테스트에서 의존성 모의 객체 만들기

protocol UserStore {
    func name(for id: Int) -> String
}
struct RealStore: UserStore {
    func name(for id: Int) -> String { "User-\(id)" }
}
print(RealStore().name(for: 5))

protocol UserStore { func name(for id: Int) -> String }
struct MockStore: UserStore {
    func name(for id: Int) -> String { "Mock" }
}
print(MockStore().name(for: 99))

protocol UserStore { func name(for id: Int) -> String }
struct MockStore: UserStore { func name(for id: Int) -> String { "Mock" } }
struct Profile {
    let store: UserStore
    func title(_ id: Int) -> String { "Hello, " + store.name(for: id) }
}
print(Profile(store: MockStore()).title(1))

protocol Analytics { func track(_ e: String) }
class SpyAnalytics: Analytics {
    var events: [String] = []
    func track(_ e: String) { events.append(e) }
}
let spy = SpyAnalytics()
spy.track("open")
spy.track("buy")
print(spy.events)

protocol Analytics { func track(_ e: String) }
class Spy: Analytics { var last = ""; func track(_ e: String) { last = e } }
struct Checkout { let analytics: Analytics; func pay() { analytics.track("paid") } }
let spy = Spy()
Checkout(analytics: spy).pay()
print(spy.last == "paid")

protocol Clock { func hour() -> Int }
struct StubClock: Clock { let value: Int; func hour() -> Int { value } }
struct Greeter { let clock: Clock; func greet() -> String { clock.hour() < 12 ? "AM" : "PM" } }
print(Greeter(clock: StubClock(value: 9)).greet())
print(Greeter(clock: StubClock(value: 18)).greet())

protocol Loader { func load() -> Int? }
struct FailingLoader: Loader { func load() -> Int? { nil } }
struct VM { let loader: Loader; func status() -> String { loader.load() == nil ? "error" : "ok" } }
print(VM(loader: FailingLoader()).status())

protocol Random { func next() -> Int }
struct FixedRandom: Random { func next() -> Int { 42 } }
struct Game { let rng: Random; func roll() -> Int { rng.next() } }
print(Game(rng: FixedRandom()).roll())

protocol Calc { func add(_ a: Int, _ b: Int) -> Int }
struct RealCalc: Calc { func add(_ a: Int, _ b: Int) -> Int { a + b } }
struct MockCalc: Calc { func add(_ a: Int, _ b: Int) -> Int { 0 } }
print(RealCalc().add(2, 2))
print(MockCalc().add(2, 2))

protocol Repo { func count() -> Int }
struct MockRepo: Repo { func count() -> Int { 3 } }
struct Stats { let repo: Repo; func total() -> Int { repo.count() * 2 } }
print(Stats(repo: MockRepo()).total())