standard for "loops" in XML?

Question

I am designing a simple XML file for animations. I often get in a situation that I write something like this:

<frame>
    <image>./image1.png<image/>
</frame>
<frame>
    <image>./image2.png<image/>
</frame>
<frame>
    <image>./image3.png<image/>
</frame>
...

Now, this of course can get annoying. So I hoped XML allowed something like this:

<frameset min=1 max=5>
    <image>./image#.png<image/>
</frameset>

Making # a placeholder for a counter variable. What I am wondering: is there already a standardized way of doing something like this? It just makes things easier if one uses methods people are already used to.

Answer 1

XML is data not instructions. If you need a kind of instruction in the xml then make it data. You could change your datastructure like this:

<frameset>
  <start>1</start>
  <end>3</end>
  <prefix>./image</prefix>
  <suffix>.png</prefix>
</frameset>

Basically your second approach.

Answer 2

There is no universal model, but what you're looking for is essentially a templating solution - a template language plus a template evaluation engine that transforms a template into final XML by executing the code embedded in the template.

So, you can take, for a model, ANY templating language you're familiar with.

The very best approach is of course to leverage an existing templating engine solution if you have access to one; so you don't have to write one. E.g. JSP for Java, EmbPerl for Perl, XSLT etc...

Here's what a sample EmbPerl template would look like:

[$ foreach my $i (1..5) $]
<frame>
    <image>./image[+$i+].png<image/>
</frame>
[$ endforeach $]

A lot of templating solutions would look very similar, with slightly different markup and obviously different logic syntax (Perl vs. Java vs. xxx)

---

If you go with homebrew solution, the templating language can of course be XML itself, something like

<frameset min=1 max=5>
    <image part=1 content_type="constant">./image<image/>
    <image part=2 content_type="iterator_value"><image/>
    <image part=3 content_type="constant">.png<image/>
</frameset>