Accidently disconnected outlets in shipping Cocoa apps are legend.
— Jonathan “Wolf” Rentzsch
From what I can tell assert() is slowly going the way of the goto in the programming world. Exceptions, unit-tests, and other modern software engineering practices, seem to have a better answer for testing something at runtime, and ensuring that you never enter a bad state.
But there is one case where I think you should put an assert() (and not an NSAssert()) in your code: in awakeFromNib assert() that every IBOutlet is connected.
It’s surprisingly easy to accidentally disconnect something in Interface Builder, or rename something in Xcode. I’ve done it before. More then once. assert()-ing IBOutlets has saved me a lot of debugging time.
Hard to Find In Code
An IBOutlet that is not connected is nil. Because Objective-C quietly ignores messages sent to nil, it’s very easy not to notice the problem for a while; then spend a long time debugging a side effect of the issue.
Hard to Find By Eye
A widget in a nib/xib file that is not connected will never change it’s state. But it typically it has a reasonable initial state. This makes it difficult to detect disconnected IBOutlets by eye, because things will look right until they are supposed have changed, but the eye is drawn to change, and de-emphasizes unchanging things.
Why assert()?
As Wolf says, the worst-case-scenario is that right before release you make a trivial change in Interface Builder (“We can’t ship with that window saying “Claculator”), an IBOutlet gets disconnected, and nobody notices in time.
But an assert() failing will be noticed with casual testing, and triggered if the nib is loaded at all, even if the series of interactions needed to cause the widget to change state are not performed.
An assert() is very light-weight and easy to do. It’s exactly one line of code, and you don’t have to add a unit testing framework to your project to do it. And that makes it perfect for test projects. I’m a believer in getting a feature working in a test project first, then copying it into your real product. (Honestly I don’t do it as much as I should, and I almost always kick myself for it.) It lets you test and learn without hacking-up your product. Plus, new things are often easier to try without the weight of a big code base.
Speaking of unit tests, I don’t have complete faith in them here. That’s because a disconnected IBOutlet in a shipping program is fundamentally an issue with the way the release-build is configured. So anything that’s not testing the the actual release build is not exhaustively testing for this problem. an OCUnit-test can catch a disconnected IBOutlet if it is loaded into the application. But in my experience, such tests are cumbersome. Running them involves launching your full application, and programatically manipulating it. This can take a while to run since all the UI will be displayed and animated. And you have to be careful about state since each test will be changing the state of your program.
Ultimately, assert() just seems to be optimal, in dependability and simplicity, for catching a disconnected IBOutlet.
But I suspect there’s a better way I don’t know.
How do you catch IBOutlets?