class AjaxCssForm

class AjaxCssForm extends FormBase {
  
  /**
   * The file URL generator.
   *
   * @var \Drupal\Core\File\FileUrlGeneratorInterface
   */
  protected $fileUrlGenerator;
  
  /**
   * Constructs an AjaxCssForm.
   *
   * @param \Drupal\Core\File\FileUrlGeneratorInterface $file_url_generator
   *   The file URL generator.
   */
  public function __construct(FileUrlGeneratorInterface $file_url_generator) {
    $this->fileUrlGenerator = $file_url_generator;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container->get('file_url_generator'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ckeditor_test_ajax_css_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    // Create an inline and iframe CKEditor instance so we can test against
    // both.
    $form['inline'] = [
      '#type' => 'container',
      '#attached' => [
        'library' => [
          'ckeditor_test/ajax_css',
        ],
      ],
      '#children' => $this->t('Here be dragons.'),
    ];
    $form['iframe'] = [
      '#type' => 'text_format',
      '#default_value' => $this->t('Here be llamas.'),
    ];
    // A pair of buttons to trigger the AJAX events.
    $form['actions'] = [
      'css_inline' => [
        '#type' => 'submit',
        '#value' => $this->t('Add CSS to inline CKEditor instance'),
        '#ajax' => [
          'callback' => [
            $this,
            'addCssInline',
          ],
        ],
      ],
      'css_frame' => [
        '#type' => 'submit',
        '#value' => $this->t('Add CSS to iframe CKEditor instance'),
        '#ajax' => [
          'callback' => [
            $this,
            'addCssIframe',
          ],
        ],
      ],
      '#type' => 'actions',
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Nothing to do here.
  }
  
  /**
   * Generates an AJAX response to add CSS to a CKEditor Text Editor instance.
   *
   * @param string $editor_id
   *   The Text Editor instance ID.
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   *   An AJAX response.
   */
  protected function generateResponse($editor_id) {
    // Build a URL to the style sheet that will be added.
    $url = \Drupal::service('extension.list.module')->getPath('ckeditor_test') . '/css/test.css';
    $url = $this->fileUrlGenerator
      ->generateString($url);
    $response = new AjaxResponse();
    return $response->addCommand(new AddStyleSheetCommand($editor_id, [
      $url,
    ]));
  }
  
  /**
   * Handles the AJAX request to add CSS to the inline editor.
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   *   An AJAX response.
   */
  public function addCssInline() {
    return $this->generateResponse('edit-inline');
  }
  
  /**
   * Handles the AJAX request to add CSS to the iframe editor.
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   *   An AJAX response.
   */
  public function addCssIframe() {
    return $this->generateResponse('edit-iframe-value');
  }

}