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Android to Windows Phone 8: Parse XML data

Now that your app can connect to a network and manage its network usage, it’s time to start working with data. Two of the most common data formats in use on the Internet today are Extensible Markup Language (XML) and JavaScript Object Notation (JSON). Website that update their content frequently, such as blogs, often provide an XML feed so that apps can keep up with the content changes. This section will show you how to parse an XML document and use its data in your apps.

The first thing you’ll need to do is decide on an XML feed you want to use in your app. For this section will use a feed from StackOverflow.com that focuses on Windows 8. The feed is available at http://stackoverflow.com/feeds/tag/windows-phone-8.

Once you’ve identified the feed you’ll want to analyze it to determine which fields are of interest to your app. Below is a sample abstract form the StackOverflow.com feed we’ll be using:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:creativeCommons="http://backend.userland.com/creativeCommonsRssModule" xmlns:re="http://purl.org/atompub/rank/1.0">
<title type="text">active questions tagged windows-phone-8 - Stack Overflow</title>
...
<entry>
...
</entry>
<entry>
 <id>http://stackoverflow.com/q/00000000</id>
 <re:rank scheme="http://stackoverflow.com">0</re:rank>
 <title type="text">Windows Phone XML Data</title>
 <category scheme="http://stackoverflow.com/feeds/tag/windows-phone-8/tags" term="c#"/>
 <category scheme="http://stackoverflow.com/feeds/tag/windows-phone-8/tags" term="xml"/>
 <category scheme="http://stackoverflow.com/feeds/tag/windows-phone-8/tags" term="windows-8"/>
 <category scheme="http://stackoverflow.com/feeds/tag/windows-phone-8/tags" term="windows-runtime"/>
 <author>
  <name>anonymous</name>
  <uri>http://stackoverflow.com/users/000000</uri>
 </author>
 <link rel="alternate" href="http://stackoverflow.com/questions/00000000/windows-store-xml-data" />
 <published>1900-01-01T00:00:00Z</published>
 <updated>1900-01-01T00:00:00Z</updated>
 <summary type="html">
  <p>How do I use XML in Windows Phone apps?</p>
 </summary>
</entry>
<entry>
...
</entry>
...
</feed>

For the example, we’ll focus on the title, link, and summary elements. The first step is to create a class that represents the data you want to work with in your app. Since we’re focusing on the title, link, and summary elements of the feed, the class will look like the following:

public class FeedEntry
{
 public string Title { get; set; }

 public string Link { get; set; }

 public string Summary { get; set; }
}

The next step is to create a class and a method that will be responsible for the parsing:

public class StackOverflowXmlParser
{
 ...
}

Since we’ll be connecting to an internet resource to obtain the XML data we’ll want to do so asynchronously to avoid blocking other operations. Once we’ve obtained the data and parsed it into a list of FeedEntry objects we’ll raise an event to signal to the caller that the operation is complete. This can be done via the following:

...
public event EventHandler<List<FeedEntry>> ParseCompleted;
...

Next we’ll need a method that to kick off the asynchronous procedure to obtain data and parse it:

...
public void ParseAsync()
{
 var connectionState = GetConnectionState();

 if (connectionState == ConnectionState.WiFi || connectionState == ConnectionState.Ethernet)
 {
  var request = WebRequest.CreateHttp("http://stackoverflow.com/feeds/tag/windows-8");

  request.BeginGetResponse(BeginGetResponseCallback, request);
 }
}
...

This code does a couple of things. First it obtains the current type of Internet connection the device has. If the device is connected to a WiFi or Ethernet connection it proceeds to create an HttpWebRequest object and call its BeginGetResponse method. This begins an asynchronous request to StackOverflow for the particular feed we’re interested in. The first parameter of this method is a callback that will be invoked when the response is received. The second parameter of the call is any state we want to utilize in the callback. In this case, we want to make sure we get access to the originating HttpWebRequest, so we simple pass it through.

We’ll utilize the callback to parse the data. The first thing we’ll want to do in this method is create a list of FeedEntry objects to store the results in.

...
private void BeginGetResponseCallback(IAsyncResult asyncResult)
{
 List<FeedEntry> results = new List<FeedEntry>();
 ...
}
...

Next we need to get the response stream which contain the data we want to parse:

...
var request = (HttpWebRequest)asyncResult.AsyncState;

var response = request.EndGetResponse(asyncResult);

var responseStream = response.GetResponseStream();
...

The next thing to do is to determine the method by which the feed will be processed. Because we’re dealing with an XML feed, we have three options in .NET:

  1. The System.Xml namespace
  2. The System.Xml.Linq namespace
  3. The System.ServiceModel.Syndication namespace

The System.Xml namespace provides standards-based support for processing XML. It gives you quite a bit of control over how you process XML and allows you to use things like XSD schemas, XPath expressions, and XSLT transformations. It’s a little more power than we need for this particular task. The SyndicationFeed class in the System.ServiceModel.Syndication namespace would actually make our job incredibly simple if we wanted work with all of the fields in the feed. However, since we’re only concerned with three fields we’ll use the System.Xml.Linq namespace. This namespace contains the classes for LINQ (Language INtegrated Query) to XML which serves as an in-memory XML programming interface that enables easy and efficient processing of XML documents.

First up when working with LINQ to XML is to create some XName objects that represent the names of the elements we want to work with. These names need to include the local and namespace names.

//Atom namespace
var atomNamespace = "http://www.w3.org/2005/Atom";

//Create the names of the XML elements including the Atom namespace
var entryXName = XName.Get("entry", atomNamespace);
var titleXName = XName.Get("title", atomNamespace);
var linkXName = XName.Get("link", atomNamespace);
var summaryXName = XName.Get("summary", atomNamespace);

The next step is to load the XML data in the response stream into a new instance of the XElement class.

...
XElement root = XElement.Load(responseStream);
...
Android tip
This is similar to using Android’s HttpURLConnection to retrieve and InputStream

private InputStream downloadXml() throws IOException {

 URL url = new URL('http://stackoverflow.com/feeds/tag/windows-8');

 HttpURLConnection conn = (HttpURLConnection) url.openConnection();

 conn.setRequestMethod("GET");

 conn.setDoInput(true);

 conn.connect();

 return conn.getInputStream();
}

You would then use the InputStream output as an input parameter into the XmlPullParser class’s setInput method.

Once the XElement object is loaded, we can use LINQ to query for just the entry elements and in the XElement and store them in a list:

//Load all "entry" elements from the XML into a list
var entries = (from e in root.Elements(entryXName) select e).ToList();

Next we’ll start iterating through the list of entries. The first step in the iteration is to instantiate a new FeedEntry object:

//Iterate all "entry" elements
foreach (var entry in entries)
{
 //Create a new FeedEntry
 var feedEntry = new FeedEntry();
 ...
}

Then we’ll assign the value of the FeedEntry’s Title property to the value of the title element:

//Get the title
var titleElement = (from e in entry.Elements(titleXName) select e).FirstOrDefault();

if (titleElement != null)
 feedEntry.Title = titleElement.Value;
Android tip
The Android equivalent would be calling the getText() method of the XmlPullParser class to get the value of the current element.

Similarly we’ll assign the value of the Link property to the value of the link element’s href attribute:

//Get the link
var linkElement = (from e in entry.Elements(linkXName) select e).FirstOrDefault();

if (linkElement != null)
{
 var hrefAttribute = linkElement.Attributes("href").FirstOrDefault();

 if (hrefAttribute != null)
  feedEntry.Link = hrefAttribute.Value;
}
Android tip
To get the attribute value in Android you would use the getAttribueValue() method of the XmlPullParser class.

Next we’ll assign the value of the Summary property to the value of the summary element and add the object to the result set to be returned to the calling method:

//Get the summary
if (summaryElement != null)
 feedEntry.Summary = WebUtility.HtmlDecode(summaryElement.Value);

//Add to the entry to the result set
results.Add(feedEntry);

Next we need to raise the event to signal to the original calling method that the parsing is complete:

if (this.ParseCompleted != null)
 this.ParseCompleted(this, results);

The last step is to call the ParseAsync method and respond the ParseCompleted event.

public void GetXmlData()
{
 var parser = new StackOverlfowParser();

 parser.ParseCompleted += parser_ParseCompleted;

 parser.ParseAsync();
}

private void parser_ParseCompleted(object sender, List<FeedEntry> e)
{
 //do something with the results
}

That’s it! Your Windows Phone application can now work with data from the Internet.

Read the original blog entry...

More Stories By Adam Grocholski

Hey there! My name is Adam Grocholski, and I'm a Technical Evangelist at Microsoft where I spend time focusing on Windows, Windows Phone, and Windows Azure. I live in the frozen tundra of Minnesota and run thinkfirstcodelater.com. You can also follow me on twitter at @codel8r.

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