"The Case of the Crimson Test Suite" is an introduction to the modern Swift Testing framework.

Testing your packages and apps for Apple platforms has never been easier. The new Swift Testing framework makes it easy for you to write tests that are clean and clear with easy-to-read feedback when a test fails. In this book we begin with tests that run as freestanding functions. Later, we gather some of them up into a suite so that we can run a test individually, as part of a suite, or as one of the entire tests in the current test plan.

You'll write expectations and requirements to test the result of calling your production code and you'll verify methods that should and do throw an error. You'll test asynchronous code and, in those rare cases when its needed, write serialized tests. By default Swift Testing runs your tests in parallel in random order for both improved performance and to catch hidden dependencies. You'll create display names and use test description to improve the test output and introduce tags so that you can run a subset of your tests without suites or across many suites.

Requires Xcode 16 (beta 5 or later) and Swift 6.