class TestAjaxBlock

class TestAjaxBlock extends BlockBase {
  
  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $form['ajax_test'] = [
      '#type' => 'radios',
      '#options' => [
        1 => $this->t('Ajax test option 1'),
        2 => $this->t('Ajax test option 2'),
      ],
      '#prefix' => '<div id="test-ajax-wrapper">',
      '#suffix' => '</div>',
      '#title' => $this->t('Time in this ajax test is @time', [
        '@time' => time(),
      ]),
      '#ajax' => [
        'wrapper' => 'test-ajax-wrapper',
        'callback' => [
          $this,
          'ajaxCallback',
        ],
      ],
    ];
    return $form;
  }
  
  /**
   * Ajax callback.
   */
  public function ajaxCallback($form, $form_state) {
    return $form['settings']['ajax_test'];
  }
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    $build['content'] = [
      '#markup' => $this->t('Every word is like an unnecessary stain on silence and nothingness.'),
    ];
    return $build;
  }

}