Existing APIs for XML processing fall into two categories:

• SAX (Simple API for XML) is a serial access parser API for XML. It provides a mechanism for reading data from an XML document. The quantity of memory that a SAX parser must use in order to function is typically much smaller than that of a DOM parser. Unlike DOM, there is no formal specification for SAX.
• The Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. It represents a tree structure based API. The Dom parser implements the DOM API and it creates a DOM tree in memory for a XML document. Because DOM supports navigation in any direction (e.g., parent and previous sibling) and allows for arbitrary modifications, an implementation must at least buffer the document that has been read so far (or some parsed form of it).

Cocoa offers a “complete” XML parser with the NSXML family of classes.

• NSXMLParser is a SAX parser, meaning that it traverses the XML tree and informs a delegate of events as they happen. This is an event-driven parser, which calls methods on a delegate to handle “events” as it parses through the XML.
• NSXMLDocument provides tree parser functionality. An instance of NSXMLDocument represents an XML document as internalized into a logical tree structure. This tree-based parser is fairly sophisticated but it is not included in Cocoa-Touch.

In the spirit of small and simple, iPhone developers have the NSXMLParser class to use. Because NSXMLParser only generates messages for a delegate, and doesn’t store data about the XML tree, it requires less memory and is better for accessing small pieces of data or parsing large XML documents. The basic operation of the NSXMLParser class will not be covered here. This is well documented in the Apple Programming Guide.

For devices like iPhone where memory usage is critical, using tree parser is not recommended. I guess that is why Apple doesn’t provide NSXMLDocument on iPhone/iPod touch. However, the model of event parsing may not be acceptable by your code. NSXMLParser is probably adequate, but it is not going to handle all your XML parsing needs. The biggest problem with this type of streaming parser is that you lose the structure of your data; your call-backs do not provide any context or hierarchy. Because of the above mentioned or by some other reason like often changing XML format [1] you may need to use something like DOM model implementation.

Available Libraries (DOM model implementations) for iPhone

• XPath/XQuery – used to navigate through elements and attributes in an XML document.
• Libxml2 – XML C parser and toolkit developed for the Gnome project. At Cocoa with Love, very nice explanation and code sample to write wrapper for the libxml2 was posted. It has a title, “Using libxml2 for XML parsing and XPath queries in Cocoa”[2]
• KissXML,TouchXML – two tree XML parsers for iPhone.
• APXML – not a perfect implementation of the W3C XML 1.0 standard, but it’s close enough for a lot of usage.
• XMLTreeParser/XMLTreeNode -  Enhancing the standard NSXMLParser class

Which parser (DOM or SAX) should you use?

Well, if you’re going to need to parse very large XML documents, then you should probably consider NSXMLParser. What “very large” means depends a lot on the available memory you have to work with? The model of iPhone and the memory already in use by your app may prevent you from being able to use DOM model.

While DOM model usage may seem easier, NSXMLParser really excels in situations where you’re loading in many instances of the same kind of data, such as an address book or twitter feed. However, if memory is not an issue, I would recommend DOM model. It makes it easy to access the data you want and also allows you to manipulate the tree.

References

1. Arash Payan(Salman),APXML: NSXMLDocument ’substitute’ for iPhone/iPod Touch – Arash Payan,Sep 27th, 2009
2. jongampark, Tree XML parser for iPhone, 2009