The Observer Pattern

According to the Gang of Four, the intent of the Observer pattern is to “Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.”^[2]

^[2] Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns: Elements of Reusable Object-Oriented Software, Reading, Mass.: Addison-Wesley, 1995, p. 293.

Often, I have a set of objects that need to be notified whenever an event occurs. I want this notification to occur automatically. However, I do not want to change the broadcasting object everytime there is a change to the set of objects listening to the broadcast. (That would be like having to change a radio transmitter every time a new car radio comes to town.) I want to decouple the notify-ors and the notify-ees.

This pattern is a very common one. It also goes by the names Dependents and Publish-Subscribe,^[3] and is analogous to the notify process in COM. It is implemented in Java with the Observer interface and the Observable class (more on these later). In rule-based, expert systems, they are often implemented with daemon rules.

^[3] ibid, p. 293.