class MessageCommand

class MessageCommand implements CommandInterface, CommandWithAttachedAssetsInterface {

  /**
   * The message text.
   *
   * @var string
   */
  protected $message;

  /**
   * Whether to clear previous messages.
   *
   * @var bool
   */
  protected $clearPrevious;

  /**
   * The query selector for the element the message will appear in.
   *
   * @var string
   */
  protected $wrapperQuerySelector;

  /**
   * The options passed to Drupal.message().add().
   *
   * @var array
   */
  protected $options;

  /**
   * Constructs a MessageCommand object.
   *
   * @param string $message
   *   The text of the message.
   * @param string|null $wrapper_query_selector
   *   The query selector of the element to display messages in when they
   *   should be displayed somewhere other than the default.
   *   @see Drupal.Message.defaultWrapper()
   * @param array $options
   *   The options passed to Drupal.message().add().
   * @param bool $clear_previous
   *   If TRUE, previous messages will be cleared first.
   */
  public function __construct($message, $wrapper_query_selector = NULL, array $options = [], $clear_previous = TRUE) {
    $this->message = $message;
    $this->wrapperQuerySelector = $wrapper_query_selector;
    $this->options = $options;
    $this->clearPrevious = $clear_previous;
  }

  /**
   * {@inheritdoc}
   */
  public function render() {
    return [
      'command' => 'message',
      'message' => $this->message,
      'messageWrapperQuerySelector' => $this->wrapperQuerySelector,
      'messageOptions' => $this->options,
      'clearPrevious' => $this->clearPrevious,
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getAttachedAssets() {
    $assets = new AttachedAssets();
    $assets
      ->setLibraries([
      'core/drupal.message',
    ]);
    return $assets;
  }

}