Swift Tutorial · Collections and Types · Lesson 17 of 30

Swift Extensions

Add methods, computed properties, initializers and protocol conformances to existing types.

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Extending existing types

An extension adds functionality to any type, including String, Int and types from frameworks you don't own. Extensions can add methods, computed properties, initializers, nested types and protocol conformances, but not stored properties.

Extensions with properties, methods and conformances

import Foundation

extension String {
    var slug: String {
        lowercased()
            .split(whereSeparator: { !$0.isLetter && !$0.isNumber })
            .joined(separator: "-")
    }
}

extension Int {
    var isEven: Bool { self % 2 == 0 }
    func times(_ action: () -> Void) { for _ in 0..<self { action() } }
}

struct Money { let cents: Int }

extension Money: CustomStringConvertible {
    var description: String { "EUR \(cents / 100).\(String(format: "%02d", cents % 100))" }
}

print("Swift Concurrency: A Guide!".slug)
print((1...6).filter(\.isEven))
3.times { print("hi", terminator: " ") }
print()
print(Money(cents: 1999))

Output

swift-concurrency-a-guide
[2, 4, 6]
hi hi hi 
EUR 19.99

Run this on your computer — see “Get Started” for setup.

Test yourself

What can't an extension add to a type?

Methods
Computed properties
Stored properties
Protocol conformances

Show answer

Answer: Stored properties

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