Parsing an RSS Feed:

• startElement() - This is called by the parser every time a new XML node is found
• endElement() - This is called by the parser every time an XML node is closed (e.g. </.. )
• chars() - this is called when characters are found between nodes

Because we only really care about capturing data from the leaf nodes, our startElement() method is left empty. The chars() element has to be watched, as there is no guarantee when it will be called (e.g. in a node like hello world  this method might be called several times between the start and end) so every time we will just append the contents to a StringBuffer - that way we can be sure that we will have captured all the data in the node.  By the time the endElement() method is called, we know that we have the contents of the node itself, and we just have to store the data.  At this point, we just quickly knocked up a POJO with the attributes that we wanted to capture - the Strings that we match on are the node names from the ATOM RSS feed (that Blogger uses) - if you are using another feed, just have a quick look at the feed and update the node names appropriately.

Using our Feed in an Android App

So, that was easy right? Once that parser has run through (and you could use that code standalone in any java app really) then you will have a list of Java objects that have the core details about the latest blog posts on the feed (title, author, datecreated, content, etc) - So now lets look at using it in an Android app.

We will assume a basic understanding of the Android SDK and the concept of Fragments, so won't go back to basics with that stuff.

What we will do, is create a basic ListFragment and an RSSService class that we will use to populate the list. In our ListFragment we will simply tell our RSS service to populate the list:



Simple right?

Let's take a look at what our helpful RSS service is doing for us.



The first thing to note is that this class is extending Android's AsyncTask- The reason for this is that since Android 3.0, you are no longer able to perform network operations in the main application thread, so being as our class is going to have to fetch some RSS feeds we are going to have to spin off a new thread.

As you can see, the constructor just sets some context info that we will use later on, and then builds a progress dialogue - this is then displayed in the onPreExecute() method - This lets us show a "loading" spinning disk whilst we fetch the data.

Android's AsyncTask's primary method that handles the actual work that you want to do asynchronously is called "doInBackground()" - In our case, this is simple - we just invoke our SAX RSS parser and fetch our feed data:



Finally, we will override the "onPostExecute()" method of the async class to use our newly fetched list to populate our ListFragment.  You note how when we overrode the doInBackground() method earlier we set the return to List of Articles (where Article is my simple POJO containing my RSS blog post info) - well this must correspond to the argument of the "onPostExecute()" method, which looks like this:



Actually, all we really needed to do in this method would be pass our new List or articles to the ListFragment and notify it of the change as below:



However, in our application we have added a bit more sugar on the app - and we have actually backed the app with a simple DB that records the unique IDs of the posts and tracks whether or not they have been read to provide a nicer highlighting of listed blog posts.

So that's it - there's plenty more you can add to your RSS reader app, such as storing posts for reading later, and supporting multiple feeds/feed types - but feel free to fork the code on GitHub, or just download on to your android device to enjoy all our NerdAbility updates!

RSS application running in an Andriod  tablet emulator