Protocols and Extensions

Protocol-oriented programming is a cornerstone of Swift. This lesson covers protocols, protocol conformance, extensions, and how they combine to create flexible, composable designs.

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Protocols

A protocol defines a blueprint of methods, properties, and other requirements:

protocol Describable {
    var description: String { get }
    func summarise() -> String
}

Conforming to a Protocol

struct Person: Describable {
    var name: String
    var age: Int

    var description: String {
        "\(name), age \(age)"
    }

    func summarise() -> String {
        "Person: \(description)"
    }
}

let alice = Person(name: "Alice", age: 30)
print(alice.summarise())  // "Person: Alice, age 30"

Property Requirements

protocol Named {
    var name: String { get }        // read-only requirement
}

protocol Editable {
    var content: String { get set } // read-write requirement
}

Method Requirements

protocol Toggleable {
    mutating func toggle()
}

struct LightSwitch: Toggleable {
    var isOn = false

    mutating func toggle() {
        isOn.toggle()
    }
}

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Protocol Conformance for Multiple Types

protocol Area {
    var area: Double { get }
}

struct Circle: Area {
    var radius: Double
    var area: Double { .pi * radius * radius }
}

struct Rectangle: Area {
    var width: Double
    var height: Double
    var area: Double { width * height }
}

// Use the protocol as a type
let shapes: [Area] = [
    Circle(radius: 5),
    Rectangle(width: 4, height: 6)
]

for shape in shapes {
    print("Area: \(shape.area)")
}

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Protocol Inheritance

Protocols can inherit from other protocols:

protocol Identifiable {
    var id: String { get }
}

protocol Named {
    var name: String { get }
}

protocol User: Identifiable, Named {
    var email: String { get }
}

struct AppUser: User {
    var id: String
    var name: String
    var email: String
}

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Extensions

Extensions add new functionality to existing types without modifying their source code:

extension Int {
    var isEven: Bool { self % 2 == 0 }
    var isOdd: Bool { !isEven }
    var squared: Int { self * self }
}

print(4.isEven)    // true
print(7.squared)   // 49

Adding Methods

extension String {
    func reversed() -> String {
        String(self.reversed() as ReversedCollection<String>)
    }

    var wordCount: Int {
        self.split(separator: " ").count
    }
}

print("Hello World".wordCount)  // 2

Adding Protocol Conformance via Extensions