觀察者模式又叫做訂閱/發布模式

• 一個Subject，多Observer
• 相互之間持有引用
• Subject可以調用Observer的函數執行相應操作
• Observer發生變動時也可以通知Subject讓其通知其他觀察者。

給個小栗子（效果圖如下）

目標被觀察者

 1 package ObserverPattern;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 
 6 public abstract class Subject {
 7 
 8     //define an ArrayList to store observers
 

 9     private List<Observer>  observers = new ArrayList<>();
10     public List<Observer> getObservers() {
11         return observers;
12     }
13 
14 
15     public void join(Observer observer) {
16         observers.add(observer);
17     }
18 
19     public void quit(Observer observer) {
 
20         observers.remove(observer);
21     }
22 
23     public abstract void notifyObservers(String name);
24 
25 }

觀察者

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);
}

戰況指揮中心

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：堅持住，你隊友正在趕來！");
            else
                observer.sysMsg(observer.getName() +"收到消息：我方\"" + name + "\"正在受到攻擊，請盡快支援！");
        }
    }
}

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：您正在受到攻擊！");
        subject.notifyObservers(getName());
    }
    @Override
    public void sysMsg(String msg) {
        System.out.println(msg);
    }
}

測試用例

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("鄧艾");
        p2.setName("甄姬");


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

        p2.beAttacked();
    }

}

觀察者模式（Observer Pattern）