You are an iOS programmer, and you’ve chosen to adopt Apple’s new language, Swift. And this means that you’ll never have to concern yourself with Apple’s old language, Objective-C, right? Wrong.
Objective-C is not dead. Far from it. You may be using Swift, but Cocoa is not. 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. Some APIs cannot be satisfactorily used with Swift alone, so your app may even have to include some Objective-C code.
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 ...