设计模式——（2）观察者模式
第二章 观察者模式
1.1 引言
在开发过程中，当遇到如下场景：
- 类似
微信公众号的消息通知模型，只要公众号更新了消息，那么所有订阅了该公众号的用户都会收到消息推送。 拍卖的时候，拍卖师观察最高标价，然后通知给其他竞价者竞价。
就上述场景而言，一方更新消息，其余用户获取消息。
由于在公众号消息通知模型中，订阅消息的用户可能增加、减少，因此，需要一种设计模式来解决此场景下的问题。
观察者模式就是应对此场景的一种设计模式。这里更新消息的一方称为主题（公众号），所有接收消息的用户称为观察者（订阅公众号的用户）。
1.2 自己手动实现主题-订阅模型
设计方案如类图1所示，Subject 是自定义主题接口，其中包含注册观察者的方法 registerObserver()；移除观察者的方法removeObserver()；消息更新后，通知所有观察者的方法notifyAllObservers()。
WeChatPublicAccount 类是微信公众号，实现了Subject 接口。维护了一个 observersList ，用于管理所有的观察者，除此，还包括文章的标题、作者和内容等属性。
Observer 接口，为所有的观察者提供了统一的接口，并具有update()方法，用于更新信息。
User1类、User2类和User3类，是订阅了微信公众号的观察者。
类图1.自己实现微信公众号的主题-订阅模型
Subject 接口：
package headfirst.designpatterns.observer.simple;
public interface Subject {
void registerObserver(Observer observer);
void removeObserver(Observer observer);
void notifyAllObservers();
}
WeChatPublicAccount 类：
package headfirst.designpatterns.observer.simple;
import java.util.ArrayList;
public class WeChatPublicAccount implements Subject {
private ArrayList<Observer> observersList;
private String title;
private String author;
private String content;
public WeChatPublicAccount() {
observersList = new ArrayList<>();
}
public ArrayList<Observer> getObserversList() {
return observersList;
}
@Override
public void registerObserver(Observer observer) {
if (observer != null) observersList.add(observer);
}
@Override
public void removeObserver(Observer observer) {
int index = -1;
if (observer != null)
index = observersList.indexOf(observer);
if (index != -1)
observersList.remove(index);
}
// 数据由主题由 “推” 的方式通知观察者
@Override
public void notifyAllObservers() {
for (Observer o : observersList) {
if (o != null)
o.update(title, author, content);
}
}
public void dataChanged() {
notifyAllObservers();
}
public void setDataChanged(String title, String author, String content) {
this.title = title;
this.author = author;
this.content = content;
dataChanged();
}
}
Observer 接口:
package headfirst.designpatterns.observer.simple;
public interface Observer {
void update(String title,String author,String content);
}
User1 类：
package headfirst.designpatterns.observer.simple;
public class User1 implements Observer {
private String title;
private String author;
private String content;
private Subject weChatPublicAccount;
public User1(Subject weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.registerObserver(this);
}
@Override
public void update(String title, String author, String content) {
this.title = title;
this.author = author;
this.content = content;
}
@Override
public String toString() {
return "User1{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
}
User2 类：
package headfirst.designpatterns.observer.simple;
public class User2 implements Observer {
private String title;
private String author;
private String content;
private Subject weChatPublicAccount;
public User2(Subject weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.registerObserver(this);
}
@Override
public void update(String title, String author, String content) {
this.title = title;
this.author = author;
this.content = content;
}
@Override
public String toString() {
return "User2{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
}
User3：
package headfirst.designpatterns.observer.simple;
public class User3 implements Observer {
private String title;
private String author;
private String content;
private Subject weChatPublicAccount;
public User3(Subject weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.registerObserver(this);
}
@Override
public void update(String title, String author, String content) {
this.title = title;
this.author = author;
this.content = content;
}
@Override
public String toString() {
return "User3{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
}
Test：
package headfirst.designpatterns.observer.simple;
public class Test {
public static void main(String[] args) {
WeChatPublicAccount weChatPublicAccount = new WeChatPublicAccount();
Observer user1 = new User1(weChatPublicAccount);
Observer user2 = new User2(weChatPublicAccount);
Observer user3 = new User3(weChatPublicAccount);
System.out.println(user1.toString());
System.out.println(user2.toString());
System.out.println(user3.toString());
// 更新数据
weChatPublicAccount.setDataChanged("午觉睡多久比较好","丁香医生","午觉睡10-20mins是极好的，一觉醒来元气满满");
System.out.println("丁香医生公众号数据更新后...");
System.out.println(user1.toString());
System.out.println(user2.toString());
System.out.println(user3.toString());
}
}
输出： 在微信公众号更新数据后，所有的观察者都将收到公众号推送的数据。
User1{title='null', author='null', content='null'}
User2{title='null', author='null', content='null'}
User3{title='null', author='null', content='null'}
丁香医生公众号数据更新后...
User1{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
User2{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
User3{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
1.3 通过Observable接口（推）实现微信公众号的主题-订阅模型
利用 JDK 自带的 Observable 类和 Observer 接口，设计的方案如类图2所示。Observable 类和 1.2节中的 Subject 接口功能类似，其维护了一个 obs 列表，用于管理所有观察者。
WeChatPublicAccount 类继承了 Observable 类，其中在公众号的数据更新后，可将所有数据封装至一个 HashMap dataMap中，然后以dataMap 参数，调用 notifyObservers(Object arg) 方法可逐一通知所有观察者，实现数据推送。
类图2.通过Observable接口（推）实现微信公众号的主题-订阅模型
WeChatPublicAccount 类：
package headfirst.designpatterns.observer.observablepush;
import java.util.HashMap;
import java.util.Observable;
public class WeChatPublicAccount extends Observable {
private String title;
private String author;
private String content;
private HashMap<String, String> dataMap;
public WeChatPublicAccount() {
dataMap = new HashMap<>();
}
// 将数据封装至 map，然后以 “推” 的方式将数据通知所有观察者。
public void dataChanged() {
// 当状态变化满足一定条件时，调用 setChanged() 标志状态已改变，可通知观察者。例如，温度变化1°，才允许标志状态已改变，可通知观察者。
setChanged();
dataMap.put("title", title);
dataMap.put("author", author);
dataMap.put("content", content);
notifyObservers(dataMap);
}
public void setDataChanged(String title, String author, String content) {
this.title = title;
this.author = author;
this.content = content;
dataChanged();
}
}
User1类：
package headfirst.designpatterns.observer.observablepush;
import java.util.HashMap;
import java.util.Observable;
import java.util.Observer;
public class User1 implements Observer {
private String title;
private String author;
private String content;
private Observable weChatPublicAccount;
public User1(Observable weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.addObserver(this);
}
@Override
public String toString() {
return "User1{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
@Override
public void update(Observable o, Object arg) {
HashMap<String,String> dataMap = (HashMap<String, String>) arg;
title = dataMap.get("title");
author = dataMap.get("author");
content = dataMap.get("content");
}
}
User2 类：
package headfirst.designpatterns.observer.observablepush;
import java.util.HashMap;
import java.util.Observable;
import java.util.Observer;
public class User2 implements Observer {
private String title;
private String author;
private String content;
private Observable weChatPublicAccount;
public User2(Observable weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.addObserver(this);
}
@Override
public String toString() {
return "User2{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
@Override
public void update(Observable o, Object arg) {
HashMap<String,String> dataMap = (HashMap<String, String>) arg;
title = dataMap.get("title");
author = dataMap.get("author");
content = dataMap.get("content");
}
}
User3类：
package headfirst.designpatterns.observer.observablepush;
import java.util.HashMap;
import java.util.Observable;
import java.util.Observer;
public class User3 implements Observer {
private String title;
private String author;
private String content;
private Observable weChatPublicAccount;
public User3(Observable weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.addObserver(this);
}
@Override
public String toString() {
return "User3{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
@Override
public void update(Observable o, Object arg) {
HashMap<String,String> dataMap = (HashMap<String, String>) arg;
title = dataMap.get("title");
author = dataMap.get("author");
content = dataMap.get("content");
}
}
Test类：
package headfirst.designpatterns.observer.observablepush;
import java.util.Observer;
public class Test {
public static void main(String[] args) {
WeChatPublicAccount weChatPublicAccount = new WeChatPublicAccount();
Observer user1 = new User1(weChatPublicAccount);
Observer user2 = new User2(weChatPublicAccount);
Observer user3 = new User3(weChatPublicAccount);
System.out.println(user1.toString());
System.out.println(user2.toString());
System.out.println(user3.toString());
weChatPublicAccount.setDataChanged("午觉睡多久比较好","丁香医生","午觉睡10-20mins是极好的，一觉醒来元气满满");
System.out.println("丁香医生公众号数据更新后...");
System.out.println(user1.toString());
System.out.println(user2.toString());
System.out.println(user3.toString());
}
}
输出：
User1{title='null', author='null', content='null'}
User2{title='null', author='null', content='null'}
User3{title='null', author='null', content='null'}
丁香医生公众号数据更新后...
User1{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
User2{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
User3{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
1.4 通过Observable接口（拉）实现微信公众号的主题-订阅模型
1.3 节中实现了主题-订阅模型，主题主动推送数据，本节主要介绍观察者主动拉取数据的模型。利用 JDK 自带的 Observable 类和 Observer 接口，设计的方案如类图3所示。
类图3.通过Observable接口（拉）实现微信公众号的主题-订阅模型
与 1.3 节中不同的是，当主题中的数据更新后，并不是主动的将数据推送至所有观察者，而是通知所有观察者，并提供了获取数据的公共方法，以供所有观察者自己获取所需的数据，getTitle()、getAuthor()和getContent()，除此，微信公众号调用的notifyObservers()方法，并不传递数据参数，只负责通知。
WeChatPublicAccount 类：
package headfirst.designpatterns.observer.observablepull;
import java.util.Observable;
public class WeChatPublicAccount extends Observable {
private String title;
private String author;
private String content;
public WeChatPublicAccount() {
}
// 通知所有观察者，可以通过 “拉” 的方式获取自己所需的数据。
public void dataChanged() {
setChanged();
notifyObservers();
}
public void setDataChanged(String title, String author, String content) {
this.title = title;
this.author = author;
this.content = content;
dataChanged();
}
public String getTitle() {
return title;
}
public String getAuthor() {
return author;
}
public String getContent() {
return content;
}
}
User1类：
package headfirst.designpatterns.observer.observablepull;
import java.util.Observable;
import java.util.Observer;
public class User1 implements Observer {
private String title;
private String author;
private String content;
private Observable weChatPublicAccount;
public User1(Observable weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.addObserver(this);
}
@Override
public String toString() {
return "User1{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
@Override
public void update(Observable o, Object arg) {
WeChatPublicAccount weChatPublicAccount = (WeChatPublicAccount) o;
title = weChatPublicAccount.getTitle();
author = weChatPublicAccount.getAuthor();
content = weChatPublicAccount.getContent();
}
}
User2 类：
package headfirst.designpatterns.observer.observablepull;
import java.util.Observable;
import java.util.Observer;
public class User2 implements Observer {
private String title;
private String author;
private String content;
private Observable weChatPublicAccount;
public User2(Observable weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.addObserver(this);
}
@Override
public String toString() {
return "User2{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
@Override
public void update(Observable o, Object arg) {
WeChatPublicAccount weChatPublicAccount = (WeChatPublicAccount) o;
title = weChatPublicAccount.getTitle();
author = weChatPublicAccount.getAuthor();
content = weChatPublicAccount.getContent();
}
}
User3类：
package headfirst.designpatterns.observer.observablepull;
import java.util.Observable;
import java.util.Observer;
public class User3 implements Observer {
private String title;
private String author;
private String content;
private Observable weChatPublicAccount;
public User3(Observable weChatPublicAccount) {
this.weChatPublicAccount = weChatPublicAccount;
weChatPublicAccount.addObserver(this);
}
@Override
public String toString() {
return "User3{" +
"title='" + title + '\'' +
", author='" + author + '\'' +
", content='" + content + '\'' +
'}';
}
@Override
public void update(Observable o, Object arg) {
WeChatPublicAccount weChatPublicAccount = (WeChatPublicAccount) o;
title = weChatPublicAccount.getTitle();
author = weChatPublicAccount.getAuthor();
content = weChatPublicAccount.getContent();
}
}
Test类：
package headfirst.designpatterns.observer.observablepull;
import java.util.Observer;
public class Test {
public static void main(String[] args) {
WeChatPublicAccount weChatPublicAccount = new WeChatPublicAccount();
Observer user1 = new User1(weChatPublicAccount);
Observer user2 = new User2(weChatPublicAccount);
Observer user3 = new User3(weChatPublicAccount);
System.out.println(user1.toString());
System.out.println(user2.toString());
System.out.println(user3.toString());
weChatPublicAccount.setDataChanged("午觉睡多久比较好","丁香医生","午觉睡10-20mins是极好的，一觉醒来元气满满");
System.out.println("丁香医生公众号数据更新后...");
System.out.println(user1.toString());
System.out.println(user2.toString());
System.out.println(user3.toString());
}
}
User1{title='null', author='null', content='null'}
User2{title='null', author='null', content='null'}
User3{title='null', author='null', content='null'}
丁香医生公众号数据更新后...
User1{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
User2{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
User3{title='午觉睡多久比较好', author='丁香医生', content='午觉睡10-20mins是极好的，一觉醒来元气满满'}
1.5 总结
观察者模式定义了对象之间的一对多依赖，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新，一个主题，多个观察者。
观察者提供了一种对象设计，让主题和观察者之间松耦合，主题只知道观察者实现了Observer 接口，而具体的观察者也仅依赖Subject 接口或Observable 超类实现注册。
要点：
- 主题用一个共同的接口来更新观察者；
- 主题与观察者间的松耦合设计，主题不知道观察者的细节，只知道观察者实现了观察者接口；
- 可使用
推和拉两种方式更新数据。
OO原则：
- 为交互对象之间的松耦合设计而努力。
GitHub 代码地址：https://github.com/SJMP1573/DesignPatterns
参考
[1] Freeman E. Head First 设计模式[M] 中国电力出版社.
[2] 菜鸟教程.