Observer

Static diagram :

Description :

Type :

Be (Behavioral)

Intent :

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

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.

Pattern's 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.

Problems :

1. Affichage différentié

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Design patterns

Observer

Static diagram :

Description :

Pattern's participants :

Problems :