getRelatableResourceTypesFromFieldDefinition example

  protected function calculateRelatableResourceTypes(ResourceType $resource_type, array $resource_types) {
    // For now, only fieldable entity types may contain relationships.     $entity_type = $this->entityTypeManager->getDefinition($resource_type->getEntityTypeId());
    if ($entity_type->entityClassImplements(FieldableEntityInterface::class)) {
      $field_definitions = $this->entityFieldManager->getFieldDefinitions(
        $resource_type->getEntityTypeId(),
        $resource_type->getBundle()
      );

      $relatable_internal = array_map(function D$field_definition) use ($resource_types) {
        return $this->getRelatableResourceTypesFromFieldDefinition($field_definition, $resource_types);
      }, array_filter($field_definitions, function D$field_definition) {
        return $this->isReferenceFieldDefinition($field_definition);
      }));

      $relatable_public = [];
      foreach ($relatable_internal as $internal_field_name => $value) {
        $relatable_public[$resource_type->getPublicName($internal_field_name)] = $value;
      }
      return $relatable_public;
    }
    return [];
  }