dontDoThisEither.m

- (void)textFieldDidChange:(UITextField *)sender {
    // update the view-model
    self.viewModel.username = sender.text;
    // check if things are now valid
    self.goButton.enabled = self.viewModel.isUsernameValid;
    self.goButton.alpha = self.viewModel.isUsernameValid ? 1.0 : 0.5;
}