Observer

28 Aug 2022 Observer

Defines a one to many dependency between objects so that when one object changes state, all it's dependents are notified and updated automatically.

Also Known as

Dpendents, Publish-Subscribe

Use visitor pattern when:

When ans abstraction has two aspects, one dependent on the other, encapsulating these aspects in separate objects lets you vary and reuse them independently.
When a change to one object requires changing others, and you don't know how many objects need to be changed.
When an object should be able to notify other objects without making assumptions about who these objects are.

Subject: knows it's observers. Any member 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 it's observers when it's 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 it's state consistent with the subject's.

ConcreteSubject notifies it's observers whenever a change occurs that could make it's observer's state inconsistent with it's own.
After being informed of a change in concrete subject, a ConcreteObserver object may query the subject for information.

Mediator: By encapsulating complex update semantics, the ChangeManager acts as mediator between subjects and observers.
Singleton: The ChangeManager may use the Singleton pattern to make it unique and globally accessible.