From Hello Swift! by Tanmay Bakshi.

Hello Swift! is a how-to guide to programming iOS Apps with the Swift language, written from a kid’s perspective. This approachable, well-illustrated, step-by-step guide takes you from very beginning programming concepts through developing complete apps. You’ll begin by learning the basics of what an App is and what you need to build one. From there, you’ll create your first simple App before learning more about programming with Swift. Along the way, you’ll conquer important programming concepts that you can use with other programming languages. By the end of this book you’ll have everything you need to dive into the world of iOS App Development. This animation displays string reversal logic.