观察者模式（Observer）（行为模式）

参考文档：

1.设计模式-可复用面向对象软件的基础

2.http://blog.youkuaiyun.com/zhshulin/article/details/38708351（设计模式——观察者模式（Observer） ）

观察者模式（Observer）（行为模式） 

详细的理论方面的知识，可以参考  参考文档。

看一下observer的结构：

协作方式：

如果，在实现是，需要封装复杂的更新语义，可以使用Mediator(5.5)模式，ChangeManager是一个Mediator(5.5)模式的实例。

代码实现，案例为书中讲的时钟的案例。

Subject.java:

package com.rick.designpattern.observer;

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

/**
 * Created by MyPC on 2017/6/19.
 */
public abstract class Subject {
    private List<Observer> observers = new ArrayList<Observer>();

    public void attach(Observer observer) {
        observers.add(observer);
    }

    public void detach(Observer observer) {
        observers.remove(observer);
    }

    public void Notify() {
        for (Observer os : observers) {
            os.update(this);
        }
    }
}

ClockTimer.java:

package com.rick.designpattern.observer;

import javax.xml.crypto.Data;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * Created by MyPC on 2017/6/19.
 */
public class ClockTimer extends Subject {
    private int hour, minute, second;

    public ClockTimer() {
    }

    public int getHour() {
        return hour;
    }

    public int getMinute() {
        return minute;
    }

    public int getSecond() {
        return second;
    }

    public void tick() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    Date date = new Date(System.currentTimeMillis());
                    hour = date.getHours();
                    minute = date.getMinutes();
                    second = date.getSeconds();
                    ClockTimer.this.Notify();
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }

            }
        }).run();
    }


}

Observer.java:

package com.rick.designpattern.observer;

/**
 * Created by MyPC on 2017/6/19.
 */
public abstract class Observer {
    public abstract void update(Subject subject);

}

AnalogClock.java:

package com.rick.designpattern.observer;


/**
 * Created by MyPC on 2017/6/19.
 */
public class AnalogClock extends Observer implements Widget {
    private ClockTimer subject;

    public AnalogClock(ClockTimer subject) {
        this.subject = subject;
        subject.attach(this);
    }

    public void detachSubject() {
        subject.detach(this);
    }

    @Override
    public void draw() {
        System.out.println("AnalogClock：当前时间：" + subject.getHour() + ":" + subject.getMinute() + ":" + subject.getSecond());
    }

    @Override
    public void update(Subject subject) {
        if (subject instanceof ClockTimer) {
            draw();
        }
    }
}

DigitalClock.java:

package com.rick.designpattern.observer;


/**
 * Created by MyPC on 2017/6/19.
 */
public class DigitalClock extends Observer implements Widget {
    private ClockTimer subject;

    public DigitalClock(ClockTimer subject) {
        this.subject = subject;
        subject.attach(this);
    }

    public void detachSubject() {
        subject.detach(this);
    }

    @Override
    public void draw() {
        System.out.println("DigitalClock：当前时间：" + subject.getHour() + ":" + subject.getMinute() + ":" + subject.getSecond());
    }

    @Override
    public void update(Subject subject) {
        if (subject instanceof ClockTimer) {
            draw();
        }
    }
}

Widget.java:

package com.rick.designpattern.observer;

/**
 * Created by MyPC on 2017/6/19.
 */
public interface Widget {


    public void draw();

}

Client.java:

package com.rick.designpattern.observer;

/**
 * Created by MyPC on 2017/6/19.
 */
public class Client {


    public static void main(String[] args) {
        ClockTimer clockTimer = new ClockTimer();
        DigitalClock digitalClock = new DigitalClock(clockTimer);
        AnalogClock analogClock = new AnalogClock(clockTimer);
        clockTimer.tick();
    }
}