Observer

😎 Observer

Observer is a behavioral design pattern that lets you define a subscription mechanism to notify multiple objects about any events that happen to the object they’re observing.

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.

General structure

• Subject :
  • knows its observers. Any number of Observer objects may observe a subject.
  • provides an interface for attachingand detaching Observer objects.
• Observer :
  • defines an updating interface for objects that should be notified of changes in a subject.
• ConcreteSubject :
  • stores state ofinterest toConcreteObserver objects.
  • sends a notification to its observers when itsstate 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.

Example

For example, a news agency can notify channels when it receives news. Receiving news is what changes the state of the news agency, and it causes the channels to be notified.

• Subject : Agency
• Observer : Channel
• ConcreteSubject : NewsAgency
• ConcreteObserver ; NewsChannel

The code source : source folder

      public static void main(String[] args) {
        NewsAgency observable = new NewsAgency();
        NewsChannel observer = new NewsChannel();

        observable.addObserver(observer);
        observable.setNews("Today's news !");

        System.out.println(observer.getNews());
    }

Output :

    Today's news !