class StylesCombo

class StylesCombo extends CKEditorPluginBase implements CKEditorPluginConfigurableInterface {
    
    /**
     * {@inheritdoc}
     */
    public function isInternal() {
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFile() {
        // This plugin is already part of Drupal core's CKEditor build.
        return FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getConfig(Editor $editor) {
        $config = [];
        $settings = $editor->getSettings();
        if (!isset($settings['plugins']['stylescombo']['styles'])) {
            return $config;
        }
        $styles = $settings['plugins']['stylescombo']['styles'];
        $config['stylesSet'] = $this->generateStylesSetSetting($styles);
        return $config;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getButtons() {
        return [
            'Styles' => [
                'label' => $this->t('Font style'),
                'image_alternative' => [
                    '#type' => 'inline_template',
                    '#template' => '<a href="#" role="button" aria-label="{{ styles_text }}"><span class="ckeditor-button-dropdown">{{ styles_text }}<span class="ckeditor-button-arrow"></span></span></a>',
                    '#context' => [
                        'styles_text' => $this->t('Styles'),
                    ],
                ],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state, Editor $editor) {
        // Defaults.
        $config = [
            'styles' => '',
        ];
        $settings = $editor->getSettings();
        if (isset($settings['plugins']['stylescombo'])) {
            $config = $settings['plugins']['stylescombo'];
        }
        $form['styles'] = [
            '#title' => $this->t('Styles'),
            '#title_display' => 'invisible',
            '#type' => 'textarea',
            '#default_value' => $config['styles'],
            '#description' => $this->t('A list of classes that will be provided in the "Styles" dropdown. Enter one or more classes on each line in the format: element.classA.classB|Label. Example: h1.title|Title. Advanced example: h1.fancy.title|Fancy title.<br />These styles should be available in your theme\'s CSS file.'),
            '#attached' => [
                'library' => [
                    'ckeditor/drupal.ckeditor.stylescombo.admin',
                ],
            ],
            '#element_validate' => [
                [
                    $this,
                    'validateStylesValue',
                ],
            ],
        ];
        return $form;
    }
    
    /**
     * #element_validate handler for the "styles" element in settingsForm().
     */
    public function validateStylesValue(array $element, FormStateInterface $form_state) {
        $styles_setting = $this->generateStylesSetSetting($element['#value']);
        if ($styles_setting === FALSE) {
            $form_state->setError($element, $this->t('The provided list of styles is syntactically incorrect.'));
        }
        else {
            $style_names = array_map(function ($style) {
                return $style['name'];
            }, $styles_setting);
            if (count($style_names) !== count(array_unique($style_names))) {
                $form_state->setError($element, $this->t('Each style must have a unique label.'));
            }
        }
    }
    
    /**
     * Builds the "stylesSet" configuration part of the CKEditor JS settings.
     *
     * @see getConfig()
     *
     * @param string $styles
     *   The "styles" setting.
     *
     * @return array|false
     *   An array containing the "stylesSet" configuration, or FALSE when the
     *   syntax is invalid.
     */
    protected function generateStylesSetSetting($styles) {
        $styles_set = [];
        // Early-return when empty.
        $styles = trim($styles);
        if (empty($styles)) {
            return $styles_set;
        }
        $styles = str_replace([
            "\r\n",
            "\r",
        ], "\n", $styles);
        foreach (explode("\n", $styles) as $style) {
            $style = trim($style);
            // Ignore empty lines in between non-empty lines.
            if (empty($style)) {
                continue;
            }
            // Validate syntax: element[.class...]|label pattern expected.
            if (!preg_match('@^ *[a-zA-Z0-9]+ *(\\.[a-zA-Z0-9_-]+ *)*\\| *.+ *$@', $style)) {
                return FALSE;
            }
            // Parse.
            list($selector, $label) = explode('|', $style);
            $classes = explode('.', $selector);
            $element = array_shift($classes);
            // Build the data structure CKEditor's stylescombo plugin expects.
            // @see https://ckeditor.com/docs/ckeditor4/latest/guide/dev_howtos_styles.html
            $configured_style = [
                'name' => trim($label),
                'element' => trim($element),
            ];
            if (!empty($classes)) {
                $configured_style['attributes'] = [
                    'class' => implode(' ', array_map('trim', $classes)),
                ];
            }
            $styles_set[] = $configured_style;
        }
        return $styles_set;
    }

}