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 }
}