class FieldItemDataDefinition

class FieldItemDataDefinition extends DataDefinition implements FieldItemDataDefinitionInterface {

  /**
   * The field definition the item definition belongs to.
   *
   * @var \Drupal\Core\Field\FieldDefinitionInterface
   */
  protected $fieldDefinition;

  /**
   * {@inheritdoc}
   */
  public static function createFromDataType($data_type) {

    // The data type of a field item is in the form of "field_item:$field_type".
    $parts = explode(':', $data_type, 2);
    if ($parts[0] != 'field_item') {
      throw new \InvalidArgumentException('Data type must be in the form of "field_item:FIELD_TYPE".');
    }
    $field_definition = BaseFieldDefinition::create($parts[1]);
    return $field_definition
      ->getItemDefinition();
  }

  /**
   * Creates a new field item definition.
   *
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The field definition the item definition belongs to.
   *
   * @return static
   */
  public static function create($field_definition) {
    $definition['type'] = 'field_item:' . $field_definition
      ->getType();
    $item_definition = new static($definition);
    $item_definition->fieldDefinition = $field_definition;
    return $item_definition;
  }

  /**
   * {@inheritdoc}
   */
  public function getPropertyDefinition($name) {
    return $this->fieldDefinition
      ->getFieldStorageDefinition()
      ->getPropertyDefinition($name);
  }

  /**
   * {@inheritdoc}
   */
  public function getPropertyDefinitions() {
    return $this->fieldDefinition
      ->getFieldStorageDefinition()
      ->getPropertyDefinitions();
  }

  /**
   * {@inheritdoc}
   */
  public function getMainPropertyName() {
    return $this->fieldDefinition
      ->getFieldStorageDefinition()
      ->getMainPropertyName();
  }

  /**
   * {@inheritdoc}
   */
  public function getFieldDefinition() {
    return $this->fieldDefinition;
  }

  /**
   * {@inheritdoc}
   */
  public function setFieldDefinition($field_definition) {
    $this->fieldDefinition = $field_definition;
    return $this;
  }

  /**
   * Gets the label of the field type.
   *
   * If the label hasn't been set, then fall back to the label of the
   * typed data definition.
   *
   * @return string
   *   The label of the field type.
   *
   * @throws \Drupal\Component\Plugin\Exception\PluginNotFoundException
   */
  public function getLabel() {
    return parent::getLabel() ?: $this
      ->getTypedDataManager()
      ->getDefinition($this
      ->getDataType())['label'];
  }

}