I love the dark theme transitions in Tweetbot and Castro, and until now those animations were difficult to recreate. This great library makes it easy to add this type of animation to your own app. Although Gagat does not specify how to write your styling logic, the sample project demonstrates a good way to approach this problem.
The announcement of the new CoreML and Vision frameworks sounded really interesting, but I assumed they would be too specialized to be accessible to the average developer. This article by Matthijs Hollemans breaks down these frameworks and includes a sample project to show how simple CoreML actually is. I also recommend reading Otto Schnurr's short blog post which explains why CoreML is such a big deal.
UIFontMetrics, a new type in iOS 11, gives developers several new tools to adapt user interfaces and custom fonts to the user's Dynamic Type settings. In the past, there was no simple way to make these changes to an app's appearance. This is a nice change that will simplify code and improve the user experience in apps.