in snippets/android-ios-tutorial/ContentView.swift [103:107]
func loginDataChanged(username: String, password: String) {
formState = LoginFormState(
usernameError: (loginValidator.checkUsername(username: username) as? LoginDataValidator.ResultError)?.message,
passwordError: (loginValidator.checkPassword(password: password) as? LoginDataValidator.ResultError)?.message)
}