All of your development so far on the passer-rating projects has left out one essential consideration—one that I worried about at the end of the last chapter:
How do you know it works?
Yes, you know generally what to expect, and you’ve used the Xcode debugger to verify that what the application does makes sense, but you don’t have the time, and most people don’t have the discipline, to monitor for every possible error. Ideally, for every change to your application, you’d verify that everything still worked. With a prompt warning, you can isolate the problem to the last thing you did.
This discipline of verifying that each little part of your application works is called unit testing. The meticulous search for errors is the sort ...