자바 디자인 패턴(4)_옵저버 패턴

옵저버 패턴

• 객체의 상태 변화를 보고있는 관찰자(주체)가 어떤 객체의 상태 변화를 관찰하다가 상태 변화가 있을 때마다 메서드 등을 추가 변화 사항이 생기는 객체들(옵저버들)에게 변화를 알리는 패턴
• MVC 패턴에서 사용된다.
  • Model에서 변경사항이 생겨 옵저버인 View에게 알려주고 이를 기반으로 Controller가 동작한다.

// topic을 기반으로 옵저버 패턴을 구현 - Observer a = new TopicSubscriber("a", topic);으로 해당 이름과 어떤 topic의 옵저버가 될지 결정
import java.util.*;

interface Subject {
    public void register(Observer obj);
    public void unregister(Observer obj);
    public void notifyObservers();
    public Object getUpdate(Observer obj);
}

interface Observer {
    public void update();
}

class Topic implements Subject {
    private List<Observer> observers;
    private String message;
    
    public Topic() {
        this.observers = new ArrayList<>();
        this.message = "";
    }
    
    @Override
    public void register(Observer obj) {
        if (!observers.contains(obj)) observers.add(obj);
    }
    
    @Override
    public void unregister(Observer obj) {
        observers.remove(obj);
    }
    
    @Override
    public void notifyObservers() {
        this.observers.forEach(Observer::update);
    }
    
    @Override
    public Object getUpdate(Observer obj) {
        return this.message;    
    }
    
    public void postMessage(String msg) {
        System.out.println("message sended to topic : " + msg);
        this.message = msg;
        notifyObservers();
    }
}

class TopicSubscriber implements Observer {
    private String name;
    private Subject topic;
    
    public TopicSubscriber(String name, Subject topic) {
        this.name = name;
        this.topic = topic;
    }
    
    @Override
    public void update() {
        String msg = (String)topic.getUpdate(this);
        System.out.println(name+" :: message >> " + msg);
    }
}

public class HelloWorld{
     public static void main(String []args){
        Topic topic = new Topic();
        
        Observer a = new TopicSubscriber("a", topic);
        Observer b = new TopicSubscriber("b", topic);
        Observer c = new TopicSubscriber("c", topic);
        
        topic.register(a);
        topic.register(b);
        topic.register(c);
        
        /*
        message sended to topic : ###
        a :: message >> ###
        b :: message >> ###
        c :: message >> ###
        */
        topic.postMessage("###");
     }
}

 

 

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