Java学习笔记——设计模式之十.观察者模式

Posted 2020-09-23 Tomas曼

 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：

 1 package cn.happy.design_pattern._10publishsubscribe;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 
 6 public class Subject {
 7 
 8     private List<Observer> observers = new ArrayList<>();
 9     
10     public void attach(Observer observer){
11         observers.add(observer);
12     }
13     
14     public void detach(Observer observer){
15         observers.remove(observer);
16     }
17     
18     public void mNotify(){
19         for (Observer item : observers) {
20             item.update();
21         }
22     }
23 }

Observer类：

1 package cn.happy.design_pattern._10publishsubscribe;
2 
3 public abstract class Observer {
4 
5     public abstract void update();
6 }

ConcreteSubject类：

 1 package cn.happy.design_pattern._10publishsubscribe;
 2 
 3 public class ConcreteSubject extends Subject {
 4 
 5     private String subjectState;
 6 
 7     String getSubjectState() {
 8         return subjectState;
 9     }
10 
11     void setSubjectState(String subjectState) {
12         this.subjectState = subjectState;
13     }
14     
15 }

ConcreteObserver类：

 1 package cn.happy.design_pattern._10publishsubscribe;
 2 
 3 public class ConcreteObserver extends Observer {
 4     
 5     private String name;
 6     private String observerState;
 7     private ConcreteSubject subject;
 8     
 9     public ConcreteObserver(ConcreteSubject subject, String name) {
10         this.subject = subject;
11         this.name = name;
12     }
13 
14     @Override
15     public void update() {
16         observerState = subject.getSubjectState();
17         System.out.println("观察者"+name+"的新状态是"+observerState);
18     }
19     //省略getter and setter
20 }

 测试类：

 1 package cn.happy.design_pattern._10publishsubscribe;
 2 
 3 public class Mmain {
 4 
 5     public static void main(String[] args) {
 6         ConcreteSubject subject = new ConcreteSubject();
 7         subject.attach(new ConcreteObserver(subject, "X"));
 8         subject.attach(new ConcreteObserver(subject, "Y"));
 9         subject.attach(new ConcreteObserver(subject, "Z"));
10         subject.setSubjectState("ABC");
11         subject.mNotify();
12     }
13 
14 }