How to check your program state using precondition()

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

You should already know that the assert() function lets you check the state of your program at runtime, and crash if things aren’t how they should be. One of the neat features of assert() is that it automatically gets removed when you build your app in release mode, but if you don’t want that to happen – if you want your app to crash if something is seriously wrong – then you should use the precondition() function instead.

precondition() works identically to assert(): give it a condition to check along with an optional message to print if the check fails. At runtime – even in release mode – Swift will run the check for you and crash if the check fails.

For example:

precondition(users.count > 0, "There must be at least one user.")

Whereas assert() should be used liberally to make sure your program is always in a sane state, precondition() should be used much more infrequently because it will still crash your app in release mode. That’s not always a bad thing, particularly if bad state means something is seriously corrupted, but you should definitely use it more carefully.