Controlled values in oXygen XML Web Author

main.xml

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="values.less"?>
<root choices="values2.xml">
    <elementWithChoices/>
</root>

values.less

// By default, load the values.xml file from the framewotk directory.
:root elementWithChoices {
    .controlled-values(oxy_url("${framework}", "values.xml"));
}
// If specified in the root element, another choices file is used.
:root[choices]  elementWithChoices {
    .controlled-values(oxy_xpath("/*/@choices", processChangeMarkers, false, evaluate, dynamic-once));
}

.controlled-values(@valuesFile) {
    //The oxy_concat expression just composes an XPath expression like the one below:
    // 
    //  string-join(doc('values.xml')//my-element-name/*/text(), ',')
    //
    @xpath: oxy_concat("string-join(doc('", @valuesFile, "')//", oxy_name(), "/*/text(), ',')");
    content: oxy_combobox( 
            edit, '#text',
            editable, false,
            values, oxy_xpath(@xpath, processChangeMarkers, false, static)
    );
    visibility: -oxy-collapse-text;
}

values.sch

<?xml version="1.0" encoding="UTF-8"?>
<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2"
    xmlns:sqf="http://www.schematron-quickfix.com/validator/process">
    <!-- General variables for the whole document. -->
    <sch:let name="doc-uri" value="base-uri(.)"/>
    <sch:let name="choices-file" value="/*/@choices"/>
    <sch:let name="values-uri" value="resolve-uri($choices-file, $doc-uri)"/>    <!-- Here we compute the location of the 'values.xml' file -->
    <sch:let name="values-for-elements" value="doc($values-uri)"/>
    
    <!-- Abstract pattern to be instantiated for each element with constrained values -->
    <sch:pattern abstract="true" id="value-constraint">
        <sch:title>Constrain element values</sch:title>
        <sch:rule context="$element-name">
            <sch:let name="crt-text" value="text()"/>
            <sch:let name="choices" value="$values-for-elements//$element-name/*/text()"/>
            <sch:let name="matches" value="$values-for-elements//$element-name/*[text() = $crt-text]/text()"/>
            <sch:assert test="not(empty($matches))">
                Invalid value for element <sch:value-of select="local-name(.)"/>: "<sch:value-of select="$crt-text"/>" is not among <sch:value-of select="string-join($choices, ',')"/> 
            </sch:assert>
            
        </sch:rule>    
    </sch:pattern>
    
    <!-- This is an instance of the rule for a particular element -->
    <sch:pattern is-a="value-constraint">
        <sch:param name="element-name" value="elementWithChoices"/>
    </sch:pattern>
</sch:schema>

values.xml

<?xml version="1.0" encoding="UTF-8"?>
<values>
    <elementWithChoices>
        <value>x1</value>
        <value>x2</value>
        <value>x3</value>
    </elementWithChoices>
</values>