观察者模式

1. 观察者模式

观察者模式就是定义对象间一种一对多的依赖关系，当一个对象状态发生变化时，所有依赖于它的对象都得到通知并被自动跟新

使用Java提供的接口可以方便的实现观察者模式

2. 股票的例子

public interface StockObserver{
	void updatePrice(String name);
	void updateCount(String name);
}

3. 电脑客户端

public class ComputerClient implements StockObserver {
	public void updatePrice(String name)
	{....}
	public void updateCount(string name)
	{....}
}

4. 手机客户端

public class PhoneClient implements StockObserver {
	public void updatePrice(String name)
	{....}
	public void updateCount(string name)
	{....}
}

5.定义一个股票的抽象类，用于动态，随意的添加删除股票客户端

public abstract class StockSubject{
	List list = new ArrayList();
	private String name;
	public void changeCount(){
		notifyCount();
	}
	public void changePrice(){
		notifyPrice();
	}
	public void add(StockObserver stockObserver){
		list.add(stockObserver);
	}
	public void remove(StockObserver stockObserver){
		list.remove(stockObserver);
	}
	public void notifyPrice(){
		for (int i = 0; i < list.size(); i++){
			StockObserver stockObserver = (StockObserver)list.get(i);
			stockObserver.updatePrice(getName());
		}
	}
	public void notifyCount(){
		for (int i = 0; i < list.size(); i++){
			StockObserver stockObserver = (StockObserver)list.get(i);
			stockObserver.updateCount(getName());
		}
	}
	
	private String getName(){ return name; }
	public void setName(String name) { this.name = name; }
}

6. 定义一个具体的股票类，在该股票价格和买卖数量发生变化时通知两个客户端

public class Stock extends StockSubject {
	.......		//其它Stock类的属性和方法
	public void changeCount(){
		setName("wanda");
		super.changeCount();
	}
	public void changePrice(){
		setName("wanda");
		super.changePrice();
	}
}

客户端调用示意代码

public static void main(String[] argv) [
	Stock stock = new Stock();
	stock.add(new PhoneClient());
	//stock.add(new ComputerClient());
	stock.changePrice();
	stock.changeCount():
}

7. JAVA 已经为观察者模式提供了接口
定义两个观察者，实现Observer接口

import java.util.Observable;
import java.util.Observer;
public class ComputerClient implements Observer{
	public void update(Observerable obj, Object arg){
		String name = (String)arg;
		.....
	}
}

import java.util.Observable;
import java.util.Observer;
public class PhoneClient implements Observer{
	public void update(Observerable obj, Object arg){
		String name = (String)arg;
		.....
	}
}

定义被观察者， 实现Observable接口

public class Stock extends Observable{
	public void changeCountandPrice(){
		setChanged();
		notifyObservers("wanda");
	}
}

客户端调用示意代码

public static void main(String[] argv) {
	Stock stock = new Stock();
	stock.addObserver(new PhoneClient);
	stock.addObserver(new ComputerClient);
	stock.changeCountandPrice();
}