Java学习笔记——设计模式之十.观察者模式

 观察者模式（Observer），定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使他们能够自动更新自己。

Subject类：

 package cn.happy.design_pattern._10publishsubscribe;

 import java.util.ArrayList;

 import java.util.List;

 public class Subject {

     private List<Observer> observers = new ArrayList<>();

     public void attach(Observer observer){

         observers.add(observer);

     }

     public void detach(Observer observer){

         observers.remove(observer);

     }

     public void mNotify(){

         for (Observer item : observers) {

             item.update();

         }

     }

 }

Observer类：

 package cn.happy.design_pattern._10publishsubscribe;

 public abstract class Observer {

     public abstract void update();

 }

ConcreteSubject类：

 package cn.happy.design_pattern._10publishsubscribe;

 public class ConcreteSubject extends Subject {

     private String subjectState;

     String getSubjectState() {

         return subjectState;

     }

     void setSubjectState(String subjectState) {

         this.subjectState = subjectState;

     }

 }

ConcreteObserver类：

 package cn.happy.design_pattern._10publishsubscribe;

 public class ConcreteObserver extends Observer {

     private String name;

     private String observerState;

     private ConcreteSubject subject;

     public ConcreteObserver(ConcreteSubject subject, String name) {

         this.subject = subject;

         this.name = name;

     }

     @Override

     public void update() {

         observerState = subject.getSubjectState();

         System.out.println("观察者"+name+"的新状态是"+observerState);

     }

     //省略getter and setter

 }

测试类：

 package cn.happy.design_pattern._10publishsubscribe;

 public class Mmain {

     public static void main(String[] args) {

         ConcreteSubject subject = new ConcreteSubject();

         subject.attach(new ConcreteObserver(subject, "X"));

         subject.attach(new ConcreteObserver(subject, "Y"));

         subject.attach(new ConcreteObserver(subject, "Z"));

         subject.setSubjectState("ABC");

         subject.mNotify();

     }

 }