only related to this instance. |