From the website raywenderlich.com, an updated version of the tutorial How To Make A Simple Drawing App with UIKit and Swift.
While trying to make it work in Swift 3, I have found this repo with a full-working update, from which I have drawn examples on how to translate some parts of the code to the new version of the language: https://github.com/dragosholban/DrawPad.
This version works fine with Swift 4.2, anyway just before publishing my version of the tutorial I have discovered a more advanced version here: UIKit Drawing Tutorial: How to Make a Simple Drawing App.