菜鸟硕枫设计模式系列之20 观察者模式

观察者模式是设计模式中非常重要的设计模式之一。主要特点是，向要获得信息的对象在观察平台注册，然后当感兴趣的事情发生时，平台通知每一个注册的观察者，观察者根据发生的事情来做出自己的行为。其中要点在于观察者要能方便注册，方便解除注册，观察平台在事情发生时通知他们。消息的发布 订阅传输方式就是采用观察者模式这种机理的。

观察者模式类图：
[img]http://dl.iteye.com/upload/attachment/364911/da16f36c-c9ac-3876-b121-d0f893b925dc.jpg[/img]


具体代码示例：
观察者接口

package oberserverPattern;

public interface Obersever {

	void changeState(int tag);
}

具体实现对象类：

package oberserverPattern;

public class Person implements Obersever {

    private String name;

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

    public String getName() {
        return name;
    }

    public void changeState(int tag){
        if(tag==1)
            System.out.println(this.name
                    +" changes his state from playing to hardworking");
        if(tag==2)
            System.out.println(this.name
                    +" changes his state from hardworking to playing");
    }
}

package oberserverPattern;

import java.util.List;

public interface Subject {

	void notify(Obersever obersever,int tag);
}

package oberserverPattern;

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

public class ConcreteSubject implements Subject{

	private  List<Obersever> obersevers= null;

	public void notify(List<Obersever> obersevers,int tag) {
		Iterator<Obersever> it = obersevers.iterator();
		while(it.hasNext()){
			notify(it.next(),tag);
		}

	}

	@Override
	public void notify(Obersever obersever,int tag) {

		obersever.changeState(tag);
	}

	public  List<Obersever> getList(){
		if (obersevers==null)
			return new ArrayList<Obersever>();
		else 
			return obersevers;
	}
}

package oberserverPattern;

public class Coder extends Person{

}

package oberserverPattern;

public class Boss extends Person{

	private int tag;

	public void changeState(int tag){
		if(tag==1)
			System.out.println(this.getName() 
					+"is coming");
		if(tag==2)
			System.out.println(this.getName() 
					+"has gone.");
	}

	public void setTag(int tag) {
		this.tag = tag;
	}
	public int getTag() {
		return tag;
	}
}

测试类：

package oberserverPattern;

public class OberseverTest {

	public static void main(String[] args){
		Boss boss = new Boss();
		boss.setName("boss wang");
		boss.setTag(1);

		Coder coder1 = new Coder();
		coder1.setName("lxy");

		Coder coder2 = new Coder();
		coder2.setName("shuofeng");

		ConcreteSubject subject = new ConcreteSubject();
		subject.getList().add(coder1);
		subject.getList().add(coder1);
		boss.changeState(boss.getTag());
		subject.notify(subject.getList(), boss.getTag());
		coder1.changeState(boss.getTag());
		coder2.changeState(boss.getTag());

		System.out.println();
		System.out.println("---------------Changer boss's state from 1 to 2-----------");
		System.out.println();
		boss.setTag(2);
		boss.changeState(boss.getTag());
		subject.notify(subject.getList(), boss.getTag());
		coder1.changeState(boss.getTag());
		coder2.changeState(boss.getTag());
	}
}

说明：观察者模式在消息订阅等其他场合应用广泛，是一种重要的设计模式。要点在于有存储订阅者的聚集，有触发通知的时间，有添加订阅，取消订阅的功能方法。优点在于只有当订阅者期望的事情发生时，订阅者才会被通知，从而完成订阅者期望的事件。