Observer

Origin:

GoF 95

Reason:

To establish dependencies between objects so that when one object changes state, its dependent objects are informed.

Synopsis:

Define and implement interfaces with methods to register dependencies between objects and to notify dependent objects of state changes.

Example:

In a security system objects that trigger alarms must notify objects that handle alarms, but at the same time they must be largely independent of each other.

Solution:

Observer	There are both an interface and one or more implementing classes. The interface has an update method called by Observable objects when their state changes.
Observable	There are both an interface and one or more implementing classes. The interface has methods to add and remove Observer objects whose update method is called upon a state change.
Multicaster	This class manages the the registration of Observers on behalf of Observables.