设计模式——观察者模式(ObserverPattern)

观察者模式(ObserverPattern):观察者模式又称发布-订阅(Publish/Subscribe)模式，定义了一个中一对多的依赖关系，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。

UML图：

Subject：

package com.cnblog.clarck;

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

/**
 * 主题/抽象通知者
 * 
 * @author clarck
 * 
 */
public class Subject {
    private List<Observer> observers = new ArrayList<Observer>();

    /**
     * 添加观察者
     * 
     * @param observer
     */
    public void attach(Observer observer) {
        observers.add(observer);
    }

    /**
     * 移除观察者
     * 
     * @param observer
     */
    public void detach(Observer observer) {
        observers.remove(observer);
    }

    /**
     * 通知
     */
    public void notification() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

Observer：

package com.cnblog.clarck;

/**
 * 抽象观察者
 * 
 * @author clarck
 * 
 */
public abstract class Observer {
    public abstract void update();
}

ConcrateSubject：

package com.cnblog.clarck;

/**
 * 具体主题/具体通知者
 * 
 * @author clarck
 * 
 */
public class ConcrateSubject extends Subject {
    private String mSubjectState;

    public String getSubjectState() {
        return mSubjectState;
    }

    public void setSubjectState(String mSubjectState) {
        this.mSubjectState = mSubjectState;
    }

}

ConcrateObserver：

package com.cnblog.clarck;

/**
 * 具体观察者
 * 
 * @author clarck
 * 
 */
public class ConcrateObserver extends Observer {
    private String mName;
    private String mObserverState;
    private ConcrateSubject mSubject;

    public ConcrateObserver(ConcrateSubject subject, String name) {
        mSubject = subject;
        mName = name;
    }

    @Override
    public void update() {
        mObserverState = mSubject.getSubjectState();
        String format = String.format("观察者%s的新状态是%s", mName, mObserverState);
        System.out.println(format);
    }

    public ConcrateSubject getmSubject() {
        return mSubject;
    }

    public void setmSubject(ConcrateSubject mSubject) {
        this.mSubject = mSubject;
    }

}

Client：

package com.cnblog.clarck;

public class Client {
    public static void main(String[] args) {
        ConcrateSubject concrateSubject = new ConcrateSubject();
        concrateSubject.attach(new ConcrateObserver(concrateSubject, "X"));
        concrateSubject.attach(new ConcrateObserver(concrateSubject, "Y"));
        concrateSubject.attach(new ConcrateObserver(concrateSubject, "Z"));
        
        concrateSubject.setSubjectState("ABC");
        concrateSubject.notification();
    }
}