Programming iOS involves communicating with Cocoa and its supplementary frameworks. The APIs for those frameworks are written in Objective-C — or in its underlying base language, C. Messages that you send to Cocoa using Swift are being translated for you into Objective-C. Objects that you send and receive back and forth across the Swift/Objective-C bridge are Objective-C objects. Some objects that you send from Swift to Objective-C are even being translated for you into other object types, or into nonobject types.
You need to understand what Objective-C expects from you when you are sending messages across the language bridge. You need to know what Objective-C is going to do with those messages. You need to know what is coming from Objective-C, and how it will be represented in Swift. Your app may even include some Objective-C code as well as Swift code, so you need to know how the parts of your own app will communicate with each other.
This appendix summarizes certain linguistic features of C and Objective-C, and describes how Swift interfaces with those features. I do not explain here how to write Objective-C! For example, I’ll talk about Objective-C methods and method declarations, because you need to know how to call an Objective-C method from Swift; but I’m not going to explain how to call an Objective-C method in Objective-C.
Objective-C is a superset of C; to put it another way, C provides the linguistic underpinnings ...