class AssignOwnerNode

class AssignOwnerNode extends ConfigurableActionBase implements ContainerFactoryPluginInterface {
  
  /**
   * The database connection.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $connection;
  
  /**
   * Constructs a new AssignOwnerNode action.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Database\Connection $connection
   *   The database connection.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, Connection $connection) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->connection = $connection;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('database'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function execute($entity = NULL) {
    /** @var \Drupal\node\NodeInterface $entity */
    $entity->setOwnerId($this->configuration['owner_uid'])
      ->save();
  }
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'owner_uid' => '',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $description = $this->t('The username of the user to which you would like to assign ownership.');
    $count = $this->connection
      ->query("SELECT COUNT(*) FROM {users}")
      ->fetchField();
    // Use dropdown for fewer than 200 users; textbox for more than that.
    if (intval($count) < 200) {
      $options = [];
      $result = $this->connection
        ->query("SELECT [uid], [name] FROM {users_field_data} WHERE [uid] > 0 AND [default_langcode] = 1 ORDER BY [name]");
      foreach ($result as $data) {
        $options[$data->uid] = $data->name;
      }
      $form['owner_uid'] = [
        '#type' => 'select',
        '#title' => $this->t('Username'),
        '#default_value' => $this->configuration['owner_uid'],
        '#options' => $options,
        '#description' => $description,
      ];
    }
    else {
      $form['owner_uid'] = [
        '#type' => 'entity_autocomplete',
        '#title' => $this->t('Username'),
        '#target_type' => 'user',
        '#selection_settings' => [
          'include_anonymous' => FALSE,
        ],
        '#default_value' => User::load($this->configuration['owner_uid']),
        // Validation is done in static::validateConfigurationForm().
'#validate_reference' => FALSE,
        '#size' => '6',
        '#maxlength' => '60',
        '#description' => $description,
      ];
    }
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    $exists = (bool) $this->connection
      ->queryRange('SELECT 1 FROM {users_field_data} WHERE [uid] = :uid AND [default_langcode] = 1', 0, 1, [
      ':uid' => $form_state->getValue('owner_uid'),
    ])
      ->fetchField();
    if (!$exists) {
      $form_state->setErrorByName('owner_uid', $this->t('Enter a valid username.'));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['owner_uid'] = $form_state->getValue('owner_uid');
  }
  
  /**
   * {@inheritdoc}
   */
  public function access($object, AccountInterface $account = NULL, $return_as_object = FALSE) {
    /** @var \Drupal\node\NodeInterface $object */
    $result = $object->access('update', $account, TRUE)
      ->andIf($object->getOwner()
      ->access('edit', $account, TRUE));
    return $return_as_object ? $result : $result->isAllowed();
  }

}