Server-Side Field Validation

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    From Wikibooks, under CC BY-SA 4.0, as the source states it. This page at Wikibooks.

    Motivation

    Steps

    Step 1: Add instances for outgoing and incoming data

    <!-- store the outgoing data for the invalid e-mail check -->
<xf:instance id="email-out">
   <data xmlns="">
      <email></email>
   </data>
</xf:instance>



    <!-- place to store the results of an invalid e-mail check -->
<xf:instance id="email-check-results">
   <data xmlns=""/>
</xf:instance>

    Step 2: Add a submission element to the model

    The submission element connects the "field changed" event to a service on the server. 

    <xf:submission id="email-check" method="get" action="email-check.xq" 
           ref="instance('email-check')" 
           replace="instance" instance="email-check-results" />

    Step 3: Add an action to your input

    <xf:input ref="email" >
    <xf:label>Email: </xf:label>
    <xf:hint>This field must contain a valid email format.</xf:hint>
    <xf:action ev:event="xforms-value-changed">
           <!-- copy from your "save-data" into the outgoing instance -->
           <xf:setvalue ref="instance('email-out')/email" value="instance('save-data')/email"/>
           <xf:send submission="email-check"/>
    </xf:action>
</xf:input>

    Step 4: Add a group that displays errors if the results are invalid

    <xf:group ref="instance('email-check-results')/message[@class='error']">
     <div class="field-warn"><xf:output value="instance('email-check-results')/message"/></div>
</xf:group>

    Step 5: Create a server-side REST GET service that returns true/false

    If there are no errors just return <ok/>

If there are errors return
<message type="error">Invalid e-mail format</message>