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java设计模式---观察者模式

2013-07-18 16:20 375 查看
在一对多依赖的对象关系中, 如果这个'一'对象状态发生了变化,那么它所有依赖的'多'对象都应该被通知,然后做相应的变化,这就是观察者模式. 就如同'多'对象一直在观察'一'对象的状态变化一样.

在观察者模式中最重要的俩个对象分别是:Observable和Observer对象.它们的关系可总结如下:

1. Observable和Observer对象是一对多的关系,也就是说一旦Observable对象状态变化,它就要负责通知所有和它有关系的Observer对象,然后做相应的改变.

2. Observable对象不会主动去通知各个具体的Observer对象其状态发生了变化,而是提供一个注册接口供Observer对象使用,任何一个Observer对象如果想要被通知,则可以使用这个接口来注册.

3. 在Observable中有一个集合和一个状态控制开关,所有注册了通知的Observer对象会被保存在这个集合中.这个控制开关就是用来控制Observable是否发生了变化,一旦发生了变化,就通知所有的Observer对象更新状态.

在java api中分别提供了Observable对象:java.util.Observable和Observer接口:java.util.Observer. 下面用实例来实现一下观察者模式: 股票系统

所有的类如下:

StockData (Observable对象,也就是所股票数据发生了变化,它就要通知所有和它有关系的交易实体做相应的变化)

BigBuyer (Observer对象, 实现了Observer接口)

TradingFool (Observer对象, 实现了Observer接口)

StockQuote 测试类

在这个例子中一旦StockData对象的状态发生了变化,那BigBuyer和TradingFool都应该受到通知:

StockData.java:

import java.util.Observable;

public class StockData extends Observable
{
private String symbol;
private float close;
private float high;
private float low;
private long volume;

public StockData()
{}

public String getSymbol()
{
return symbol;
}

public float getClose()
{
return close;
}

public float getHigh()
{
return high;
}

public float getLow()
{
return low;
}

public long getVolume()
{
return volume;
}

public void sendStockData()
{
setChanged();
notifyObservers();
}

public void setStockData(String symbol,float close,float high,float low,long volume)
{
this.symbol = symbol;
this.close = close;
this.high = high;
this.low = low;
this.volume = volume;
sendStockData();
}
}

BigBuyer.java:

public class BigBuyer implements Observer
{
private String symbol;
private float close;
private float high;
private float low;
private long volume;

public BigBuyer(Observable observable)
{
observable.addObserver(this); //注册关系
}

public void update(Observable observable,Object args)
{
if(observable instanceof StockData)
{
StockData stockData = (StockData)observable;
this.symbol = stockData.getSymbol();
this.close = stockData.getClose();
this.high = stockData.getHigh();
this.low = stockData.getLow();
this.volume = stockData.getVolume();
display();
}
}

public void display()
{
DecimalFormatSymbols dfs = new DecimalFormatSymbols();
DecimalFormat volumeFormat = new DecimalFormat("###,###,###,###",dfs);
DecimalFormat priceFormat = new DecimalFormat("###.00",dfs);
System.out.println("Big Buyer reports... ");
System.out.println("\tThe lastest stock quote for " + symbol + " is:");
System.out.println("\t$" + priceFormat.format(close) + " per share (close).");
System.out.println("\t$" + priceFormat.format(high) + " per share (high).");
System.out.println("\t$" + priceFormat.format(low) + " per share (low).");
System.out.println("\t" + volumeFormat.format(volume) + " shares traded.");
System.out.println();
}
}

TradingFool.java:

public class TradingFool implements Observer
{
private String symbol;
private float close;

public TradingFool(Observable observable)
{
observable.addObserver(this);//注册关系
}

public void update(Observable observable,Object args)
{
if(observable instanceof StockData)
{
StockData stockData = (StockData)observable;
this.symbol = stockData.getSymbol();
this.close = stockData.getClose();
display();
}
}

public void display()
{
DecimalFormatSymbols dfs = new DecimalFormatSymbols();
DecimalFormat priceFormat = new DecimalFormat("###.00",dfs);
System.out.println("Trading Fool says... ");
System.out.println("\t" + symbol + " is currently trading at $" + priceFormat.format(close) + " per share.");
System.out.println();
}
}

StokeQuote.java

public class StockQuotes
{
public static void main(String[] args)
{
System.out.println();
System.out.println("-- Stock Quote Application --");
System.out.println();

StockData stockData = new StockData();

// register observers...
new TradingFool(stockData);
new BigBuyer(stockData);

// generate changes to stock data...
stockData.setStockData("JUPM",16.10f,16.15f,15.34f,(long)481172);
stockData.setStockData("SUNW",4.84f,4.90f,4.79f,(long)68870233);
stockData.setStockData("MSFT",23.17f,23.37f,23.05f,(long)75091400);
}
}

在测试类中我们可以看到俩个Observer对象都注册了Observable对象,而当Observable对象发生改变时,这俩个Observable对象就会做相应的更新了, 运行结果如下:

Big Buyer reports...
The lastest stock quote for JUPM is:
$16.10 per share (close).
$16.15 per share (high).
$15.34 per share (low).
481,172 shares traded.

Trading Fool says...
JUPM is currently trading at $16.10 per share.

Big Buyer reports...
The lastest stock quote for SUNW is:
$4.84 per share (close).
$4.90 per share (high).
$4.79 per share (low).
68,870,233 shares traded.

Trading Fool says...
SUNW is currently trading at $4.84 per share.

Big Buyer reports...
The lastest stock quote for MSFT is:
$23.17 per share (close).
$23.37 per share (high).
$23.05 per share (low).
75,091,400 shares traded.

Trading Fool says...
MSFT is currently trading at $23.17 per share.

我们通过Observable源码可以看到,其实Observable对象不关心具体的Observer的实例类型. 只要是实现了Observer接口的Observer对象都可以得到通知,这就为我们如果想要对模型进行扩展提供了方便,使Observable对象和Observer对象实现了松耦合. 如果我们需要添加一个新的Observer对象时,我们只要注册一下,当Observable对象发生变化时就可以得到通知,而不要做其他任何改变,非常方便.
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