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('');
        }
    }

}