JAVA设计模式—观察者模式Observer的演变

观察者模式基础

以下是小孩哭这个动作，通知所观察者(Mum Dad Dog)做出不同的反应

/**
 * 分离观察者与被观察者
 */

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

    {
        observers.add(new Dad());
        observers.add(new Mum());
        observers.add(new Dog());
    }


    public boolean isCry() {
        return cry;
    }

    public void wakeUp() {
        cry = true;
        for(Observer o : observers) {
            o.actionOnWakeUp();
        }
    }
}
/**定义主题对象接口 观察者接口**/
interface Observer {
    void actionOnWakeUp();
}

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

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

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

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

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

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

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

dad feeding...
dog wang...
mum hugging...

观察者模式本身比较简单 就是多态的应用

很多时候 观察者需要根据这件事的具体情况做出相应的处理：比如小孩哭的时间和地点，观察者做出的反应应该不同

下面对观察者模式进行进一步的封装

抽象出处理事件类

把小孩哭得所有状况传到一个事件里面

//事件类 fire Event
class wakeUpEvent{
    long timestamp;
    String loc;

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

interface Observer {
    void actionOnWakeUp(wakeUpEvent event);
}

有点类似发布订阅，发布者发布一个事件，订阅者接收这个事件

事件类增加事件源对象，并形成继承体系

大多数时候，我们处理事件的时候，需要事件源对象(有时观察者需要事件原对象的一些资源)

class wakeUpEvent{
    long timestamp;
    String loc;
    Child source;

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

因为我们都是面向接口编程 所以把事件定义一个接口或者抽象类。这也是多态的应用

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;
    }
}

其实Observer Listener Hook CallBack 这些都是观察者模式

最终代码

package com.mashibing.dp.observer.v8;

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

/**
 * 有很多时候，观察者需要根据事件的具体情况来进行处理
 * 大多数时候，我们处理事件的时候，需要事件源对象
 * 事件也可以形成继承体系
 */

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();
    }
}

ApplicationListener

Spring中的ApplicationListener也是使用的观察者模式，下面两种方式做比较

https://www.cnblogs.com/ssskkk/p/12942375.html#_label4