设计模式--观察者模式“拉”数据

 

• 前面讲了观察者模式的“推”数据的方式，接下来时观察者模式的“拉”数据方式。

拉数据方式是指具体主题不将变化后的数据交给具体观察者，而是提供获得这些数据的方法，具体主题在得到通知后，调用具体主题的方法更新自己的数据（即观察者把数据拉过来）。

 

例子，一家商店发布打折商品信息，包括商品的价格、折后价、名称。有两位顾客，一位关注打折的商品，另一位关心商品折后价格。

 

/**
 * 
 */
package org.rico.pattern.demo.observer2;

/**
 * @author rico 2013-3-3
 *  主题：定义具体主题需要实现的方法，如：添加、删除、通知观察者的方法。
 */
public interface Subject {
	
	/**
	 * 新增观察者
	 * @param observer
	 */
	public void add(Observer observer);
	
	/**
	 * 删除观察者
	 * @param observer
	 */
	public void delete(Observer observer);
	
	/**
	 * 主题通知观察者更新数据
	 */
	public void notifyObserver();
	
}

 

/**
 * 
 */
package org.rico.pattern.demo.observer2.impl;

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

import org.rico.pattern.demo.observer2.Observer;
import org.rico.pattern.demo.observer2.Subject;

/**
 * @author rico 2013-3-3
 * 
 */
public class ShopSubject implements Subject {
	private List<Observer> observers = null;
	private int oldPrice, newPrice;
	private String goodName;
	
	public ShopSubject() {
		observers = new ArrayList<Observer>();
	}

	/* (non-Javadoc)
	 * @see org.rico.pattern.demo.observer.Subject#add(org.rico.pattern.demo.observer.Observer)
	 */
	@Override
	public void add(Observer observer) {
		if(!observers.contains(observer)) {
			observers.add(observer);
		}
	}

	/* (non-Javadoc)
	 * @see org.rico.pattern.demo.observer.Subject#delete(org.rico.pattern.demo.observer.Observer)
	 */
	@Override
	public void delete(Observer observer) {
		if(observers.contains(observer)) {
			observers.remove(observer);
		}
	}

	/* (non-Javadoc)
	 * @see org.rico.pattern.demo.observer.Subject#notifyObserver()
	 */
	@Override
	public void notifyObserver() {
		for(Observer ob : observers) {
			ob.update();
		}
	}
	
	//设置商品信息
	public void setGoods(String goodName, int oldPrice, int newPrice) {
		this.setGoodName(goodName);
		this.setOldPrice(oldPrice);
		this.setNewPrice(newPrice);
		
		this.notifyObserver();
	}

	public List<Observer> getObservers() {
		return observers;
	}

	public void setObservers(List<Observer> observers) {
		this.observers = observers;
	}

	public int getOldPrice() {
		return oldPrice;
	}

	public void setOldPrice(int oldPrice) {
		this.oldPrice = oldPrice;
	}

	public int getNewPrice() {
		return newPrice;
	}

	public void setNewPrice(int newPrice) {
		this.newPrice = newPrice;
	}

	public String getGoodName() {
		return goodName;
	}

	public void setGoodName(String goodName) {
		this.goodName = goodName;
	}

}

 

/**
 * 
 */
package org.rico.pattern.demo.observer2;

/**
 * @author rico 2013-3-4
 * 观察者
 */
public interface Observer {
	public void update();
}

 

/**
 * 
 */
package org.rico.pattern.demo.observer2.impl;

import org.rico.pattern.demo.observer2.Observer;
import org.rico.pattern.demo.observer2.Subject;

/**
 * @author rico 2013-3-4
 * 顾客（具体观察者）
 */
public class CustomerOne implements Observer {
	private Subject subject;
	private String name;
	
	public CustomerOne(Subject subject, String name) {
		this.subject = subject;
		this.name = name;
		subject.add(this);
	}

	/* 顾客从商店“拉去”数据，更新自己的数据(non-Javadoc)
	 * @see org.rico.pattern.demo.observer2.Observer#update()
	 */
	@Override
	public void update() {
		if(subject instanceof ShopSubject) {
			ShopSubject su  = (ShopSubject) subject;
			System.out.println("顾客：" + name + ",");
			System.out.println("只对商品名称感兴趣，");
			System.out.println("商品名称：" + su.getGoodName());
		} else {
			System.out.println("获取到任何数据...");
		}
	}
}

 

/**
 * 
 */
package org.rico.pattern.demo.observer2.impl;

import org.rico.pattern.demo.observer2.Observer;
import org.rico.pattern.demo.observer2.Subject;

/**
 * @author rico 2013-3-4
 * 顾客（具体观察者）
 */
public class CustomerTwo implements Observer {
	private Subject subject;
	private String name;
	
	public CustomerTwo(Subject subject, String name) {
		this.subject = subject;
		this.name = name;
		subject.add(this);
	}

	/* 顾客从商店“拉去”数据，更新自己的数据(non-Javadoc)
	 * @see org.rico.pattern.demo.observer2.Observer#update()
	 */
	@Override
	public void update() {
		if(subject instanceof ShopSubject) {
			ShopSubject su  = (ShopSubject) subject;
			System.out.println("顾客：" + name + ",");
			System.out.println("只对商品价格感兴趣，");
			System.out.println("商品价格：" + su.getNewPrice() + "-" + su.getOldPrice());
		} else {
			System.out.println("获取到任何数据...");
		}
	}
}

 

/**
 * 
 */
package org.rico.pattern.demo.observer2;

import org.rico.pattern.demo.observer2.impl.CustomerOne;
import org.rico.pattern.demo.observer2.impl.CustomerTwo;
import org.rico.pattern.demo.observer2.impl.ShopSubject;

/**
 * @author rico 2013-3-3
 * 
 */
public class ObserverTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		ShopSubject subject = new ShopSubject();
		Observer university = new CustomerOne(subject, "rico");
		Observer haigui = new CustomerTwo(subject, "admin");
		
		System.out.println("###################感冒999#######################");
		subject.setGoods("感冒999", 10, 8);
		
		System.out.println("###################可口可乐#######################");
		subject.setGoods("可口可乐", 6, 5);
		
	}
}