在看原始碼的時候我們經常會看到Observer、Listener、Hook、callback等單詞，不要被這些詞所迷惑，實際上說的都是觀察者模式，

觀察模式解決方案圖如下:

具體程式碼:

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

/**
 * 有很多時候，觀察者需要根據事件的具體情況來進行處理
 * 大多數時候，我們處理事件的時候，需要事件源物件
 * 事件也可以形成繼承體系
 */

class Child {
    private boolean cry = false;
    private List<Observer> observers = new
 
 ArrayList<>();

    {
        observers.add(new Dad());
        observers.add(new Mum());
        observers.add(new Dog());
        observers.add((e)->{
            System.out.println("ppp");
        });
        //hook callback function
    }


    public boolean isCry() {
        return cry;
    }

    
 
public void wakeUp() {
        cry = true;

        wakeUpEvent event = new wakeUpEvent(System.currentTimeMillis(), "bed", this);

        for(Observer o : observers) {
            o.actionOnWakeUp(event);
        }
    }
}

abstract class Event<T> {
    abstract T getSource();
}

class wakeUpEvent extends
 
 Event<Child>{
    long timestamp;
    String loc;
    Child source;

    public wakeUpEvent(long timestamp, String loc, Child source) {
        this.timestamp = timestamp;
        this.loc = loc;
        this.source = source;
    }

    @Override
    Child getSource() {
        return source;
    }
}

interface Observer {
    void actionOnWakeUp(wakeUpEvent event);
}

class Dad implements Observer {
    public void feed() {
        System.out.println("dad feeding...");
    }

    @Override
    public void actionOnWakeUp(wakeUpEvent event) {
        feed();
    }
}

class Mum implements Observer {
    public void hug() {
        System.out.println("mum hugging...");
    }

    @Override
    public void actionOnWakeUp(wakeUpEvent event) {
        hug();
    }
}

class Dog implements Observer {
    public void wang() {
        System.out.println("dog wang...");
    }

    @Override
    public void actionOnWakeUp(wakeUpEvent event) {
        wang();
    }
}

public class Main {
    public static void main(String[] args) {
        Child c = new Child();
        //do sth
        c.wakeUp();
    }
}