class SystemTest {

    class AuthorizerDoSomethingMock: Authorizer {
        private var receivedUsername: String?
        private var receivedPassword: String?
        func authorize(username: String, password: String) -> Bool {
            receivedUsername = username
            receivedPassword = password
            return true
        }

        func verify(expectedUser: User) -> Bool {
            return receivedUsername == expectedUser.username &&
                receivedPassword == expectedUser.password
        }
    }

    func testDoSomethingCallAuthorizerWithAandB() {
        // Given
        let mock = AuthorizerAcceptingMock()
        let sut = System(with: mock)

        // When
        let user = User(username: "A", password: "B")
        sut.doSomething(user: user)

        // Then
        assert(mock.verify(expectedUser: user))
    }
}