This is a nice collection of the changes in Swift 3.1. From language changes to a big Swift Package Manager update, this post will keep you up to date on what's new.
Support is also ending for Swift 2.3, so if you haven't updated that code, you need to soon.
What’s New in Swift 3.1? by Cosmin Pupăză
With many of Apple's APIs singleton based, it is difficult to fully test your code. John walks us through an efficient way to test singletons that are out of your control. He shows us a normal example, then converts that to code that is easily testable.
Why worry about testing singletons?
Singletons are also a quite common source of bugs, where state ends up being shared and mutations not propagated properly throughout the system
Testing Swift code that uses system singletons in 3 easy steps by John Sundell