Minimal Cake Pattern

mcake.swift

import Foundation

// See also: https://qiita.com/pab_tech/items/1c0bdbc8a61949891f1f

protocol Clock {
    var now: Date { get }
}

struct SystemClock: Clock {
    let now: Date = Date()
}

/// Uses
protocol UsesClock {
    var clock: Clock { get }
}

/// MixIn
protocol MixInClock: UsesClock {}
extension MixInClock {
    var clock: Clock { return SystemClock() }
}

/// Service
protocol ClockService: UsesClock {}

/// Production

struct SystemClockService: ClockService, MixInClock {
    static let shared = SystemClockService()
    private init () {}
}

/// Test

protocol MockMixInClock: UsesClock {}
extension MockMixInClock {
    var clock: Clock { return MockClock() }
}

struct MockClock: Clock {
    var now: Date { return Date(timeIntervalSince1970: 0) }
}

struct MockClockService: ClockService, MockMixInClock {
    static let shared = MockClockService()
    private init () {}
}

/// Run

func printDate(service: ClockService) {
    print(service.clock.now)
}

printDate(service: SystemClockService.shared)
printDate(service: MockClockService.shared)