Callbacks.php

<?php

namespace Drupal\form_test;

use Drupal\Core\Form\FormStateInterface;

/**
 * Simple class for testing methods as Form API callbacks.
 */
class Callbacks {

  /**
   * Form element validation handler for 'name' in form_test_validate_form().
   */
  public function validateName(&$element, FormStateInterface $form_state) {
    $triggered = FALSE;
    if ($form_state
      ->getValue('name') == 'element_validate') {

      // Alter the form element.
      $element['#value'] = '#value changed by #element_validate';

      // Alter the submitted value in $form_state.
      $form_state
        ->setValueForElement($element, 'value changed by setValueForElement() in #element_validate');
      $triggered = TRUE;
    }
    if ($form_state
      ->getValue('name') == 'element_validate_access') {
      $form_state
        ->set('form_test_name', $form_state
        ->getValue('name'));

      // Alter the form element.
      $element['#access'] = FALSE;
      $triggered = TRUE;
    }
    elseif ($form_state
      ->has('form_test_name')) {

      // To simplify this test, just take over the element's value into $form_state.
      $form_state
        ->setValueForElement($element, $form_state
        ->get('form_test_name'));
      $triggered = TRUE;
    }
    if ($triggered) {

      // Output the element's value from $form_state.
      \Drupal::messenger()
        ->addStatus(t('@label value: @value', [
        '@label' => $element['#title'],
        '@value' => $form_state
          ->getValue('name'),
      ]));

      // Trigger a form validation error to see our changes.
      $form_state
        ->setErrorByName('');
    }
  }

}