farm-2.x-dev/modules/asset/structure/farm_structure.module

<?php

/**
 * @file
 * Structure asset module.
 */

use Drupal\Core\Entity\ContentEntityInterface;
use Drupal\Core\Field\FieldStorageDefinitionInterface;

/**
 * Structure type options helper.
 *
 * @return array
 *   Returns an array of structure types for use in form select options.
 */
function farm_structure_type_options() {
  /** @var \Drupal\farm_structure\Entity\FarmStructureTypeInterface[] $types */
  $types = \Drupal::entityTypeManager()->getStorage('structure_type')->loadMultiple();
  $options = [];
  foreach ($types as $id => $type) {
    $options[$id] = $type->getLabel();
  }
  return $options;
}

/**
 * Allowed values callback function for the structure type field.
 *
 * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $definition
 *   The field definition.
 * @param \Drupal\Core\Entity\ContentEntityInterface|null $entity
 *   The entity being created if applicable.
 * @param bool $cacheable
 *   Boolean indicating if the allowed values can be cached. Defaults to TRUE.
 *
 * @return array
 *   Returns an array of allowed values for use in form select options.
 */
function farm_structure_type_field_allowed_values(FieldStorageDefinitionInterface $definition, ContentEntityInterface $entity = NULL, bool &$cacheable = TRUE) {
  return farm_structure_type_options();
}