class SystemTest {

    class AuthorizerDoSomethingMock: Authorizer {
        var authorizeCalled = false
        func authorize(username: String, password: String) -> Bool {
            authorizeCalled = true
            return true
        }

        func verify() -> Bool {
            return authorizeCalled
        }
    }

    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())
    }
}