Swift is Apple's new, native, fast, and easy to learn programming language for iOS, watchOS, tvOS and OS X app development. It's their "Objective-C without the C". If you are an iOS developer or planning to become one, learning Swift is your #1 priority, and Learn Swift 2 on the Mac tells you everything you need to get up to speed, well, swiftly. The language is evolving very quickly, Apple has released version 2.1 of the langugae.
You'll start with the Swift Playground and an introduction to object-oriented programming so you can immediately see Swift in action. You then learn about all of the key language features like functions, closures, protocols, classes, methods, extensions, and how Swift works just as well as Objective-C when it comes to easy memory management with ARC.
Finally, you'll learn how to use Swift alongside Objective-C as well as with Core Data, and you'll learn how to put all of the pieces together with REST base application.