Observer Pattern

The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.

This pattern is useful for the loosely coupled way it disseminates the fact of a data event or data change in a single subject to as many observers of that subject as have subscribed to receive such notifications.

Apparently, usual practice is for observers to "pull" data from the subject rather than for the subject to "push" the data to observers; in other words, usual practice is for the subject to have getter methods for its state rather than send its data as a parameter in a notifyObservers() call.

A concrete subject implements an interface to register, unregister and notify observers and has a list of registered observers that receive notifications. Concrete observers of the subject implement an interface to update themselves or other objects upon receiving notification of data events or changes in the subject.