Head First Swift

Chapter 8. Protocols and Extensions: A Swift Lesson in Protocol

Note

You will also extend your knowledge! Get it? No? You will soon.

You know all about classes and inheritance, but Swift has a few more tricks for structuring programs, and they’re a lot Swiftier. Meet protocols and extensions. Protocols in Swift let you define a blueprint specifying methods and properties that are required for some purpose, or some piece of functionality. A protocol is adopted by a class, structure, or enumeration, and the actual implementation happens there. Types that provide the functionality needed, and adopt a protocol, are referred to as conforming to that protocol. Extensions simple let you add new functionality to existing types.

That’s a fabulous question, and we’re glad you asked.

Swift has a whole other way of building related logic and objects, using protocols and extensions. It’s very Swifty, and it’s kind of unique.

Protocols let you define a set of functionality that something can conform to, without having to provide the implementation until you conform something to it.

A protocol is kind of like a contract: if you define a protocol that requires something to have a property called color, storing a String representing a color, then any type that conforms to that protocol absolutely ...