Java学习笔记——设计模式之十.观察者模式

 观察者模式（Observer），定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使他们能够自动更新自己。

Subject类：

package cn.happy.design_pattern._10publishsubscribe;

import java.util.ArrayList;
import java.util.List;

public class Subject {

    private List<Observer> observers = new ArrayList<>();
    
    public void attach(Observer observer){
        observers.add(observer);
    }
    
    public void detach(Observer observer){
        observers.remove(observer);
    }
    
    public void mNotify(){
        for (Observer item : observers) {
            item.update();
        }
    }
}

Observer类：

package cn.happy.design_pattern._10publishsubscribe;

public abstract class Observer {

    public abstract void update();
}

ConcreteSubject类：

package cn.happy.design_pattern._10publishsubscribe;

public class ConcreteSubject extends Subject {

    private String subjectState;

    String getSubjectState() {
        return subjectState;
    }

    void setSubjectState(String subjectState) {
        this.subjectState = subjectState;
    }
    
}

ConcreteObserver类：

package cn.happy.design_pattern._10publishsubscribe;

public class ConcreteObserver extends Observer {
    
    private String name;
    private String observerState;
    private ConcreteSubject subject;
    
    public ConcreteObserver(ConcreteSubject subject, String name) {
        this.subject = subject;
        this.name = name;
    }

    @Override
    public void update() {
        observerState = subject.getSubjectState();
        System.out.println("观察者"+name+"的新状态是"+observerState);
    }
    //省略getter and setter
}

 测试类：

package cn.happy.design_pattern._10publishsubscribe;

public class Mmain {

    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
        subject.attach(new ConcreteObserver(subject, "X"));
        subject.attach(new ConcreteObserver(subject, "Y"));
        subject.attach(new ConcreteObserver(subject, "Z"));
        subject.setSubjectState("ABC");
        subject.mNotify();
    }

}