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# Extract Composite Talk Andy

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### Extract Composite Talk Andy

1. 1. Andy Bulka Technical Director Austhink Software www.austhink.com March 2008 Refactoring to patterns…
2. 2. Simple Composite pattern <ul><li>Class with a “children” collection </li></ul><ul><li>Children are of type Node (i.e. you point to yourself) </li></ul>
3. 3. Classic Composite pattern (GOF) <ul><li>Distinguish between Composites and Leaves </li></ul><ul><li>Benefit of “classic” is that you can distinguish leafs and override display() or doubleClick() or whatever for leafs. </li></ul>
4. 4. The Problem Code <ul><li>We have two classes that loop through their children – duplicate code </li></ul><ul><li>The children attribute is named differently in both classes – inconsistent </li></ul><ul><li>Both classes are “composites” </li></ul>
5. 5. The Solution <ul><li>Create a common base class which has the generic “composite” looping behaviour </li></ul><ul><li>That’s where the name of the refactoring comes from “Extract Composite” </li></ul>
6. 6. Original Problem code class Node (object): def __init__(self, name): self.name = name def toPlainTextString (self): return self.name class FormTag (Node): def __init__(self): self.allNodesVector = [] def toPlainTextString (self): result = &quot;&quot; for node in self.allNodesVector: result += node.toPlainTextString() return result class LinkTag (Node): def __init__(self): self.linkData = [] def toPlainTextString (self): result = &quot;&quot; for node in self.linkData: result += node.toPlainTextString() return result f = FormTag() f.allNodesVector.append(Node(&quot;a&quot;)) f.allNodesVector.append(Node(&quot;b&quot;)) f.allNodesVector.append(Node(&quot;c&quot;)) l = LinkTag() l.linkData += [Node(&quot;x&quot;), Node(&quot;y&quot;), Node(&quot;z&quot;)] print f.toPlainTextString() print l.toPlainTextString() abc xyz output
7. 7. Step 1 <ul><li># Create a Composite Class - compile </li></ul><ul><li>class Composite(Node): </li></ul><ul><li>pass </li></ul>
8. 8. Step 2 <ul><li>Make each child container (a class in the hierarchy that </li></ul><ul><li>contains duplicate child-handling code) a subclass of </li></ul><ul><li>your composite – compile </li></ul><ul><li>class Composite(Node): </li></ul><ul><li>pass </li></ul><ul><li>class FormTag(Composite): </li></ul><ul><li>… . </li></ul><ul><li>class LinkTag(Composite): </li></ul><ul><li>… . </li></ul>
9. 9. Step 3 <ul><li>For each method with duplicated looping code </li></ul><ul><li>1. move & rename the child reference field UP </li></ul><ul><li>to the composite using &quot;Pull Up Field&quot; </li></ul><ul><li>2. Move the method UP to the composite using &quot;Pull Up Method&quot; </li></ul><ul><li>3. Pull up any relevant constructor code too. </li></ul>
10. 10. Everything moved to Composite <ul><li>class Composite (Node): </li></ul><ul><li>def __init__(self): </li></ul><ul><li>self.children = [] </li></ul><ul><li>def toPlainTextString(self): </li></ul><ul><li>result = &quot;&quot; </li></ul><ul><li>for node in self.children: </li></ul><ul><li>result += node.toPlainTextString() </li></ul><ul><li>return result </li></ul><ul><li>class FormTag(Composite): </li></ul><ul><li>pass </li></ul><ul><li>class LinkTag(Composite): </li></ul><ul><li>pass </li></ul>
11. 11. Step 4 <ul><li>Check interfaces so that client code using the old composites still works. </li></ul>f = FormTag() f. children .append(Node(&quot;a&quot;)) f. children .append(Node(&quot;b&quot;)) f. children .append(Node(&quot;c&quot;)) l = LinkTag() l. children += [Node(&quot;x&quot;), Node(&quot;y&quot;), Node(&quot;z&quot;)] print f.toPlainTextString() print l.toPlainTextString() f = FormTag() f. allNodesVector .append(Node(&quot;a&quot;)) f. allNodesVector .append(Node(&quot;b&quot;)) f. allNodesVector .append(Node(&quot;c&quot;)) l = LinkTag() l. linkData += [Node(&quot;x&quot;), Node(&quot;y&quot;), Node(&quot;z&quot;)] print f.toPlainTextString() print l.toPlainTextString()
12. 12. New UML <ul><li>We have inserted a new class called Composite above the LinkTag and FormTag, which does the common looping work. </li></ul>
13. 13. Before and After