Visitor
From APIDesign
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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]] | ||
| + | [[Category:APIDesignPatterns:Evolution]] | ||
Revision as of 06:46, 4 November 2009
The 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:
does not exists: visitor.clientprovider.v1
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. Visitors written using this style are easily extensible. For example when adding support for Minus operation in version 2.0 one just adds:
does not exists: visitor.clientprovider.v2
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:
does not exists: visitor.clientprovider.dispatch.v3.l2
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.
<comments/>
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