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XML is one of the most common formats for raw data on the Internet today. It's used for XHTML, SOAP, RSS, and many other data delivery applications. The iOS SDK provides a powerful event driven XML Parser, like many iOS Services, it's event driven, and it works with the delegate protocol. The parsing process can take some time, so we use a separate execution thread for it. This allows the application to continue to respond to touch events, while the parsing takes place in the background.
This provides a better user experience as it keeps the application from seeming unresponsive. Before we can parse the XML, we need to get it from the network using the NSURLConnection Class. As we get the data from the network, we'll be displaying the activity indicator in the status bar. This gives the user some feedback so they know that something is happening and data is coming. Using the Activity Indicator is not an excuse to block User Input. The user must still be able to use the app. They will expect the user interface to still respond to input.
XML data is parsed and added to a local database. We store the data for two reasons: one, a portable device has limited available memory, so we cannot carry around a lot of data at once. Storing data in the database freezes up valuable RAM, while keeping data readily available. And two, this gives our application data persistence. So the user can maintain a context between sessions and they don't have to read and parse the data from the net every time they need it. So we drive the app from the database.
This allows the user to use the app without having to wait for data from the net every time they need it. This is the lifecycle of the data in our application. Read XML from the net, parse it, store the results in the database, and drive the application from the data in the database. This is a very common model for a data driven application as it allows the app to provide a great user experience.
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