Applied Design Patterns with Java

Behavioral :: Observer (293) {C ch 22}

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 allows varying and reusing them independently.
• When a change to one object requires changing others, and how many objects need to be changed is not known in advance.
• When an object should be able to notify other objects without making assumptions about who these objects are. That is, these objects should NOT be tightly coupled.

Structure

Participants

• Subject knows its Observers. Any number of Observer objects may observe a subject. The 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, and it sends a notification to its Observers when its state changes.
• ConcreteObserver maintains a reference to a ConcreteSubject object, it stores a state that should stay consistent with the Subject's, and it implements the Observer updating interface to keep its state consistent with the Subject's.

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