I set out a couple of weeks ago to port the Killer Deals
application from the iPhone 2.2.1 SDK to 3.0. “No problem”, I thought; the
application doesn’t veer far from the standard APIs and controls, and it is a
relatively straightforward application, implementation-wise.

As I was making my updates (some minor method and property
name changes here and there), I stumbled upon a show stopper: The NSXMLParser
was no longer parsing my XML data. After scratching my head for a few hours, I
finally figured out the problem.

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There are times when building an iPhone application that you
need to inform other parts of the program to do something, without regard to
what or where those parts are. The NSNotificationCenter (aka “notification
center”) is the way to go in these cases. Using notifications supports ‘loose
coupling’, which is a good thing in software design.

A notification center object provides a mechanism for
broadcasting information (“notifications”) within a task. An
NSNotificationCenter object is basically a notification dispatch table. For
those familiar with Windows programming, this pattern is similar to the Windows
Message Loop architecture.

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