Including content

Using xbl:content

The xbl:content element allows copying elements which are descendant elements from the bound node. The selectors work as if applied to an XML document rooted at the bound node.

Say you have this markup:

<fr:inline-input ref="name">
    <xf:label>Name</xf:label>
</fr:inline-input>

The implementation of the fr:inline-input component can copy the nested xf:label element under a group as follows:

<xf:group>
    <xbl:content includes="xf|label"/>

The CSS selection expression above is exactly equivalent to writing in XPath:

descendant-or-self::xf:label

Note the difference of notation in XML/XPath and in CSS to refer to qualified element names:

  • XML/XPath uses a "colon" character: foo:bar
  • CSS uses a "pipe" character: foo|bar

Both XPath and CSS are expression languages allowing selecting nodes from XML documents, but they have a quite different syntax!

Now here is a more complex scenario:

<fr:link-select1 ref="gender">
    <xf:label>Gender</xf:label>
    <xf:itemset ref="instance('genders')/gender">
        <xf:label ref="label"/>
        <xf:value ref="value"/>
    </xf:itemset>
</fr:link-select1>

You would think that the implementation of the fr:link-select1 component could simply copy the nested xf:label element as follows:

<xf:group>
    <xbl:content includes="xf|label"/>

But this doesn't work properly because the CSS selector xf|label actually returns all descendant label elements, including the xf:label element under xf:itemset.

The recommend way to express this is as follows:

<xf:group>
    <xbl:content includes=":root > xf|label"/>

The :root pseudo-class refers to the bound element (here fr:link-select1). The > combinator "describes a childhood relationship between two elements", like the XPath / axis. So the result is equivalent to the XPath:

/*/xf:label

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