📄 templatemethod.htm
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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: Template Method 模式</a></h1>
不要将设计模式想得高不可攀，好像高手才会使用的东西，事实上如果您在下手程式之前，能稍稍对程式作个分析规划，或多或少都会用到一些模式了，模式不是教条，它只是前人的经验成果，而 Gof 的书则是择前人之精华持续改进而来罢了。<br>
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Template Method模式就是一个很简单的模式，但可能是使用最广泛的模式，也许您也一直在使用这样的模式，看它的 UML 类别结构图就知道了：<br>
<div style="text-align: center;"><img style="width: 338px; height: 205px;" alt="TemplateMethod" title="TemplateMethod" src="images/templateMethod-1.jpg"><br>
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仅仅是抽象类别与具体类别实作的关系而已，有些人常问抽象类别与介面的区别为何，Template Method模式可以提供其中一个答案，例如：<br>
<ul>
<li>AbstractClass.java</li>
</ul>
<pre>public abstract class AbstractClass { <br> public void templateMethod() { <br> // step by step template to solve something <br> // implementor should follow those step <br> opStep1(); <br> opStep2(); <br> opStep3(); <br> } <br><br> public abstract void opStep1(); <br> public abstract void opStep2(); <br> public abstract void opStep3(); <br>} <br></pre>
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<ul>
<li>ConcreteClass.java</li>
</ul>
<pre>public class ConcreteClass extends AbstractClass { <br> public abstract void opStep1() { <br> // implement the real operation <br> } <br><br> public abstract void opStep2() { <br> // implement the real operation <br> } <br><br> public abstract void opStep3() { <br> // implement the real operation <br> } <br>}</pre>
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对于一些程式而言，我们希望规定一些处理的步骤、流程或骨架，就像是上例中的step1到step3一样，至于流程中的step1到step3如何实作并不规定，而留给实作的人自行决定，这就是Template Method模式的目的。<br>
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抽象类别与介面的差别之一，也正在于抽象类别可以先实作其中一些方法，而介面则是完全仅规定接口，使用Template Method模式就可以看出两者之间在应用上的一个差别。<br>
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仅以step1到step3这样的操作来看Template Method模式，似乎彰显示不出其实作骨架，而将实作部份留待子类的实用性，在 Gof
书中所举的例子是与 Factory Method 模式
结合的一个例子；通常开启一个档案的流程是相似的，例如文字档或二进位档，不外乎检查档案是否可开启、读取档案、设定显示等流程，可以使用
Template Method模式来规范这个流程： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public abstract class Application { </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 openDocument(String name) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // Template Method </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> if(!canOpenDocument(name)) { // unable to open file </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // show error message, throw exception </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> return; </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;"> Document doc = createDocument(); // Factory Method </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;"> if(doc != null) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> _docs.addDocument(doc); </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // Template Method </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> aboutToOpenDocument(doc); </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> doc.open(); </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> doc.doRead(); </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;"> // Factory Method </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public abstract Document createDocument(); </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;"> // Template Method </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public abstract boolean canOpenDocument(String name); </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public abstract void aboutToOpenDocument(Document doc); </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;"> public class MyApplication extends Application { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // implement Factory Method </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void Document createDocument() { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> return new MyDocument(); </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;"> // implement Template Method </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> public void boolean canOpenDocument(String name) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // implemented code here </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 aboutToOpenDocument(Document doc) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> // implemented code here </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>
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<a href="FactoryMethod.htm">Factyro Method模式</a>将实际要创建的物件推迟至子类中决定，而
Template
Method模式则是将流程框架的实作留待子类来解决，事实上在这个例子中，您也可以将createDocument()看作是Template
Method模式中的一个方法，从物件创建的角度来看它是Factory Method，而从流程框架的角度来看，它则是Template
Method模式的一个方法实作。<br>
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