Observer

Intent, Motivation, Applicability, Structure, Participants, Collaborations, Consequences, Implementation, Sample Code, Known Uses, Related Patterns

Participants

• Subject

• knows its observers. Any number of Observer objects may observe a subject.
• provides an interface for attaching and detaching Observer objects.
• Observer

• defines an updating interface for objects that should be notified of changes in a subject.
• ConcreteSubject

• stores state of interest to ConcreteObserver objects.
• sends a notification to its observers when its state changes.
• ConcreteObserver

• maintains a reference to a ConcreteSubject object.
• stores state that should stay consistent with the subject's.
• implements the Observer updating interface to keep its state consistent with the subject's.

Participants of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor