D8:configFactory - Example

get_names.php

<?php
/**
 * Get the names of all categories.

 *
 * @return array

 *   A list of existing category IDs.

 */
function wysiwyg_template_category_get_names() {
  $names = &drupal_static(__FUNCTION__);
  if (!isset($names)) {
    $names = array();
    $config_names = \Drupal::configFactory()
      ->listAll('wysiwyg_template_content.category.');
    foreach ($config_names as $config_name) {
      $id = substr($config_name, strlen('wysiwyg_template_content.category.'));
      $names[$id] = $id;
    }
  }
  return $names;
}