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Android to Windows 8: Parse XML data

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Android to Windows 8: Parse XML data

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It’s more than likely that a Windows Store app will need to work with data that resides on the Internet. For example, an app may aggregate data from various news sources using RSS feeds. Consuming this type of data in a Windows Store app is very similar to consuming it in an Android app. In this post I’ll show you how parse XML data from a feed on StackOverflow.

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-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-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 Store XML Data</title>
  <category scheme="http://stackoverflow.com/feeds/tag/windows-8/tags" term="c#"/>
  <category scheme="http://stackoverflow.com/feeds/tag/windows-8/tags" term="xml"/>
  <category scheme="http://stackoverflow.com/feeds/tag/windows-8/tags" term="windows-8"/>
  <category scheme="http://stackoverflow.com/feeds/tag/windows-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 Store 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. The return type of the method should be a list of FeedEntry objects. A possibility would look like this:

public class StackOverflowXmlParser
{
 ...
 public async Task<List<FeedEntry>> Parse()
 {
  ...
 }
 ...
}

Notice that the async modifier has been added to the method and the method is return a Task which has been typed as a list of FeedEntry objects. This allows the method to use the await operator to designate suspension points. The await operator tells the compiler that the async method can’t continue past that point until the awaited asynchronous operation is complete. Additionally, this makes it very easy for calling methods to call the Parse method in an asynchronous manner by simply using the await keyword.

Android tip
Usage of the async modifier and the await operator in Windows 8 to simplify asynchronous programming is similar to the use of the AsyncTask class in Android developer

The first task in the Parse method will be to instantiate a result set to return to the calling method and check the status of network connectivity using the GetConnectionState method described in the previous section. Because this could be dealing with a large amount of data, we’ll want to make sure that we only try to access the feed if the device is on a non-metered connection:

List<FeedEntry> results = new List<FeedEntry>();

//Determine connection state
var connectionState = GetConnectionState();

//Proceed if a non-metered connection is detected
if (connectionState == ConnectionState.NonMetered)
{
 ...

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 into an XElement class asynchronously. To do this, create an HttpWebRequest instance and call its GetResponseAsync() method using the await keyword. The GetResponseStream() of the HttpWebResponse object can then be used in the Load(…) method of the XElement class.

//Load the XML data asynchronously
var request = HttpWebRequest.CreateHttp("http://stackoverflow.com/feeds/tag/windows-8");
var response = await request.GetResponseAsync();
XElement root = XElement.Load(response.GetResponseStream());

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:

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

The last step is to call the Parse method. Since the Parse method was modified with the async keyword, it can be called asynchronously by using the await operator like so:

var parser = new StackOverflowXmlParser();
var entries = await parser.Parse();
//do something with the entries

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