Last week, Apple updated their Swift programming language to version 4. Initially released during WWDC in 2014, Swift replaced the Objective C language developers were working with to create iOS, macOS, watchOS and tvOS applications. There's a compatibility mode that supports Swift 3 code and you can combine new and old code as the previous version and the new can peacefully coexist.
The migration process is reasonably straight forward with an option in the Edit menu of Xcode 9 or through the Issue Navigator to access the Swift Migration Assistant. Any errors or issues show up in the Report navigator as "Convert" entries.
A set of release notes goes through all the ins and outs of the update and, if you've got a spare hour up your sleeve, there's a video going through all the new tidbits in Apple's programming language of choice.
Swift Playgrounds, the iPad app that gamifies software development with Swift, was recently updated as well, to take advantage of the new feature in the updated language.