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();
            }
        }
    }

}