XML is a highly flexible and widely used general-purpose text file format for storage and retrieval of structured data. You might use it yourself to store data that you’ll need to retrieve later, or you could encounter it when obtaining information from elsewhere, such as the Internet.

Mac OS X Cocoa provides a set of classes (NSXMLDocument and so forth) for reading, parsing, maintaining, searching, and modifying XML data in a completely general way, but iOS does not include these. I think the reason must be that their tree-based approach is too memory-intensive. Instead, iOS provides NSXMLParser, a much simpler class that walks through an XML document, sending delegate messages as it encounters elements. With this, you can parse an XML document once, but what you do with the pieces as they arrive is up to you. The general assumption here is that you know in advance the structure of the particular XML data you intend to read and that you have provided classes for storage of the same data in object form and for transforming the XML pieces into that storage.

To illustrate, let’s return to our earlier example of a Person class with a firstName and a lastName property. Imagine that as our app starts up, we would like to populate it with Person objects, and that we’ve stored the data describing these objects as an XML file in our app bundle, like this:

<?xml version="1.0" encoding="utf-8"?> <people> <person> <firstName>Matt</firstName> <lastName>Neuburg</lastName> </person> <person> ...

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