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() {
        return ($this->alias === NULL || $this->alias === '') && ($this->pid === NULL || $this->pid === '') && ($this->langcode === NULL || $this->langcode === '');
    }
    
    /**
     * {@inheritdoc}
     */
    public function preSave() {
        if ($this->alias !== NULL) {
            $this->alias = trim($this->alias);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function postSave($update) {
        $path_alias_storage = \Drupal::entityTypeManager()->getStorage('path_alias');
        $entity = $this->getEntity();
        // If specified, rely on the langcode property for the language, so that the
        // existing language of an alias can be kept. That could for example be
        // unspecified even if the field/entity has a specific langcode.
        $alias_langcode = $this->langcode && $this->pid ? $this->langcode : $this->getLangcode();
        // If we have an alias, we need to create or update a path alias entity.
        if ($this->alias) {
            if (!$update || !$this->pid) {
                $path_alias = $path_alias_storage->create([
                    'path' => '/' . $entity->toUrl()
                        ->getInternalPath(),
                    'alias' => $this->alias,
                    'langcode' => $alias_langcode,
                ]);
                $path_alias->save();
                $this->pid = $path_alias->id();
            }
            elseif ($this->pid) {
                $path_alias = $path_alias_storage->load($this->pid);
                if ($this->alias != $path_alias->getAlias()) {
                    $path_alias->setAlias($this->alias);
                    $path_alias->save();
                }
            }
        }
        elseif ($this->pid && !$this->alias) {
            // Otherwise, delete the old alias if the user erased it.
            $path_alias = $path_alias_storage->load($this->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';
    }

}