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| <!-- $Id$ --> |
| <s1 title="<xsl:choose> / <xsl:when> / <xsl:otherwise>"> |
| |
| <s2 title="Contents"> |
| <ul> |
| <li><link anchor="functionality">Functionality</link></li> |
| <li><link anchor="implementation">Implementation</link></li> |
| </ul> |
| </s2> |
| |
| <anchor name="functionality"/> |
| <s2 title="Functionality"> |
| |
| <p>The <code><xsl:choose></code> element is used to determine one |
| course of action based on a series of tests. Each test is done inside an |
| <code><xsl:when></code> element. If a test succeeds, the body of the |
| <code><xsl:when></code> element is executed. If no tests fail then |
| a <code><xsl:otherwise></code> element can be used to specify a |
| default action:</p> |
| |
| <source> |
| <xsl:choose> |
| <xsl:when test="element-available('some-extension')"> |
| ... |
| </xsl:when> |
| <xsl:when test="function-availabe('saxon:nodeset')"> |
| ... |
| </xsl:when> |
| <xsl:otherwise> |
| ... |
| </xsl:otherwise> |
| </xsl:choose></source> |
| |
| </s2> |
| |
| <anchor name="implementation"/> |
| <s2 title="Implementation"> |
| |
| <p>The <code>Choose</code> class places all <code>When</code> child-nodes |
| in a vector. The <code>Choose</code> class translates the "test"-attribute |
| of all <code>When</code> nodes (in strict order) and chains them together |
| in an if-else style. The expression that holds each test contains a true- |
| and a false-list. These lists are vectors of branch targets that should be |
| used if the test succeeds or fails, respectively. The first test's |
| false-list is pointed to the start of the next test (ie. if the first test |
| fails, then we run the next test). The last test's false-list points directly |
| to the code for the body of the <code><xsl:otherwise></code> element. |
| </p> |
| |
| <p>Just as with the <code><xsl:if></code>-element, special care is |
| taken for the <code>element-available()</code> and |
| <code>function-available()</code> functions. These functions are evaluated at |
| compile-time (this can be done since all parameters for these functions are |
| literals) and the body of a <code><xsl:when></code> element is not |
| compiled if we know that it will never be needed at runtime.</p> |
| |
| </s2> |
| |
| </s1> |