class AjaxFormBlock

class AjaxFormBlock extends BlockBase implements FormInterface, ContainerFactoryPluginInterface {
  
  /**
   * The form builder.
   *
   * @var \Drupal\Core\Form\FormBuilderInterface
   */
  protected $formBuilder;
  
  /**
   * The messenger.
   *
   * @var \Drupal\Core\Messenger\MessengerInterface
   */
  protected $messenger;
  
  /**
   * Constructs a new AjaxFormBlock.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
   *   The form builder.
   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
   *   The messenger.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, FormBuilderInterface $form_builder, MessengerInterface $messenger) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->formBuilder = $form_builder;
    $this->messenger = $messenger;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('form_builder'), $container->get('messenger'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    return $this->formBuilder
      ->getForm($this);
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ajax_forms_test_block';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['test1'] = [
      '#type' => 'select',
      '#title' => $this->t('Test 1'),
      '#required' => TRUE,
      '#options' => [
        'option1' => $this->t('Option 1'),
        'option2' => $this->t('Option 2'),
      ],
      '#ajax' => [
        'callback' => '::updateOptions',
        'wrapper' => 'edit-test1-wrapper',
      ],
      '#prefix' => '<div id="edit-test1-wrapper">',
      '#suffix' => '</div>',
    ];
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];
    return $form;
  }
  
  /**
   * Updates the options of a select list.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   *
   * @return array
   *   The updated form element.
   */
  public function updateOptions(array $form, FormStateInterface $form_state) {
    $form['test1']['#options']['option1'] = $this->t('Option 1!!!');
    $form['test1']['#options'] += [
      'option3' => $this->t('Option 3'),
      'option4' => $this->t('Option 4'),
    ];
    return $form['test1'];
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->messenger
      ->addStatus('Submission successful.');
  }

}