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');
    }

}