class ShapeItem

class ShapeItem extends FieldItemBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultStorageSettings() {
    return [
      'foreign_key_name' => 'shape',
    ] + parent::defaultStorageSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['shape'] = DataDefinition::create('string')->setLabel(t('Shape'));
    $properties['color'] = DataDefinition::create('string')->setLabel(t('Color'));
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    $foreign_keys = [];
    // The 'foreign keys' key is not always used in tests.
    if ($field_definition->getSetting('foreign_key_name')) {
      $foreign_keys['foreign keys'] = [
        // This is a dummy foreign key definition, references a table that
        // doesn't exist, but that's not a problem.
$field_definition->getSetting('foreign_key_name') => [
          'table' => $field_definition->getSetting('foreign_key_name'),
          'columns' => [
            $field_definition->getSetting('foreign_key_name') => 'id',
          ],
        ],
      ];
    }
    return [
      'columns' => [
        'shape' => [
          'type' => 'varchar',
          'length' => 32,
        ],
        'color' => [
          'type' => 'varchar',
          'length' => 32,
        ],
      ],
    ] + $foreign_keys;
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $item = $this->getValue();
    return empty($item['shape']) && empty($item['color']);
  }
  
  /**
   * {@inheritdoc}
   */
  public static function mainPropertyName() {
    return 'shape';
  }

}