XForms: Using calculate and xxforms:default attribute in nodeset binding

Question

I have form1 passing a query string parameter(param1) to form2. I pass param1=true from form1. In form 2, I am trying to set the value of the param1 into a local instance. If I DO NOT pass param1 from form 1, I want form2 to take the value of param1 to be false. Here is my instance.

<xforms:instance id="querystring-instance">
    <query-string>
        <param1></param1>
    </query-string>
</xforms:instance>

I am using the following line to set the value of param1 by fetching it from the query string. If query string does not have param1, I want to use the default value of false.

<xforms:bind nodeset="instance('querystring-instance')/param1" calculate="xxforms:get-request-parameter('param1')" xxforms:default="false()" />

This does not work. If param1 is present it works and sets the in querystring-instance to true. If param1 is not present, it sets to nothing. I want it to be set to false. How do I do this?

Answer 1

You probably want to set the parameter upon initialization only, so use xxforms:default instead of calculate. calculate will be evaluated at each XForms recalculate, and that will fail because xxforms:get-request-parameter() is only available during XForms initialization.

You can implement the condition in XPath.

xxforms:get-request-parameter() returns and empty sequence if the parameter is missing. This should work:

<xforms:bind nodeset="instance('querystring-instance')/param1"
             xxforms:default="(xxforms:get-request-parameter('param1'), 'false')[1]"/>

What this does is that if xxforms:get-request-parameter() returns an empty sequence, then the first value of the sequence will be 'false', and that's what will be used to set the value.