Monday, August 14, 2006

Observer pattern brief notes

event object? Ignore for now. First focus on the 2 basic players


Subject and Observer HAS-A pointer to each other. (tight coupling@@ NO. Thanks to interfaces)

- Subject HAS-A reference to each observer [1] in order to send them updates.
- Observer HAS-A rcontrol linked to his subject(s) in order to subscribe/unsubscribe. Usually a field in the observer.

It seems to me that they don't normally receive these pointers by argument-passing like

- Subject doesn't HAS-A such pointer but receives a list of observers in notifyObserver() method. However, this way, subscription has no meaning.
- Observer doesn't HAS-A such pointer but receives it as argument when calling subscribe().

[1] Static collection or a collection as a field in the subject object.

[[ Head First Design Patterns ]] is good. Helps me internalize
- registration
- data passing during notification

--Usage@@ I think most (if not every) events, listeners, subscribers, publishers ... take some hints from the Observer pattern.
* swing -- /Intimate/ knowledge helps you chip away at swing. Events and listeners are fundamental to swing.
* java beans
* servlet -- This pattern is also relevant for servlet listeners.
* threads -- Observer pattern helps inter-thread communications and data sharing
* JMS, MDB @@ Confident. No need to verify

-- keywords:
subject/observer
pub/sub -- the observer subscribes to news published from the Subject object
listener aka observer
registration aka subscribing

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