xsl:accumulator-rule

Description

xsl:accumulator-rule specifies the update logic for an xsl:accumulator. Each rule has a match pattern (identical in syntax to the match attribute of xsl:template) and a phase that determines whether it fires when the processor enters (start) or exits (end) a matched element.

When a rule fires, the body expression (either the select attribute or the sequence constructor content) is evaluated and its result becomes the new accumulator value. Inside the rule, the special variable $value holds the current accumulator value before this rule fires. This lets you write incremental updates: select="$value + xs:decimal(@amount)" adds the current node’s amount to the running total.

Rules within one accumulator are evaluated in document order; if multiple rules could match the same node (due to broad patterns), all matching rules fire in specificity order, similar to template priority. The most specific match wins, just as with xsl:template.

The phase attribute is optional for non-element nodes: attributes, text nodes, comments, and processing instructions cannot have a start/end phase distinction, so phase defaults to end for them. For element nodes, start fires before any descendants are processed; end fires after all descendants have been processed.

Attributes

Examples

Tracking nested depth with start and end rules

Input XML:

<?xml version="1.0" encoding="UTF-8"?>
<tree>
  <node id="1">
    <node id="1.1">
      <node id="1.1.1"/>
    </node>
    <node id="1.2"/>
  </node>
</tree>

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:output method="xml" indent="yes"/>

  <xsl:accumulator name="depth" as="xs:integer" initial-value="0">
    <xsl:accumulator-rule match="node" phase="start" select="$value + 1"/>
    <xsl:accumulator-rule match="node" phase="end" select="$value - 1"/>
  </xsl:accumulator>

  <xsl:mode use-accumulators="depth"/>

  <xsl:template match="/tree">
    <flat-list>
      <xsl:apply-templates select="//node"/>
    </flat-list>
  </xsl:template>

  <xsl:template match="node">
    <item id="{@id}" depth="{accumulator-before('depth')}"/>
  </xsl:template>
</xsl:stylesheet>

Output:

<flat-list>
  <item id="1" depth="1"/>
  <item id="1.1" depth="2"/>
  <item id="1.1.1" depth="3"/>
  <item id="1.2" depth="2"/>
</flat-list>

Accumulating a list of all ancestor titles

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:output method="xml" indent="yes"/>

  <xsl:accumulator name="breadcrumb" as="xs:string*" initial-value="()">
    <xsl:accumulator-rule match="section" phase="start"
      select="($value, string(@title))"/>
    <xsl:accumulator-rule match="section" phase="end"
      select="$value[position() lt last()]"/>
  </xsl:accumulator>

  <xsl:mode use-accumulators="breadcrumb"/>

  <xsl:template match="para">
    <item>
      <path><xsl:value-of select="accumulator-before('breadcrumb')" separator=" > "/></path>
      <xsl:value-of select="."/>
    </item>
  </xsl:template>
</xsl:stylesheet>

Notes

• The $value variable is always available inside an xsl:accumulator-rule and refers to the current accumulator value before this rule fires.
• For phase="start", $value is the accumulator value before the element’s opening tag was reached.
• For phase="end", $value includes all updates made by descendants of the current element.
• Rules with equal priority for the same node follow the same conflict-resolution logic as xsl:template.

See also

• xsl:accumulator
• xsl:use-accumulators
• accumulator-before()
• accumulator-after()