Notice: Undefined index: understanding-swift in /var/sites/ on line 89
When are protocol extensions useful in Swift? - a free tutorial

NEW: Subscribe to Hacking with Swift+ and accelerate your learning! >>

When are protocol extensions useful in Swift?

Paul Hudson    @twostraws   

Protocol extensions are used everywhere in Swift, which is why you’ll often see it described as a “protocol-oriented programming language.” We use them to add functionality directly to protocols, which means we don’t need to copy that functionality across many structs and classes.

For example, Swift’s arrays have an allSatisfy() method that returns true if all the items in the array pass a test. So, we could create an array of numbers and check that they are all even:

let numbers = [4, 8, 15, 16]
let allEven = numbers.allSatisfy { $0.isMultiple(of: 2) }

That’s really useful, but wouldn’t it be more useful if it also worked on sets? Sure it would, and that’s why it does:

let numbers2 = Set([4, 8, 15, 16])
let allEven2 = numbers2.allSatisfy { $0.isMultiple(of: 2) }

The underlying principle is identical: pass each item in the array or set through a test you provide, and if that returns true for all items then the result of the method is true.

How about dictionaries – can they use this too? Sure they can, and it works identically: each key/value pair gets passed into the closure, and you need to return either true or false. It looks like this:

let numbers3 = ["four": 4, "eight": 8, "fifteen": 15, "sixteen": 16]
let allEven3 = numbers3.allSatisfy { $0.value.isMultiple(of: 2) }

Of course, the Swift developers don’t want to write this same code again and again, so they used a protocol extension: they wrote a single allSatisfy() method that works on a protocol called Sequence, which all arrays, sets, and dictionaries conform to. This meant the allSatisfy() method immediately became available on all those types, sharing exactly the same code.

Hacking with Swift is sponsored by NSSpain

SPONSORED Announcing NSSpain 2020: Remote Edition! An online, continuous conference for iOS developers. We’ll start on Thursday and finish on Friday, with talks, activities, and lots of fun for 36 hours, non-stop. Sound good? Join us!

Find out more

Sponsor Hacking with Swift and reach the world's largest Swift community!

Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Link copied to your pasteboard.