class SystemTest {

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

    func testDoSomethingCallAuthorizerWithUsernameAndPassword() {
        // Given
        let spy = AuthorizerSpy()
        let sut = System(with: spy)

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

        // Then
        assert(spy.receivedUsername == "A")
        assert(spy.receivedPassword == "B")
    }
}