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<h3><a href="http://caterpillar.onlyfun.net/GossipCN/index.html">From
Gossip@caterpillar</a></h3>
<h1><a href="CppGossip.html">Design Pattern: Decorator 模式</a></h1>
在Java
Swing中的JTextArea元件預(yù)設(shè)并沒(méi)有卷軸，因?yàn)樵O(shè)計(jì)人員認(rèn)為卷軸的功能并不是一定需要的，而決定讓程式人員可以動(dòng)態(tài)選擇是否增加卷軸功能，卷
軸的功能是由JScrollPane元件提供，如果您要加入一個(gè)具有卷軸功能的JTextArea，您可以如下進(jìn)行設(shè)計(jì)：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">JTextArea textArea = new JTextArea();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">JScrollPane scrollPane = new JScrollPane(textArea);</span><br>
</div>
<br>
JScrollPane對(duì)JTextArea即是個(gè)容器，而它對(duì)JFrame來(lái)說(shuō)又是個(gè)元件，可以如下這般將之加入JFrame中：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">getContentPane().add(scrollPane);</span><br>
</div>
<br>
像這樣動(dòng)態(tài)的為JTextArea加入功能的方法，我們可以使用Decorator模式來(lái)組織結(jié)構(gòu)，您可以動(dòng)態(tài)的為一個(gè)物件加入一些功能（像是為
JTextArea加上卷軸），而又不用修改JTextArea的功能。對(duì)JTextArea來(lái)說(shuō)，JScrollPane就好像是一個(gè)卷軸外框，直接套
在JTextArea上作裝飾，就好比您在照片上加上一個(gè)相框的意思。<br>
<br>
先以上面這個(gè)例子來(lái)說(shuō)明Decorator模式的一個(gè)實(shí)例：<br>
<div style="text-align: center;"><img style="width: 455px; height: 255px;" alt="Decorator" title="Decorator" src="images/decorator-1.jpg"><br>
</div>
<br>
如上圖所示的，無(wú)論是TextView或是Decorator類別，它們都是VisualComponent的一個(gè)子類，也就是說(shuō)它們都是一個(gè)可視元件，
而Decorator類又聚合了VisualComponent，所以又可以當(dāng)作TextView容器，ScrollDecorator類別實(shí)作了
Decorator類，它可能是這樣設(shè)計(jì)的：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">public abstract class Decorator extends VisualComponent {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> protected VisualComponent component;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public Decorator(VisualComponent component) {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> this.component = component;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void draw() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> component.draw();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">}</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">public class ScrollDecorator extends Decorator {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public ScrollDecorator(VisualComponent component) {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> super(component);</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void draw() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> super.draw();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> scrollTo();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void scrollTo() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // ....</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">}</span><br>
</div>
<br>
要將新功能套用至TextView上，可以這樣設(shè)計(jì)：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">ScrollDecorator scrollDecorator = </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
new ScrollDecorator(new TextView());</span><br>
</div>
<br>
super.draw()會(huì)先呼叫component也就是TextView物件的draw()方法先繪制TextView，然后再進(jìn)行
ScrollPanel的scrollTo()，也就是卷動(dòng)的方法。在圖中也表示了一個(gè)BorderDecorator，它可能是這樣設(shè)計(jì)的：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">public class BorderDecorator extends Decorator {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public BorderDecorator(VisualComponent component) {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> super(component);</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void draw() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> super.draw();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> drawBorder();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void drawBorder() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // ....</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">}</span><br>
</div>
<br>
要將ScrollDecorator與BorderDecorator加至TextView上，我們可以這樣設(shè)計(jì)：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">BorderDecorator borderDecorator =</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> new BorderDecorator(</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> new ScrollDecorator(new TextView()));</span><span style="font-weight: bold; font-family: Courier New,Courier,monospace;"></span><br>
</div>
<br>
所以當(dāng)BorderDecorator調(diào)用draw()方法時(shí)，它會(huì)先調(diào)用ScrollDecorator的draw()方法，而 ScrollDecorator的draw()方法又會(huì)先調(diào)用TextView的draw()方法，所以繪制的順序變成：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">TextDraw.draw();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">ScrollDecorator.scrollTo();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">BorderDecorator.drawBorder();</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"></span><br>
</div>
下圖為物件之間的調(diào)用關(guān)系：<br>
<div style="text-align: center;"><img style="width: 403px; height: 71px;" alt="Decorator" title="Decorator" src="images/decorator-2.jpg"><br>
</div>
<br>
Decorator模式的 UML 結(jié)構(gòu)圖如下所示：<br>
<div style="text-align: center;"><img style="width: 486px; height: 259px;" alt="Decorator" title="Decorator" src="images/decorator-3.jpg"><br>
</div>
<br>
在Gof的書(shū)中指出另一個(gè)范例，它設(shè)計(jì)一個(gè)Stream抽象類，而有一個(gè)StreamDecorator類，Stream的子類有處理記憶體串流的
MemoryStream與FileStream，有各種方法可以處理串流，也許只是單純的處理字元，也許會(huì)進(jìn)行壓縮，也許會(huì)進(jìn)行字元轉(zhuǎn)換，最基本的處理
可能是處理字元，而字元壓縮被視為額外的功能，這個(gè)時(shí)候我們可以使用裝飾模式，在需要的時(shí)候?yàn)镾tream物件加上必要的功能，事實(shí)上在java.io中
的許多輸入輸出物件，就是采取這樣的設(shè)計(jì)。<br>
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