trait FieldAPIHandlerTrait

trait FieldAPIHandlerTrait {
    
    /**
     * The field definition.
     *
     * @var \Drupal\Core\Field\FieldDefinitionInterface
     */
    protected $fieldDefinition;
    
    /**
     * The field storage definition.
     *
     * @var \Drupal\field\FieldStorageConfigInterface
     */
    protected $fieldStorageDefinition;
    
    /**
     * The entity field manager.
     *
     * @var \Drupal\Core\Entity\EntityFieldManagerInterface
     */
    protected $entityFieldManager;
    
    /**
     * Gets the field definition.
     *
     * A View works on an entity type across bundles, and thus only has access to
     * field storage definitions. In order to be able to use widgets and
     * formatters, we create a generic field definition out of that storage
     * definition.
     *
     * @see BaseFieldDefinition::createFromFieldStorageDefinition()
     *
     * @return \Drupal\Core\Field\FieldDefinitionInterface
     *   The field definition used by this handler.
     */
    protected function getFieldDefinition() {
        if (!$this->fieldDefinition) {
            $field_storage_config = $this->getFieldStorageDefinition();
            $this->fieldDefinition = BaseFieldDefinition::createFromFieldStorageDefinition($field_storage_config);
        }
        return $this->fieldDefinition;
    }
    
    /**
     * Gets the field storage configuration.
     *
     * @return \Drupal\field\FieldStorageConfigInterface
     *   The field storage definition used by this handler
     */
    protected function getFieldStorageDefinition() {
        if (!$this->fieldStorageDefinition) {
            $field_storage_definitions = $this->getEntityFieldManager()
                ->getFieldStorageDefinitions($this->definition['entity_type']);
            $this->fieldStorageDefinition = $field_storage_definitions[$this->definition['field_name']];
        }
        return $this->fieldStorageDefinition;
    }
    
    /**
     * Returns the entity field manager.
     *
     * @return \Drupal\Core\Entity\EntityFieldManagerInterface
     *   The entity field manager.
     */
    protected function getEntityFieldManager() {
        if (!isset($this->entityFieldManager)) {
            $this->entityFieldManager = \Drupal::service('entity_field.manager');
        }
        return $this->entityFieldManager;
    }

}