Introduction to XSLT

XSLT stands for Extensible Stylesheet Language
XSLT provides a means for visually formatting an XML file
1. CSS can also be used to visually format an XML file. However CSS is limited in what it can do:
 1. CSS can only specify property/value pairs for elements, such as a margin or a font. It does not provide a way to specify the structural layout for elements.
 2. CSS works with HTML because browsers have built-in understandings of how to display HTML elements, such as tables.
 3. The XML user is free to make up arbitrary element names and hence a browser does not have a built-in understanding of how to display these elements
 4. XSLT provides a means for telling a browser how to display XML elements
2. XSLT is a rule-based or functional language.
 1. A XSLT stylesheet contains a set of "rules" or "templates" for transforming XML elements into html code (or other presentable code such as pdf).
 2. A rule consists of a pattern that matches an XML element, a body that specifies a visual presentation, and an operator for applying that pattern to XML elements. Here is a simple example of a rule: The pattern is catalog/cd, the operator is for-each, and the body is the html code that creates a row in an html table. The rough meaning of the operator and the pattern is that for each cd element that is a child of catalog we want to format that cd element as a row in an html table. The value-of operator selects the value of the title and artist elements that should be children of each cd element. We will talk more about the various xslt operators in a moment.
 3. XSLT works by matching patterns against element tags and replacing the elements with the rule body.
 4. Be careful when outputting html to terminate all elements with the appropriate </> tag. Not doing so may cause your entire output to appear blank because many browsers expect strict xhtml as output.

Starting an XSLT stylesheet

All XSLT files start with a stylesheet element: <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
1. The version attribute is required and should be 1.0.
2. The xmlns:xsl attribute creates a namespace with a shorthand notation of "xsl" and a longhand notation of "http://www.w3.org/1999/XSL/Transform". The longhand notation is called a URI for Uniform Resource Indicator. A URI is used to locate resources on the web, but paradoxically, it does not need to be a pointer to a legitimate web-site, nor does it need to specify how to use that resource. For example, if you type http://www.w3.org/1999/XSL/Transform into your browser, your browser will display the line "Someday a schema for XSL Transforms will live here". Your browser actually has an XSLT processor that understands the XSL elements and knows how to apply them.
3. For your stylesheet to work you must declare your xsl URI namespace exactly as shown above--if you alter even one character in the URI then your stylesheet will be broken
4. xsl elements need to be prefixed with xsl:. If you forget to do so, the elements will be interpreted as xhtml elements.
5. it is technically ok to use a prefix other than xsl but you use xsl by convention.
Output element: The output element specifies the desired output, such as text or html. For example: <xsl:output method="text" /> <xsl:output method="html" />
1. The method attribute specifies the desired output.
2. The output element immediately follows the xsl:stylesheet element

Linking a stylesheet to an XML document

You specify a stylesheet for an XML document using the xml_stylesheet element. For example: <?xml-stylesheet type="text/xsl" href="cdcatalog.xsl"?>
1. The href attribute specifies where to find the stylesheet
2. The type attribute specifies the MIME type for the stylesheet file, which is plain text, and is to be interpreted as an xsl file.
 1. MIME types were originally used by mailers to determine what type of application should be opened in order to display an attachment. For example, "img/gif" would indicate that the file was an image file in gif format.
 2. MIME types are now also used by browsers to determine an application that should be used to display a web-page (e.g., it might open Acrobat to display a pdf file or Word to display a .doc file).
 3. Just copy the type attribute and do not worry about it further. The browser will know that it should interpret the stylesheet file as an xsl file.
The xml_stylesheet declaration is placed immediately after the <?xml?> tag.

Rule syntax

The template element introduces a new rule in XSL. The format is:
The match pattern is an XPath expression.
1. Matching is done from the current node
2. Matching recursively descends through the tree in the sense that the body will be asked to match additional templates, which will be further down the tree
The priority attribute selects the highest priority template when more than one template matches an element.
1. default values are -0.5 to 0.5, depending on the XPath expression in the match attribute
2. XSLT may not reliably assign higher priority numbers to more detailed XPath expressions so it is a good idea to use priority numbers to distinguish finer-grained XPath matches from courser-grained XPath matches.
3. example: The priority of 1 for the second template guarantees that that template will be applied to cd's with a price of more than $10.

XSL operators that appear in the body of a rule

The apply-templates element is the workhorse operator for XSL.

Syntax: <xsl:apply-templates select="pattern">
For each child XML element contained within the currently matched XML element it attempts to find a template (i.e., rule) that matches this XML child element.

Example:

<xsl:template match="/catalog">
  <html>
  <body>
    <h2>My CD Collection</h2>
    <table border="1">
      <tr bgcolor="gray">
        <th>Title</th>
        <th>Artist</th>
      </tr>
      <xsl:apply-templates />
    </table>
  </body>
  </html>
</xsl:template>
<xsl:template match="cd">
  <tr>
  <td><xsl:apply-templates select="title" /></td>
  <td><xsl:apply-templates select="artist"/></td>
  </tr>
</xsl:template>

<xsl:template match="title">
<span style="font-family: roman; color: red">
<xsl:value-of select="."/>
</span>
</xsl:template>

<xsl:template match="artist">
<xsl:value-of select="."/>
</xsl:template>

If you omit a template for an element then the default action is to return its data content
The select attribute is an XPath expression that restricts the selected elements to the specified elements

The value-of element returns the data content of an element and any subelements
1. Since the content of subelements is returned, the value-of element should only be used with atomic elements
2. Examples:
 - <xsl:value-of select="."/>
 - <xsl:value-of select="title"/>
The for-each element returns every node in a specified node set. For example:
```
 <xsl:for-each select="cd"> 
 <tr>
 <td><xsl:value-of select="title"/></td>
 <td><xsl:value-of select="artist"/></td>
 </tr>
 </xsl:for-each>
 
```
This for-each statement returns all cd elements that are children of the current node.
1. It is possible to use predicates to further restrict the nodes that are chosen by for-each For example, to restrict the nodes to cds with a price that is greater than $10, we could write:
2. When you use predicates, the < and > signs need to be written as < and > to avoid confusion with tags.
Conditional constructs: XSLT provides an if element for a single test and a choose element for multiple tests.
1. Syntax: <xsl:if test="expression">
 1. Expression can be any valid predicate.
 2. ```
 	 <xsl:template match="cd">
 	 <xsl:if test="price &gt; 10">
 		 ...
 	 </xsl:if>
 	 </xsl:template>
 
 	 <xsl:template match="cd">
 	 <xsl:if test="country = 'UK'">
 		 ...
 	 </xsl:if>
 	 </xsl:template>
 	 
```
3. Typically the same test could be placed with the match or select attribute of the enclosing operator so in single-case situations it's probably clearer to put the test in the match or select attribute and not use the if operator.
4. if's are best used when you want to do the same formatting for most of the tag but want to tweak the formatting in the middle. For example, for a cd I might want to always print the title and artist, but in addition I might want to print an asterisk next to the title if the cd's price is greater than 10. Here's how I might do it with an if element:
```
 <xsl:for-each select="cd">
 <tr>
 <td><xsl:value-of select="title"/>
 <xsl:if test="price &gt; 10">*</xsl:if></td>
 <td><ol>
 <xsl:value-of select="artist" /> 
 </xsl:for-each>
```
5. if components must obey xml nesting requirements, which can prevent you from accomplishing seemingly simple tasks. For example, suppose we want to boldface titles that are associated with cd's whose price is $10. We might want to put an if clause before and after the title that adds a boldface element and a matching boldface terminator. For example:
```
 <td><xsl:if test="price &gt; 10"></xsl:if> 
 <xsl:value-of select="title" />
 <xsl:if test="price &gt; 10"></xsl:if>
 </td>
```
 This code represents an improperly nested piece of xml code because the  was not terminated before the <xsl:if> element. So even though the code would produce a properly nested html document if it were executed, the code will never get executed. To get the title to be properly bold-faced, I would need to use a choose statement: <td><xsl:choose> <xsl:when test="price > 10"> <xsl:value-of select="title" /> </xsl:when> <xsl:otherwise> <xsl:value-of select="title" /> </xsl:otherwise> </xsl:choose> </td> The choose operator is discussed next.
2. choose: The choose operator is comparable to the if-else construct used in most languages and is useful for applying different visual formatting to different values of an element.
 1. If you want to distinguish between different elements, use match/select rather than when
 2. Syntax:
 3. Expression can be any valid predicate and < and > signs should be escaped as < and >
 4. Example
```
<xsl:template match="cd">
 <xsl:choose>
 <xsl:when test="country = 'UK'">
 <tr>
 <td bgcolor="white"><xsl:apply-templates select="title" /></td>
 <td><xsl:apply-templates select="artist"/></td>
 </tr>
 </xsl:when>
 <xsl:when test="country = 'USA'">
 ...
 </xsl:when>
 <xsl:otherwise>
 ...
 </xsl:otherwise>
 </xsl:choose>
</xsl:template>
 
```
 5. sort: The sort operator allows you to sort elements in a variety of ways.
 1. Positioning the sort operator: Place the sort operator immediately after an operator whose elements you want sorted. For example:
```
	 <xsl:for-each select="cd">
	 <xsl:sort select="artist" />
	 
```
 2. Sorting alphabetically: The default for the sort operator is to sort alphabetically so nothing further is needed
 3. Sorting in reverse alphabetical order: Use the order attribute:
```
	 <xsl:sort select="artist" order="descending" />
	 
```
 4. Sorting numerically: Use the data-type attribute:
```
	 <xsl:sort select="price" data-type="number" />
	 
```
 Valid values for data-type are text and number (there is also something called a qname but we will not cover it in this course).
 5. Sorting using a primary key and a secondary key: Cascade the sort operators. For example, to sort the cdcatalog first by price and then by artist name, write:
```
	 <xsl:sort select="price" data-type="number" />
	 <xsl:sort select="artist" />
	 
```
 6. Counting elements: You can use the XPath operator count to count the number of elements that match an XPath expression. For example:
 - <xsl:value-of select="count(cd)"/>: print the number of cds. Note that the value-of element is required to extract the value of the count operator.
 - <xsl:if test="count(cd) > 1">: test whether the number of cds is greater than 1.
 7. Numbering elements: You can number elements using an html ordered list but if you are outputting a file as plain text or you do not want to bother with the extra formatting required by ordered lists, you can use xsl's number operator to number a list.
 1. Syntax: <xsl:number value="position()" format="format string" />
 1. The format string should contain a single-character code, as well as any other punctuation or formatting characters you might want, such as spaces or periods
 2. The value attribute can be set to an arbitrary number but typically you want to output the elements position
 2. Valid format codes
 
 Code Result
 
 1 1 2 3 4 ...
 
 a a b c d ...
 
 A A B C D ...
 
 i i ii iii iv ...
 
 I I II III IV ...
 3. Example:
```
	 <xsl:for-each select="cd">
	 <xsl:number value="position()" format="1. "/>
	 ... output ...
 
```

Code	Result
1	1 2 3 4 ...
a	a b c d ...
A	A B C D ...
i	i ii iii iv ...
I	I II III IV ...