iOS App development & Swift Links:

Apple Developer Forum
https://forums.developer.apple.com/welcome
Apple API Reference
https://developer.apple.com/reference/
Apple Guides and Sample Code
https://developer.apple.com/library/content/navigation/

Swift book:
https://goo.gl/pJzqTJ

StackOverflow
https://stackoverflow.com/questions/tagged/ios

Core library:
UIKit
https://developer.apple.com/reference/uikit

Foundation
https://developer.apple.com/reference/foundation

Swift Standard Library
https://developer.apple.com/reference/swift