Friday, October 26, 2007

Parsing XML in Silverlight 1.0 - Cross Browser

One problem we have seen and dealt with allot lately is parsing XML client side. My favorite solution I must admit is to just toss out XML and use JSON as it is so much more elegant. In the case of the ET Minisite we did as of late this turned out to solve a lot of our problems. But still what if I can't do JSON for whatever reason? if you only care about IE or even firefox that seems to be much less of a problem. But for a really cross browser experience I my solution is the XML document mini me. Granted this is a bit of hack (like much of JavaScript and Silverlight)... :) but it works well... IF your XML is pretty stable and IF there isn't to much XML weirdness like multipul nested levels and such but it works.

So the XML Mini me is a simple XML parseing engine written in Javascript. It only supports a limited subset of something like the real xml parser in .NET like XMLReader or even XMLDocument but it does so a few simple things including some simple xPath like behavior. I hestiate to say a subset of xpath as the mini me is so extremely limited. I also suspect that the silverlight team at some point will have a good XML parser built in to silverlight at some point but we will see. there is of course Silverlight 1.1 but if you can't do the .NET part it doesn't do much goo.

In Silverlight, you have the downloader object so its really reasonable get XML easily no matter what. So then using the XML mini me looks something like this:

var MyDom = new XMLDocument();
MyDom.LoadXML( SomeXML);

I tried to keep the syntax as famliar as possible. So now we have our XMLDOM called 'MyDom' and then to get data out we can do this:

var ArrayOfVAlues = MyDom.SelectNodes("//items");

In this case the mini me will return all the text values of all the items nodes in the dom in the form of an array. Underneath the covers the parser is just doing some 'string' manipulation but for small groups or data or simple RSS feeds or config files it seems to work well. large XML parseing would scary me preformance wise as would writing a more full featured parser that is entirely written in JavaScript.

Some things to note on the class is that it has an XML property like the XML parser we used to use in classic ASP but otherwise it only has two methods currently as those are the ones I need to get it working for my particular project. lets look at the XML it can deal with for starters, here is a sample:

So in this sample using some simple xml our parse deals with it easily. In this case when we make a call to SelectNodes("//url") we will get an array of all the urls values with all the < ! [ CDATA stuff stripped off into an array. simple, easy and fast but easly breakable if you muck up the xml to much. So for the time being its a great stop gap and if JavaScript works then this works at it uses only intrinsic language features.

So the little XML mini me is great for simple stuff but when you really need to do real work with XML you really need a more robust solution that includes a real XML parser. Now that can prove problematic especially when it has to work in lots of browsers. To address this problem with my own work I wrote another class that wraps the cross browser functionality and wraps some key elements that I use. Typically I need things like all of properties of a given node in a list in the form of an array. Plus things like xPath and the like for Silverlight.

So the attached javascript class wraps the core functionality. It includes the HTTP request functionality and XML parsing with the other bits I 'might' need. Basically the class works like this:

var NewDOM = new XML();

This code fires when the html page script code behind fires. When you call 'Load' and pass a url to an xml file it then depending on the browser it uses the appropriate HTTP request object to load the file and assign the onload event. On the XML files load the call back is called and the object does the appropriate processing. Once this does its thing we now can use it to parse our XML.

On the onload event for the page in my little test I make two calls to different methods. The same methods are supported on this class as the XML mini me but additional ones are included like 'SelectNodesAttributes'. Also I noticed in firefox by 'onload event gets called twice so I put a flag on it so it only fires once. So in this sample I call the two methods that return arrays but you can have direct access to the DOM and use xPath etc using NewDOM.XmlDocument.

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