100 Days of Swift by Hacking with Swift is a free, daily course teaching iOS development with Swift. Paul Hudson walks through 100 lessons covering Swift syntax, UIKit (or SwiftUI), and building real iOS apps. The entire course is free on hackingwithswift.com. After finishing, you can build iOS apps and understand Swift. The main limitation: the course is self-paced over 100 days, so it requires discipline. Also, iOS development requires a Mac and Xcode, limiting accessibility.
Swift is used by 5.7% of professional developers and is the primary language for iOS, macOS, and Apple ecosystem development (source: Stack Overflow Developer Survey 2025).