Import from module config/optional

ImportConfigForm.php

<?php

// ...

/**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    $module_path = $this->extensionListModule->getPath('cpct_config');
    $directory = $module_path . '/config/optional';
    $change_list_create = $this->getChangeListCreate();

    $batch['title'] = t('Import config ...');
    foreach ($change_list_create AS $config_file_name) {
      $config_name = str_replace('.yml', '', $config_file_name);
      $batch['operations'][] = [
        '\Drupal\cpct_config\Batch\BatchImportConfig::process', [
          [
            'config_name' => $config_name,
            'directory' => $directory,
          ],
        ],
      ];
    }
    $batch['finished'] ='\Drupal\cpct_config\Batch\BatchImportConfig::finished';
    batch_set($batch);

    // in the batch process use mike-potter's code to import each config (https://gist.github.com/crittermike/2d2c6734c506d509505fa79142125757?permalink_comment_id=3423368#gistcomment-3423368)
  }

  function getChangeListCreate() {
    $module_path = $this->extensionListModule->getPath('cpct_config');
    $directory = $module_path . '/config/optional';
    $config_file_storage = new FileStorage($directory);

    $storage_comparer = new StorageComparer(
      $config_file_storage,
      $this->configStorage
    );
    // this create a list with a proper order for importing config
    $storage_comparer->createChangelist();

    return $storage_comparer->getChangelist('create');
  }