🔶 Swift LanguageIntermediateSwift 2.0+

Protocols & Protocol Extensions

Define capabilities as contracts, add default behavior with extensions, and use some/any correctly.

A protocol describes what a type can do. Structs, classes and enums can all conform, which enables protocol-oriented design and easy test doubles.

Protocol extensions provide default implementations. Use some Protocol for an opaque concrete type (fast, static dispatch) and any Protocol when you truly need a heterogeneous box.

protocols.swift

1protocol WeatherService {
2    func temperature(in city: String) async throws -> Double
3}
4
5struct LiveWeatherService: WeatherService {
6    func temperature(in city: String) async throws -> Double {
7        // call your API here
8        return 31.5
9    }
10}
11
12struct MockWeatherService: WeatherService {
13    func temperature(in city: String) async throws -> Double { 20 }
14}
15
16extension WeatherService {
17    func summary(for city: String) async -> String {
18        let t = (try? await temperature(in: city)) ?? .nan
19        return "\(city): \(t)°C"
20    }
21}
22
23func makeService(preview: Bool) -> any WeatherService {
24    preview ? MockWeatherService() : LiveWeatherService()
25}

Common pitfalls

Prefer some over any when a single concrete type is returned; any adds boxing and dynamic dispatch.

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Common pitfalls

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