class LocaleConfigSubscriber

class LocaleConfigSubscriber implements EventSubscriberInterface {

  /**
   * The configuration factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;

  /**
   * The typed configuration manager.
   *
   * @var \Drupal\locale\LocaleConfigManager
   */
  protected $localeConfigManager;

  /**
   * The language manager.
   *
   * @var \Drupal\Core\Language\LanguageManagerInterface
   */
  protected $languageManager;

  /**
   * Constructs a LocaleConfigSubscriber.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The configuration factory.
   * @param \Drupal\locale\LocaleConfigManager $locale_config_manager
   *   The typed configuration manager.
   */
  public function __construct(ConfigFactoryInterface $config_factory, LocaleConfigManager $locale_config_manager) {
    $this->configFactory = $config_factory;
    $this->localeConfigManager = $locale_config_manager;
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events[LanguageConfigOverrideEvents::SAVE_OVERRIDE] = 'onOverrideChange';
    $events[LanguageConfigOverrideEvents::DELETE_OVERRIDE] = 'onOverrideChange';
    $events[ConfigEvents::SAVE] = 'onConfigSave';
    return $events;
  }

  /**
   * Updates the locale strings when a translated active configuration is saved.
   *
   * @param \Drupal\Core\Config\ConfigCrudEvent $event
   *   The configuration event.
   */
  public function onConfigSave(ConfigCrudEvent $event) {

    // Only attempt to feed back configuration translation changes to locale if
    // the update itself was not initiated by locale data changes.
    if (!InstallerKernel::installationAttempted() && !$this->localeConfigManager
      ->isUpdatingTranslationsFromLocale()) {
      $config = $event
        ->getConfig();
      $langcode = $config
        ->get('langcode') ?: 'en';
      $this
        ->updateLocaleStorage($config, $langcode);
    }
  }

  /**
   * Updates the locale strings when a configuration override is saved/deleted.
   *
   * @param \Drupal\language\Config\LanguageConfigOverrideCrudEvent $event
   *   The language configuration event.
   */
  public function onOverrideChange(LanguageConfigOverrideCrudEvent $event) {

    // Only attempt to feed back configuration override changes to locale if
    // the update itself was not initiated by locale data changes.
    if (!InstallerKernel::installationAttempted() && !$this->localeConfigManager
      ->isUpdatingTranslationsFromLocale()) {
      $translation_config = $event
        ->getLanguageConfigOverride();
      $langcode = $translation_config
        ->getLangcode();
      $reference_config = $this->configFactory
        ->getEditable($translation_config
        ->getName())
        ->get();
      $this
        ->updateLocaleStorage($translation_config, $langcode, $reference_config);
    }
  }

  /**
   * Update locale storage based on configuration translations.
   *
   * @param \Drupal\Core\Config\StorableConfigBase $config
   *   Active configuration or configuration translation override.
   * @param string $langcode
   *   The language code of $config.
   * @param array $reference_config
   *   (Optional) Reference configuration to check against if $config was an
   *   override. This allows us to update locale keys for data not in the
   *   override but still in the active configuration.
   */
  public function updateLocaleStorage(StorableConfigBase $config, $langcode, array $reference_config = []) {
    $name = $config
      ->getName();
    if ($this->localeConfigManager
      ->isSupported($name) && locale_is_translatable($langcode)) {
      $translatables = $this->localeConfigManager
        ->getTranslatableDefaultConfig($name);
      $this
        ->processTranslatableData($name, $config
        ->get(), $translatables, $langcode, $reference_config);
    }
  }

  /**
   * Process the translatable data array with a given language.
   *
   * @param string $name
   *   The configuration name.
   * @param array $config
   *   The active configuration data or override data.
   * @param array|\Drupal\Core\StringTranslation\TranslatableMarkup[] $translatable
   *   The translatable array structure.
   *   @see \Drupal\locale\LocaleConfigManager::getTranslatableData()
   * @param string $langcode
   *   The language code to process the array with.
   * @param array $reference_config
   *   (Optional) Reference configuration to check against if $config was an
   *   override. This allows us to update locale keys for data not in the
   *   override but still in the active configuration.
   */
  protected function processTranslatableData($name, array $config, array $translatable, $langcode, array $reference_config = []) {
    foreach ($translatable as $key => $item) {
      if (!isset($config[$key])) {
        if (isset($reference_config[$key])) {
          $this
            ->resetExistingTranslations($name, $translatable[$key], $reference_config[$key], $langcode);
        }
        continue;
      }
      if (is_array($item)) {
        $reference_config_item = $reference_config[$key] ?? [];
        $this
          ->processTranslatableData($name, $config[$key], $item, $langcode, $reference_config_item);
      }
      else {
        $this
          ->saveCustomizedTranslation($name, $item
          ->getUntranslatedString(), $item
          ->getOption('context'), $config[$key], $langcode);
      }
    }
  }

  /**
   * Reset existing locale translations to their source values.
   *
   * Goes through $translatable to reset any existing translations to the source
   * string, so prior translations would not reappear in the configuration.
   *
   * @param string $name
   *   The configuration name.
   * @param array|\Drupal\Core\StringTranslation\TranslatableMarkup $translatable
   *   Either a possibly nested array with TranslatableMarkup objects at the
   *   leaf items or a TranslatableMarkup object directly.
   * @param array|string $reference_config
   *   Either a possibly nested array with strings at the leaf items or a string
   *   directly. Only those $translatable items that are also present in
   *   $reference_config will get translations reset.
   * @param string $langcode
   *   The language code of the translation being processed.
   */
  protected function resetExistingTranslations($name, $translatable, $reference_config, $langcode) {
    if (is_array($translatable)) {
      foreach ($translatable as $key => $item) {
        if (isset($reference_config[$key])) {

          // Process further if the key still exists in the reference active
          // configuration and the default translation but not the current
          // configuration override.
          $this
            ->resetExistingTranslations($name, $item, $reference_config[$key], $langcode);
        }
      }
    }
    elseif (!is_array($reference_config)) {
      $this
        ->saveCustomizedTranslation($name, $translatable
        ->getUntranslatedString(), $translatable
        ->getOption('context'), $reference_config, $langcode);
    }
  }

  /**
   * Saves a translation string and marks it as customized.
   *
   * @param string $name
   *   The configuration name.
   * @param string $source
   *   The source string value.
   * @param string $context
   *   The source string context.
   * @param string $new_translation
   *   The translation string.
   * @param string $langcode
   *   The language code of the translation.
   */
  protected function saveCustomizedTranslation($name, $source, $context, $new_translation, $langcode) {
    $locale_translation = $this->localeConfigManager
      ->getStringTranslation($name, $langcode, $source, $context);
    if (!empty($locale_translation)) {

      // If this code is triggered during installation never set the translation
      // to the source string.
      if (InstallerKernel::installationAttempted() && $source === $new_translation) {
        return;
      }

      // Save this translation as custom if it was a new translation and not the
      // same as the source. (The interface prefills translation values with the
      // source). Or if there was an existing (non-empty) translation and the
      // user changed it (even if it was changed back to the original value).
      // Otherwise the translation file would be overwritten with the locale
      // copy again later.
      $existing_translation = $locale_translation
        ->getString();
      if ($locale_translation
        ->isNew() && $source != $new_translation || !$locale_translation
        ->isNew() && (empty($existing_translation) && $source != $new_translation || !empty($existing_translation) && $new_translation != $existing_translation)) {
        $locale_translation
          ->setString($new_translation)
          ->setCustomized(TRUE)
          ->save();
      }
    }
  }

}