There is a free ebook out there that teaches you the basics of Combine, Apple's reactive framework introduced at WWDC 2019. It is pretty long already, given the time author Joseph Heck (@heckj) had to learn about Combine and then write about it. From what I saw, I think it's a good introduction to reactive programming in general.
Seriously, you should read it: https://heckj.github.io/swiftui-notes/
And the book's source is available, too! https://github.com/heckj/swiftui-notes
From the repository's name,
swiftui-notes, and this description:
A collection of notes, project pieces, playgrounds and ideas on learning and using SwiftUI and Combine.
… I can only assume it gets expanded even more over time to include both SwiftUI and Combine!
Check it out and share it; Joe Heck deserves some internet love for his efforts.