The Output control and calculated expressions

The first thing we need to do is create the full URL for each image. The URL consists of the Flickr domain name, a number for a server to retrieve the image from, an ID, a 'secret number', and a size:

http://static.flickr.com/{server}/{id}_{secret}_{size}.jpg

The following mark-up creates a URL like this for each photo in the search results:

      <xf:repeat nodeset="instance('inst-rs')/photos/photo">
        <xf:output
         value="concat('
            'http://static.flickr.com/',
            @server, '/',
            @id, '_',
            @secret, '_s.jpg'
         )"
        />
      </xf:repeat>

The main new feature to note is that we're using the attribute value on our output control, rather than the ref attribute. The distinction is that ref refers to an item in an instance, whilst value can be any calculated expression.

In the value attribute we have an XPath expression that is making use of the concat function. All this function does is to join together all of its arguments to produce a string:

concat(
  'http://static.flickr.com/',
  @server,
  '/',
  @id,
  '_',
  @secret,
  '_s.jpg'
)

This is an XPath function that combines each of its parameters together to create a string. If any of the parameters are themselves XPath expressions then they are evaluated first, so in this case the values for the attributes server, id and secret are obtained from each photo and placed into the result.