观察者模式（Observer Pattern）

观察者模式又叫做订阅/发布模式

• 一个Subject，多Observer
• 相互之间持有引用
• Subject可以调用Observer的函数执行相应操作
• Observer发生变动时也可以通知Subject让其通知其他观察者。

给个小栗子（效果图如下）

目标被观察者

package ObserverPattern;

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

public abstract class Subject {

    //define an ArrayList to store observers
    private List<Observer>  observers = new ArrayList<>();
    public List<Observer> getObservers() {
        return observers;
    }


    public void join(Observer observer) {
        observers.add(observer);
    }

    public void quit(Observer observer) {
        observers.remove(observer);
    }

    public abstract void notifyObservers(String name);

}

观察者

package ObserverPattern;

public abstract class Observer {
    protected Subject subject;
    public Observer(Subject subject) {
        this.subject = subject;
    }
    public abstract void sysMsg(String msg);
    public abstract void beAttacked();

    public abstract String getName();
    public abstract void setName(String name);
}

战况指挥中心

package ObserverPattern;

public class ActionControlCenter extends Subject {
    @Override
    public void notifyObservers(String name) {
        for (Observer observer : getObservers()) {
            if (name.equals(observer.getName()))
                observer.sysMsg(observer.getName() +"收到消息：坚持住，你队友正在赶来！");
            else
                observer.sysMsg(observer.getName() +"收到消息：我方"" + name + ""正在受到攻击，请尽快支援！");
        }
    }
}

玩家

package ObserverPattern;

public class Player extends Observer {

    public Player(Subject subject) {
        super(subject);
    }

    private String name;
    public void setName(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }

    @Override
    public void beAttacked() {
        System.out.println(getName() + "收到消息：您正在受到攻击！");
        subject.notifyObservers(getName());
    }
    @Override
    public void sysMsg(String msg) {
        System.out.println(msg);
    }
}

测试用例

package ObserverPattern;

import org.junit.Test;

public class MainTest {

    @Test
    public void myTest() {

        ActionControlCenter controlCenter = new ActionControlCenter();

        Player p1 = new Player(controlCenter);
        Player p2 = new Player(controlCenter);
        p1.setName("邓艾");
        p2.setName("甄姬");


        controlCenter.join(p1);
        controlCenter.join(p2);

        p2.beAttacked();
    }

}