class ListPlugin

class ListPlugin extends CKEditor5PluginDefault implements CKEditor5PluginConfigurableInterface, CKEditor5PluginElementsSubsetInterface {
  use CKEditor5PluginConfigurableTrait;
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'reversed' => TRUE,
      'startIndex' => TRUE,
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['reversed'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Allow the user to reverse an ordered list'),
      '#default_value' => $this->configuration['reversed'],
    ];
    $form['startIndex'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Allow the user to specify the start index of an ordered list'),
      '#default_value' => $this->configuration['startIndex'],
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    $form_value = $form_state->getValue('reversed');
    $form_state->setValue('reversed', (bool) $form_value);
    $form_value = $form_state->getValue('startIndex');
    $form_state->setValue('startIndex', (bool) $form_value);
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['reversed'] = $form_state->getValue('reversed');
    $this->configuration['startIndex'] = $form_state->getValue('startIndex');
  }
  
  /**
   * {@inheritdoc}
   */
  public function getDynamicPluginConfig(array $static_plugin_config, EditorInterface $editor) : array {
    $static_plugin_config['list']['properties'] = array_merge($static_plugin_config['list']['properties'], $this->getConfiguration());
    return $static_plugin_config;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getElementsSubset() : array {
    $subset = $this->getPluginDefinition()
      ->getElements();
    $subset = array_diff($subset, [
      '<ol reversed start>',
    ]);
    $reversed_enabled = $this->getConfiguration()['reversed'];
    $start_index_enabled = $this->getConfiguration()['startIndex'];
    $subset[] = "<ol" . ($reversed_enabled ? ' reversed' : '') . ($start_index_enabled ? ' start' : '') . '>';
    return $subset;
  }

}