abstract class ConfigFactoryOverrideBase

abstract class ConfigFactoryOverrideBase implements EventSubscriberInterface {

  /**
   * Reacts to the ConfigCollectionEvents::COLLECTION_INFO event.
   *
   * @param \Drupal\Core\Config\ConfigCollectionInfo $collection_info
   *   The configuration collection info event.
   */
  public abstract function addCollections(ConfigCollectionInfo $collection_info);

  /**
   * Actions to be performed to configuration override on configuration save.
   *
   * @param \Drupal\Core\Config\ConfigCrudEvent $event
   *   The config CRUD event.
   */
  public abstract function onConfigSave(ConfigCrudEvent $event);

  /**
   * Actions to be performed to configuration override on configuration delete.
   *
   * @param \Drupal\Core\Config\ConfigCrudEvent $event
   *   The config CRUD event.
   */
  public abstract function onConfigDelete(ConfigCrudEvent $event);

  /**
   * Actions to be performed to configuration override on configuration rename.
   *
   * @param \Drupal\Core\Config\ConfigRenameEvent $event
   *   The config rename event.
   */
  public abstract function onConfigRename(ConfigRenameEvent $event);

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events[ConfigCollectionEvents::COLLECTION_INFO][] = [
      'addCollections',
    ];
    $events[ConfigEvents::SAVE][] = [
      'onConfigSave',
      20,
    ];
    $events[ConfigEvents::DELETE][] = [
      'onConfigDelete',
      20,
    ];
    $events[ConfigEvents::RENAME][] = [
      'onConfigRename',
      20,
    ];
    return $events;
  }

  /**
   * Filters data in the override based on what is currently in configuration.
   *
   * @param \Drupal\Core\Config\Config $config
   *   Current configuration object.
   * @param \Drupal\Core\Config\StorableConfigBase $override
   *   Override object corresponding to the configuration to filter data in.
   */
  protected function filterOverride(Config $config, StorableConfigBase $override) {
    $override_data = $override
      ->get();
    $changed = $this
      ->filterNestedArray($config
      ->get(), $override_data);
    if (empty($override_data)) {

      // If no override values are left that would apply, remove the override.
      $override
        ->delete();
    }
    elseif ($changed) {

      // Otherwise set the filtered override values back.
      $override
        ->setData($override_data)
        ->save(TRUE);
    }
  }

  /**
   * Filters data in nested arrays.
   *
   * @param array $original_data
   *   Original data array to filter against.
   * @param array $override_data
   *   Override data to filter.
   *
   * @return bool
   *   TRUE if $override_data was changed, FALSE otherwise.
   */
  protected function filterNestedArray(array $original_data, array &$override_data) {
    $changed = FALSE;
    foreach ($override_data as $key => $value) {
      if (!isset($original_data[$key])) {

        // The original data is not there anymore, remove the override.
        unset($override_data[$key]);
        $changed = TRUE;
      }
      elseif (is_array($override_data[$key])) {
        if (is_array($original_data[$key])) {

          // Do the filtering one level deeper.
          // Ensure that we track $changed along the way.
          if ($this
            ->filterNestedArray($original_data[$key], $override_data[$key])) {
            $changed = TRUE;
          }

          // If no overrides are left under this level, remove the level.
          if (empty($override_data[$key])) {
            unset($override_data[$key]);
            $changed = TRUE;
          }
        }
        else {

          // The override is an array but the value is not, this will not go
          // well, remove the override.
          unset($override_data[$key]);
          $changed = TRUE;
        }
      }
    }
    return $changed;
  }

}