For Swift, SpriteKit, SceneKit, and Metal, Apple has provided very good documentation. All of these can be accessed from the Apple Developer Portal for free.
All the API and functions are very cleanly explained, and if you want a deeper understanding of the functions and variables used, then links are also provided for each. You can click on them and read through them in order to understand their implementation.
You can go to https://developer.apple.com/library/mac/navigation/ and search for SpriteKit or SceneKit for the respective documentation.
To learn more about Metal, you can visit the site at https://developer.apple.com/metal/:
For more information about Swift, you can visit the Apple site at https://developer.apple.com/swift/: