class PathItem

class PathItem extends FieldItemBase {
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['alias'] = DataDefinition::create('string')->setLabel(t('Path alias'));
    $properties['pid'] = DataDefinition::create('integer')->setLabel(t('Path id'));
    $properties['langcode'] = DataDefinition::create('string')->setLabel(t('Language Code'));
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [];
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $alias = $this->get('alias')
      ->getValue();
    $pid = $this->get('pid')
      ->getValue();
    $langcode = $this->get('langcode')
      ->getValue();
    return ($alias === NULL || $alias === '') && ($pid === NULL || $pid === '') && ($langcode === NULL || $langcode === '');
  }
  
  /**
   * {@inheritdoc}
   */
  public function preSave() {
    $alias = $this->get('alias')
      ->getValue();
    if ($alias !== NULL) {
      $this->set('alias', trim($alias));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function postSave($update) {
    $path_alias_storage = \Drupal::entityTypeManager()->getStorage('path_alias');
    $entity = $this->getEntity();
    $alias = $this->get('alias')
      ->getValue();
    $pid = $this->get('pid')
      ->getValue();
    $langcode = $this->get('langcode')
      ->getValue();
    // If an explicit alias langcode is provided, use it. This allows code to
    // preserve an existing alias language or set a specific language,
    // including language-neutral, when creating or updating an alias.
    // Otherwise, fall back to the field/entity langcode.
    $alias_langcode = $langcode ?: $this->getLangcode();
    // If we have an alias, we need to create or update a path alias entity.
    if ($alias) {
      $properties = [
        'path' => '/' . $entity->toUrl()
          ->getInternalPath(),
        'alias' => $alias,
        'langcode' => $alias_langcode,
      ];
      if (!$pid) {
        // Try to load it from storage before creating it. In some cases the
        // path alias could be created before this function runs. For example,
        // \Drupal\workspaces\EntityOperations::entityTranslationInsert will
        // create a translation, and an associated path alias will be created
        // with it.
        $query = $path_alias_storage->getQuery()
          ->accessCheck(FALSE);
        foreach ($properties as $field => $value) {
          $query->condition($field, $value);
        }
        $ids = $query->execute();
        $pid = $ids ? reset($ids) : $pid;
      }
      if (!$pid) {
        $path_alias = $path_alias_storage->create($properties);
        $path_alias->save();
        $this->set('pid', $path_alias->id());
      }
      else {
        $path_alias = $path_alias_storage->load($pid);
        $path_alias_changed = FALSE;
        if ($alias != $path_alias->getAlias()) {
          $path_alias->setAlias($alias);
          $path_alias_changed = TRUE;
        }
        // Persist langcode-only updates when the alias text is unchanged.
        if ($alias_langcode !== $path_alias->language()
          ->getId()) {
          $path_alias->set('langcode', $alias_langcode);
          $path_alias_changed = TRUE;
        }
        if ($path_alias_changed) {
          $path_alias->save();
        }
      }
    }
    elseif ($pid) {
      // Otherwise, delete the old alias if the user erased it.
      $path_alias = $path_alias_storage->load($pid);
      if ($entity->isDefaultRevision()) {
        $path_alias_storage->delete([
          $path_alias,
        ]);
      }
      else {
        $path_alias_storage->deleteRevision($path_alias->getRevisionID());
      }
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
    $random = new Random();
    $values['alias'] = '/' . str_replace(' ', '-', strtolower($random->sentences(3)));
    return $values;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function mainPropertyName() {
    return 'alias';
  }

}