All of your development so far on the passer-rating projects has left out one essential consideration:
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 nothing broke. With a prompt warning, you can isolate the problem to the last thing you did.
This discipline of verifying each little part of your application is called unit testing. The meticulous search for errors is the sort of mind-numbing, repetitive, perfectionist task you bought a computer to do. ...