设计模式-观察者模式（Observer）

摘要: 观察者模式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上发生变化时，会通知所有观察者对象，使它们能够自动更新自己。

<p>Observer.java</p> <div id="scid:9D7513F9-C04C-4721-824A-2B34F0212519:613cf5c0-e32f-4bc0-9128-4562a774f1f0" class="wlWriterEditableSmartContent" style="float: none; padding-bottom: 0px; padding-top: 0px; padding-left: 0px; margin: 0px; display: inline; padding-right: 0px"><pre class="brush: java; gutter: true; first-line: 1; tab-size: 4; toolbar: true; width: 400px; height: 116px;" style=" width: 400px; height: 116px;overflow: auto;">package cn.foxeye.design.observer;

public interface Observer {

void update(String state);

}
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<p>Subject.java</p>

<div id="scid:9D7513F9-C04C-4721-824A-2B34F0212519:85481259-b3a5-4731-9e08-0edbd5847752" class="wlWriterEditableSmartContent" style="float: none; padding-bottom: 0px; padding-top: 0px; padding-left: 0px; margin: 0px; display: inline; padding-right: 0px"><pre class="brush: java; gutter: true; first-line: 1; tab-size: 4; toolbar: true; width: 602px; height: 418px;" style=" width: 602px; height: 418px;overflow: auto;">package cn.foxeye.design.observer;

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

public abstract class Subject {

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

public void attach(Observer observer) {
this.observers.add(observer);
System.out.println("Attached an Observer. ");
}

public void dettach(Observer observer) {
this.observers.remove(observer);
System.out.println("Remove an Observer.");
}

public void notifyObservers(String state) {
for (Observer observer : observers) {
observer.update(state);
}
}

}
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<p>ConcreteObserver.java</p>

<div id="scid:9D7513F9-C04C-4721-824A-2B34F0212519:20b56b5f-32bd-4aa2-a61c-cc433a457380" class="wlWriterEditableSmartContent" style="float: none; padding-bottom: 0px; padding-top: 0px; padding-left: 0px; margin: 0px; display: inline; padding-right: 0px"><pre class="brush: java; gutter: true; first-line: 1; tab-size: 4; toolbar: true; width: 605px; height: 540px;" style=" width: 605px; height: 540px;overflow: auto;">package cn.foxeye.design.observer;

public class ConcreteObserver implements Observer {

private String name;
private String state;

public ConcreteObserver(String name) {
this.name = name;
}

public void update(String state) {
setState(state);
System.out.println(name + "状态修改为：" + state);
}

public String getState() {
return state;
}

public void setState(String state) {
this.state = state;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

}
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<p>ConcreteSubject.java</p>

<div id="scid:9D7513F9-C04C-4721-824A-2B34F0212519:d7aa01ec-f20a-4b25-ae4b-0563db4042ea" class="wlWriterEditableSmartContent" style="float: none; padding-bottom: 0px; padding-top: 0px; padding-left: 0px; margin: 0px; display: inline; padding-right: 0px"><pre class="brush: java; gutter: true; first-line: 1; tab-size: 4; toolbar: true; width: 605px; height: 264px;" style=" width: 605px; height: 264px;overflow: auto;">package cn.foxeye.design.observer;

public class ConcreteSubject extends Subject {

private String state;

public String getState() {
return state;
}

public void changeState(String state) {
this.state = state;
System.out.println("主题状态变为：" + state);
this.notifyObservers(state);
}

}
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<p>ObserverDesignDemo.java</p>

<div id="scid:9D7513F9-C04C-4721-824A-2B34F0212519:c2553ba7-f9fb-4b0f-bc8a-6bc25875ff04" class="wlWriterEditableSmartContent" style="float: none; padding-bottom: 0px; padding-top: 0px; padding-left: 0px; margin: 0px; display: inline; padding-right: 0px"><pre class="brush: java; gutter: true; first-line: 1; tab-size: 4; toolbar: true; width: 628px; height: 309px;" style=" width: 628px; height: 309px;overflow: auto;">package cn.foxeye.design.observer;

public class ObserverDesignDemo {

public static void main(String[] args) {

ConcreteSubject subject = new ConcreteSubject();

for (int i = 0; i < 10; i++) {
Observer observer = new ConcreteObserver("张三" + i);
subject.attach(observer);
}

subject.changeState("已读");

}

}
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