处理撒克逊人：评估异常

Question

I have an XBL file where I can pass some arguments from my form, let's say :

<my-xbl arg1='' arg2=''>
</my-xbl>

<xf:input ref='@arg1'/>
<xf:input ref='@arg2'/>

Then I have something like this :

<xf:model>
    <xf:action id="populate-data-binding">                         
        <xf:action context="instance('sub')"
                   ev:event="xforms-submit-done"
                   ev:observer="my-submission">                     
            <xf:action>        
                <xf:var as="xs:string" name="control-value" value="saxon:evaluate(instance('arg1'))"/>  
                <xf:var as="xs:string" name="control-name"  value="saxon:evaluate(instance('arg2'))"/>
                <xf:setvalue ref="xxf:instance('fr-form-instance')//*[name() = $control-name]"
                             value="$control-value"/>
           </xf:action>
    </xf:action>
</xf:model>

The problem is when arg1 is empty string (it means that in my form when I press Control settings and skipp arg1 field, just leaving it empty) then my form crashes. When I insert '' or anything but empty string (asd, q, anything) as arg1 everything is ok. So now I would like to do something like

<xf:var as="xs:string" name=".." value="if(@arg1 ='null') then 'asd'
 else 'saxon:evaluate(instance('arg1'))'/>

It doesn't work, obviously because quotes issue (condition must be between '' and I cannot put any quotes inside) and probably something else. Could someone help me fix this ? Thanks in advance.

UPDATE :

I guess I was a little bit unprecise. To make things clear, arg1 is an ID of xf:input in my form. When form gets loaded I call a REST service (you've helped me with this once, Event xforms-model-construct-done behaviour ) and it sends me back some data. Now I would like to populate data to my xf:inputs and the code above does it very well. The problem is that in my XBL I have plenty of parameters, not just arg1 , arg2 , it looks more like

<my-xbl arg1 = '' arg2 = '' ... arg19 = '' arg20 = ''/>

Now, when in my form I go to control's basic settings I have to fill all the paraemters arg1...20 . I would like to fill only those I need, which is usually 3 or 4 and leave the others blank. The problem is that I cannot do that, because that way my form crashes and it's because saxon:evalute throws an exception if its argument is null.

(Also to mention everything, but I don't think it's relevant here but it may help you to understand my code above : in real, arg2 is an ID of xf:input but arg1 is something like a path to resource in webservice, fe I have arg2 = "'firstname'" which refers to my xf:input and arg1 =" //CUSTOMER/FIRSTNAME" (no quotes) and my REST service URL is http://www.thomas-bayer.com/sqlrest/CUSTOMER/2/ . This way I can choose that under xf:input of id 'firstname' I want to insert only firstname of the whole data I got from webservice. Then again, arg4 would be like 'lastname' and arg3 would be //CUSTOMER/LASTNAME and so on. As I said, don't think it's relevant here)

That's why I wanted to check empty value and I guess your code should have helped me - unfortunately it's not working. My form doesn't crash anymore, but also data is not populating into xf:inputs . Maybe because this misunderstanding ? Thanks.

Answer 1

<xf:var
    as="xs:string"
    name="..."
    value="
        if(@arg1 ='null')
        then 'asd'
        else 'saxon:evaluate(instance(''arg1''))"/>

To test for a blank string, better use normalize-space() . Also, I guess that instance('arg1') is where the value is stored in both cases:

<xf:var
    as="xs:string"
    name="..."
    value="
        if (normalize-space(instance('arg1')) = '')
        then 'asd'
        else 'saxon:evaluate(instance(''arg1''))'
"/>

But I am a bit unclear on why you need to quote the saxon:evaluate() call. Do you want the variable to hold:

1. the result of the evaluation?
2. or an XPath expression?