class ExcludedModulesEventSubscriber

final class ExcludedModulesEventSubscriber implements EventSubscriberInterface {
    
    /**
     * The key in settings and state for listing excluded modules.
     *
     * @var string
     */
    const EXCLUDED_MODULES_KEY = "config_exclude_modules";
    
    /**
     * @var \Drupal\Core\Config\StorageInterface
     */
    private $activeStorage;
    
    /**
     * @var \Drupal\Core\Site\Settings
     */
    private $settings;
    
    /**
     * @var \Drupal\Core\Config\ConfigManagerInterface
     */
    private $manager;
    
    /**
     * EnvironmentModulesEventSubscriber constructor.
     *
     * @param \Drupal\Core\Config\StorageInterface $active_storage
     *   The active config storage.
     * @param \Drupal\Core\Site\Settings $settings
     *   The Drupal settings.
     * @param \Drupal\Core\Config\ConfigManagerInterface $manager
     *   The config manager.
     */
    public function __construct(StorageInterface $active_storage, Settings $settings, ConfigManagerInterface $manager) {
        $this->activeStorage = $active_storage;
        $this->settings = $settings;
        $this->manager = $manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        // React early on export and late on import.
        return [
            'config.transform.import' => [
                'onConfigTransformImport',
                -500,
            ],
            'config.transform.export' => [
                'onConfigTransformExport',
                500,
            ],
        ];
    }
    
    /**
     * Transform the storage which is used to import the configuration.
     *
     * Make sure excluded modules are not uninstalled by adding them and their
     * config to the storage when importing configuration.
     *
     * @param \Drupal\Core\Config\StorageTransformEvent $event
     *   The transformation event.
     */
    public function onConfigTransformImport(StorageTransformEvent $event) {
        $storage = $event->getStorage();
        if (!$storage->exists('core.extension')) {
            // If the core.extension config is not present there is nothing to do.
            // This means that probably the storage is empty or non-functional.
            return;
        }
        foreach (array_merge([
            StorageInterface::DEFAULT_COLLECTION,
        ], $this->activeStorage
            ->getAllCollectionNames()) as $collectionName) {
            $collection = $storage->createCollection($collectionName);
            $activeCollection = $this->activeStorage
                ->createCollection($collectionName);
            foreach ($this->getDependentConfigNames() as $configName) {
                if (!$collection->exists($configName) && $activeCollection->exists($configName)) {
                    // Make sure the config is not removed if it exists.
                    $collection->write($configName, $activeCollection->read($configName));
                }
            }
        }
        $extension = $storage->read('core.extension');
        $existing = $this->activeStorage
            ->read('core.extension');
        $modules = $extension['module'];
        foreach ($this->getExcludedModules() as $module) {
            if (array_key_exists($module, $existing['module'])) {
                // Set the modules weight from the active store.
                $modules[$module] = $existing['module'][$module];
            }
        }
        // Sort the extensions.
        $extension['module'] = module_config_sort($modules);
        // Set the modified extension.
        $storage->write('core.extension', $extension);
    }
    
    /**
     * Transform the storage which is used to export the configuration.
     *
     * Make sure excluded modules are not exported by removing all the config
     * which depends on them from the storage that is exported.
     *
     * @param \Drupal\Core\Config\StorageTransformEvent $event
     *   The transformation event.
     */
    public function onConfigTransformExport(StorageTransformEvent $event) {
        $storage = $event->getStorage();
        if (!$storage->exists('core.extension')) {
            // If the core.extension config is not present there is nothing to do.
            // This means some other process has rendered it non-functional already.
            return;
        }
        foreach (array_merge([
            StorageInterface::DEFAULT_COLLECTION,
        ], $storage->getAllCollectionNames()) as $collectionName) {
            $collection = $storage->createCollection($collectionName);
            foreach ($this->getDependentConfigNames() as $configName) {
                $collection->delete($configName);
            }
        }
        $extension = $storage->read('core.extension');
        // Remove all the excluded modules from the extensions list.
        $extension['module'] = array_diff_key($extension['module'], array_flip($this->getExcludedModules()));
        $storage->write('core.extension', $extension);
    }
    
    /**
     * Get the modules set as excluded in the Drupal settings.
     *
     * @return string[]
     *   An array of module names.
     */
    private function getExcludedModules() {
        return $this->settings
            ->get(self::EXCLUDED_MODULES_KEY, []);
    }
    
    /**
     * Get all the configuration which depends on one of the excluded modules.
     *
     * @return string[]
     *   An array of configuration names.
     */
    private function getDependentConfigNames() {
        $modules = $this->getExcludedModules();
        $dependencyManager = $this->manager
            ->getConfigDependencyManager();
        $config = [];
        // Find all the configuration depending on the excluded modules.
        foreach ($modules as $module) {
            foreach ($dependencyManager->getDependentEntities('module', $module) as $dependent) {
                $config[] = $dependent->getConfigDependencyName();
            }
            $config = array_merge($config, $this->activeStorage
                ->listAll($module . '.'));
        }
        // Find all configuration that depends on the configuration found above.
        foreach ($this->manager
            ->findConfigEntityDependencies('config', array_unique($config)) as $dependent) {
            $config[] = $dependent->getConfigDependencyName();
        }
        return array_unique($config);
    }

}