观察者模式（Observer）

Posted 2022-12-13 yvhqbat

1. 定义

观察者模式很好理解，类似于邮件订阅和RSS订阅，当我们浏览一些博客或wiki时，经常会看到RSS图标，就这的意思是，当你订阅了该文章，如果后续有更新，会及时通知你。

其实，简单来讲就一句话：当一个对象变化时，其它依赖该对象的对象都会收到通知，并且随着变化！对象之间是一对多的关系。

2. the class diagram

3. 实例

import java.util.*;

public class ObserverPattern 
    public static void main(String[] args)
        Subject sub=new ConcreteSubject();
        Observer observer1=new Observer1();
        sub.registerObserver(observer1);
        Observer observer2=new Observer2();
        sub.registerObserver(observer2);
        sub.operation();
    




//观察者模式

//观察者接口
interface Observer
    public void update();


//观察者1
class Observer1 implements Observer
    @Override
    public void update()
        System.out.println("observer1 has received!");
    


//观察者2
class Observer2 implements Observer
    @Override
    public void update()
        System.out.println("observer2 has received!");
    


//Subject接口
interface Subject
    public void registerObserver(Observer observer);
    public void removeObserver(Observer observer);
    public void notifyObserver();

    public void operation();


//Subject实现类
class ConcreteSubject implements Subject
    private Vector<Observer> observers=new Vector<Observer>();

    @Override
    public void registerObserver(Observer observer)
        observers.add(observer);
    

    @Override
    public void removeObserver(Observer observer)
        observers.remove(observer);
    

    @Override
    public void notifyObserver()
        for(Observer obs:observers)
            obs.update();
        
    

    @Override
    public void operation()
        System.out.println("subject changed!");
        this.notifyObserver();
    

输出

subject changed!
observer1 has received!
observer2 has received!