Unit testing refers to a software validation methodology that allows programmers to test individual program units for correctness. It's been an ongoing question in the iPhone developer community as to whether the iPhone's view controller class is testable or not.

In response to these discussions, iPhone developer Jonah Williams has written up a view controller unit testing how-to over at the Carbon Five web blog. His write-up offers examples that show how to incorporate some best practices into your code.

Williams points out how broken NIB bindings are a common problem for iPhone OS applications. To address these issues, he regularly adds simple assertions that test that each IB outlet and action are set properly from inside his view controller class implementations. These assertions check that IBOutlet instance variables are not set to nil and that IBAction targets have been assigned, adding a layer of protection against broken bindings.

Another typical view controller issue involves responding to application memory warnings. To respond, he adds tests that ensure that each view-dependent property gets correctly released and re-created as views unload and then later reload. By building these into test methods, he can execute this behavior on demand, and ensure that the sequence will execute flawlessly in real world conditions.

Finally, Williams discusses view controller interdependencies. Often instances are tightly intertwined, with objects acting as clients for each other. For example, a simple table view controller, living within a navigation controller, might present a detail view via yet another view controller when a row is selected. That's three separate controllers to account for, when you really only want to test one at a time. Williams suggests isolating these view controllers away from their interdependencies to test each component separately and provides examples of how you can do so.

What made Williams' approach pop for me is how he carefully exposes and isolates dependencies for testing. These are features that can otherwise be hard to inspect and validate in the normal course of programming. His write-up is well worth reading through, and provides an excellent jumping off point for investigating view controller unit testing.