Observer Pattern

Observer Pattern

Overview

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically

Participants

Abstract Subject

knows its observers. Any number of Observer objects may observe a subject.
provides an interface for attaching and detaching Observer objects.
this participant can not be read-only.

Abstract Observer

defines an updating interface for objects that should be notified of changes in a subject.
this participant can not be read-only.

Concrete Subject

stores state of interest to ConcreteObserver objects.
sends a notification to its observers when its state changes.
this participant can not be read-only.
this participant can not be an interface.

Observer implementation

maintains a reference to a Concrete Subject object.
stores state that should stay consistent with the subjects.
implements the Observer updating interface to keep its state consistent with the subjects.
this participant can not be read-only.

Parameters

Attach Observer method

name of the method in the Subject for attaching Observers.

Detach Observer method

name of the method in the Subject for detaching Observers.

Notify Observer method

name of the method in the Subject for notifying all the Observers of this Subject.

Observer update method

name of the update method in the Observer, the method that is called when a observed Subject has been changed. The parameters of which are the Subject itself.

Applicability

Use the Observer pattern in any of the following situations:

when an 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. In other words, you don't want these objects tightly coupled.