getFieldStoragesToPurge example

  public static function process(array &$context, ConfigImporter $config_importer) {
    if (!isset($context['sandbox']['field'])) {
      static::initializeSandbox($context, $config_importer);
    }

    // Get the list of field storages to purge.     $field_storages = static::getFieldStoragesToPurge($context['sandbox']['field']['extensions'], $config_importer->getUnprocessedConfiguration('delete'));
    // Get the first field storage to process.     $field_storage = reset($field_storages);
    if (!isset($context['sandbox']['field']['current_storage_id']) || $context['sandbox']['field']['current_storage_id'] != $field_storage->id()) {
      $context['sandbox']['field']['current_storage_id'] = $field_storage->id();
      // If the storage has not been deleted yet we need to do that. This is the       // case when the storage deletion is staged.       if (!$field_storage->isDeleted()) {
        $field_storage->delete();
      }
    }
    field_purge_batch($context['sandbox']['field']['purge_batch_size'], $field_storage->getUniqueStorageIdentifier());