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Re: Binding : Do I need a nodeset?

From: Mark Birbeck <mark.birbeck@webbackplane.com>
Date: Thu, 6 Aug 2009 15:54:15 +0200
Message-ID: <ed77aa9f0908060654t78e06651sf016699d5bf8fcf@mail.gmail.com>
To: adam.flinton@nhs.net
Cc: "www-forms@w3.org" <www-forms@w3.org>
HI Adam,

> I wonder if there is scope within any improvements to XForms to have a
> <xforms:variable/> element
>
> Much as with XSLT or similar......

It's been discussed many times over the years, and I'm sure it will happen.


> The normal use cases are wrt checking values e.g. my "check unique" check:
>
>       <xforms:bind
>           calculate="if(instance('OIDVals')/oids/@active = 1,true(),if
> (count(instance('OIDVals')//ov[@full = instance('main')/@full]) &lt;=
> instance('OIDVals')//oids[1]/@exist,true(),false()))"
>           id="unique_OID" nodeset="instance('uniqueOID')/@val"/>
>
> Or as stated in the previous post wrt displaying one group vs another.

If you have multiple groups, and multiple possible values (obtained
via @calculate) then you could use a clever little trick that I first
saw used by David Landwehr.

The idea is to bind multiple groups to the same node (in your case it
might be @val), but to then add an XPath filter with the expression
you want relevance to be based on:

  <xf:group ref="instance('uniqueOID')">
    <xf:group ref="@val[. = 1]">
      ...
    </xf:group>

    <xf:group ref="@val[. = 2]">
      ...
    </xf:group>

    <xf:group ref="@val[. = 3]">
      ...
    </xf:group>
  </xf:group>

This has the effect of creating an empty nodeset for each @ref that
contains a filter that evaluates to false. The default behaviour in
XForms, for a binding to a node that doesn't exist, is to act as if
the binding is to a non-relevant node.

In short, you should only ever get a maximum of one of these groups
being relevant at any one time, and with this approach there is no
need to use xf:bind, other than for the calculation to set the
selection criteria.

Regards,

Mark

-- 
Mark Birbeck, webBackplane

mark.birbeck@webBackplane.com

http://webBackplane.com/mark-birbeck

webBackplane is a trading name of Backplane Ltd. (company number
05972288, registered office: 2nd Floor, 69/85 Tabernacle Street,
London, EC2A 4RR)
Received on Thursday, 6 August 2009 13:54:59 GMT

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