Swift documentation
Apple and the Swift community, as a whole, have released a number of resources to help developers program in Swift. Apple's official documentation, which can be found at https://developer.apple.com/documentation/, includes API documentation for Swift as well as all of Apple's frameworks. Only a small portion of Apple's frameworks are open source and work across all platforms; however if you are looking to get started on one of Apple's frameworks this is definitely the place to start. However it can be hard to find documentation on specific Swift APIs.
To quickly find documentation on Swift's API, my favorite site is https://swiftdoc.org. This site is incredibly easy to navigate and has autogenerated documentation for all types, protocols, operators, and global functions that make up the Swift language. I have noticed that this site does not always stay up to date; however, it is a great reference for any Swift developer. The code that...