观察者模式应用场景

在我们开发的过程中，经常会遇到一些当什么什么事情发生的时候，然后做什么什么事。比如某种商品的物价上涨时会导致部分商家高兴，而消费者伤心。

代码示例

抽象被观察者类

package com.demo.test.observe.abs;

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

public abstract class Subject {
    private List<Observe> observeList = new ArrayList<>();

    public  void attach(Observe observe){
        this.observeList.add(observe);
    };
    public void detach(Observe observe){
        this.observeList.remove(observe);
    };

    public void allNotify(){
        for(Observe observe:observeList){
            observe.update();
        }
    }

}

抽象观察者类

package com.demo.test.observe.abs;

public abstract class Observe {

    public abstract void update();

}

具体被观察者类

package com.demo.test.observe;

import com.demo.test.observe.abs.Subject;

import java.util.List;

public class SubjectA extends Subject {


}

具体观察者1

package com.demo.test.observe;


import com.demo.test.observe.abs.Observe;

public class StudentObserve extends Observe {

    public void update(){
        System.out.println("老师来了，快坐好");
    }
}

具体观察者2

package com.demo.test.observe;

import com.demo.test.observe.abs.Observe;

public class StudentBObserve extends Observe {

    @Override
    public void update() {
        System.out.println("老师来了，别吃了");
    }
}

客户端

package com.demo.test.observe;

import com.demo.test.observe.abs.Observe;
import com.demo.test.observe.abs.Subject;

public class Client {

    public static void main(String[] args){
       Subject subjectA =  new SubjectA();
       Observe studentObserve = new StudentObserve();

       subjectA.attach(studentObserve);
       subjectA.attach(new StudentBObserve());
       subjectA.allNotify();
    }
}

JAVA 自带的观察者类

被观察者

package com.demo.test.observe.javaObserve;

import java.util.Observable;


public class Subject1 extends Observable {


    public void change(){
        setChanged(); //如果不执行setChanged，就不会调用update函数。
    }
}

观察者A

package com.demo.test.observe.javaObserve;

import java.util.Observable;
import java.util.Observer;

public class ObserveA implements Observer {

    @Override
    public void update(Observable o, Object arg) {
        System.out.println("java 自带观察者：A");
    }
}

观察者B

package com.demo.test.observe.javaObserve;

import java.util.Observable;
import java.util.Observer;

public class ObserveB implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("java 自带观察者：B");
    }
}

客户端

package com.demo.test.observe.javaObserve;


import java.util.Observable;
import java.util.Observer;

public class Client {

    public static void main(String[] args){
        Subject1 subject =  new Subject1();

        Observer observerA = new ObserveA();
        Observer observerB = new ObserveB();

        subject.addObserver(observerA);
        subject.addObserver(observerB);
//这一步主要调用里面的setChanged，否则update不执行。setChanged必须在Observable继承类内执行，因为它是protected修饰的。
        subject.change();
        subject.notifyObservers();
    }
}