namespace Drupal\num\Form;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\HtmlCommand;
use Drupal\Core\Session\AccountInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Implementing a numerology calculator form.
 */
class TestForm extends FormBase {
   
  /**
   * @var AccountInterface $account
   */
  protected $account;
   
  /**
   * Class constructor.
   */
  public function __construct(AccountInterface $account) {
    $this->account = $account;
  }
 
   /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    // Instantiates this form class.
    return new static(
      // Load the service required to construct this class.
      $container->get('current_user')
    );
  }
 
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'test_form';
  }

  /**
   * {@inheritdoc}.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

     $form['actions']['submit'] = [
        '#id' => 'button',
        '#type' => 'submit',
        '#value' => $this->t('Определить'),
        '#ajax' => [
        'callback' => '::getID',
         ],
    ];
     
    $form['message'] = [
        '#type' => 'markup',
        '#markup' => '<div class="result"></div>',
    ];
     
    return $form;
  }

  public function getID(array $form, FormStateInterface $form_state) {
     
        $uid = $this->account->id();

        $response = new AjaxResponse();
        $response->addCommand(
        new HtmlCommand(
        '.result',  
            '<div class="my_mess_notlink">' .'UID - Пользователя' . $uid . '</div>'
                       ),);    
        return $response;
  }

  /**
   * Submitting the form.
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  }

}