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Developing complex, XSLT scripts: design patterns address flexibility and scalability. (XSLT).

Article Excerpt
XSLT is a declarative language designed for transforming XML documents into documents in any format. In developing large-scale XSLT scripts, software qualities such as flexibility and maintainability become issues. To address those issues, this article will first discuss a design approach that emphasizes a decomposition and recomposition view of transformations; then, a technique for modularizing XSLT scripts will be described; and finally, a few design patterns will be introduced.

Identify Responsibilities and Assign Them to Code Units

The goal of any kind of application design is to divide responsibilities into smaller pieces and conquer them in separated code units. In XSLT, call-by-name templates, call-by-context templates, and global variables are code units, which are equivalent to functions in structured programming languages. Global variables are equivalent to call-by-name templates without any parameters. Call-by-context templates require callers and callees to share context information. Examples of three types of code units are shown in Listing 1.

LISTING 1 * Examples of three types of code units An example of call-by-name templates: The definition of a call-by-name template: The invocation of the call-by-name template: An example of call-by-context templates: The definition of a call-by-context template: The invocation of the call-by-context templates: An example of global variables: The definition of a global variable: The invocation of the global variable: $global-variable

Having identified code units in XSLT, we should study how to divide a complex transformation into smaller ones and assign each to a code unit.

At the most abstract level, due to the stateless nature of XSLT, the responsibility of any XSLT script can be described as transforming a set of input streams to an output stream. Intuitively, outputting a segment of the output stream can be the candidate of a smaller responsibility.

Outputting each line in Listing 2 could be a responsibility. However, abstraction is required to identify reusable and semantically significant segments. Identifying those segments, which may not even be continuous, requires insight into the output stream. It's easy for a C programmer to find outputting the conditional expressions a meaningful responsibility. The template that fulfills this responsibility is implemented in Listing 3.

LISTING 2 * The C++ code to output bool AClass::operator== ( const AClass & rhs ) const { if (intField1_!=rhs.intField1_) return false; if (intField2_!=rhs.intField2_) return false; if (strcmp (stringField_,rhs.stringField_) != 0) return false; return true; } LISTING 3 * The code unit outputting conditional expressions

Identifying smaller responsibilities is a decomposition process, which decomposes an output...

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