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Case Study of Writing the Extensible Visitor Pattern

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Do you wonder whether knowledge of proper design patterns makes you good API designer? Yes and no. Of course knowing regular design patterns simplifies communication, understanding, etc. However there are hidden catches. Not every design pattern belongs among [[APIDesignPatterns]] - it may not be ready for evolution. For example the well known [[wikipedia::Visitor_pattern|Visitor pattern]] is really not evolvable easily as analysed by [[Case Study of Writing the Extensible Visitor Pattern|Chapter 18]].
Do you wonder whether knowledge of proper design patterns makes you good API designer? Yes and no. Of course knowing regular design patterns simplifies communication, understanding, etc. However there are hidden catches. Not every design pattern belongs among [[APIDesignPatterns]] - it may not be ready for evolution. For example the well known [[wikipedia::Visitor_pattern|Visitor pattern]] is really not evolvable easily as analysed by [[Case Study of Writing the Extensible Visitor Pattern|Chapter 18]].
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== Tripple Dispatch ==
 
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The [[Case Study of Writing the Extensible Visitor Pattern|Chapter 18]] discusses various approaches to implement [[visitor]] in an evolvable [[API]]. The learning path itself is important, but to stress the important point, here is the code for the final solution of the [[expression problem]]:
 
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<source lang="java" snippet="visitor.clientprovider.v1"/>
 
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The solution is using [[Java]] interfaces to represent expression elements and yet it is fully evolvable (one can always define new expression element interface). [[Visitor]] is not just a single class, but one [[Java]] interface and one [[Java]] final class. [[Visitor]]s written using this style are easily extensible. For example when adding support for ''Minus'' operation in version 2.0 one just adds:
 
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<source lang="java" snippet="visitor.clientprovider.v2"/>
 
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This is the most flexible solution. It uses a form of ''tripple'' dispatch - e.g. the final visit method called on the (implementation of the [[visitor]] interface) is determined by the expression type, version of the expression language and implementation of the [[visitor]] interface:
 
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<source lang="java" snippet="visitor.clientprovider.dispatch.v3.l2"/>
 
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This solution to the [[expression problem]] is another realization of the general principle to separate [[ClientAPI]] from [[ProviderAPI]]. Client part of the [[visitor]] is enriched by new and new ''dispatch'' methods with every version. The interface part of [[visitor]] is immutable, a fixed point, which stays the same for those who implement it. Each version defines its own unique interface (according to the list of expression types it supports). The internals of the [[API]] then bridge the ''dispatch'' methods to appropriate interface ''visit'' methods.
 
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<comments/>
 
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[[Category:APIDesignPatterns]]
 
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[[Category:APIDesignPatterns:Evolution]]
 

Revision as of 06:45, 4 November 2009

Have You Ever Wondered...?

Do you wonder whether knowledge of proper design patterns makes you good API designer? Yes and no. Of course knowing regular design patterns simplifies communication, understanding, etc. However there are hidden catches. Not every design pattern belongs among APIDesignPatterns - it may not be ready for evolution. For example the well known Visitor pattern is really not evolvable easily as analysed by Chapter 18.

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