class AjaxFormsTestValidationForm

class AjaxFormsTestValidationForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ajax_forms_test_validation_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['driver_text'] = [
      '#title' => $this->t('AJAX-enabled textfield.'),
      '#description' => $this->t("When this one AJAX-triggers and the spare required field is empty, you should not get an error."),
      '#type' => 'textfield',
      '#default_value' => $form_state->getValue('driver_text', ''),
      '#ajax' => [
        'callback' => [
          Callbacks::class,
          'validationFormCallback',
        ],
        'wrapper' => 'message_area',
        'method' => 'replaceWith',
      ],
      '#suffix' => '<div id="message_area"></div>',
    ];
    $form['driver_number'] = [
      '#title' => $this->t('AJAX-enabled number field.'),
      '#description' => $this->t("When this one AJAX-triggers and the spare required field is empty, you should not get an error."),
      '#type' => 'number',
      '#default_value' => $form_state->getValue('driver_number', ''),
      '#ajax' => [
        'callback' => [
          Callbacks::class,
          'validationNumberFormCallback',
        ],
        'wrapper' => 'message_area_number',
        'method' => 'replaceWith',
      ],
      '#suffix' => '<div id="message_area_number"></div>',
    ];
    $form['spare_required_field'] = [
      '#title' => $this->t("Spare Required Field"),
      '#type' => 'textfield',
      '#required' => TRUE,
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->messenger()
      ->addStatus($this->t("Validation form submitted"));
  }

}