waitForMessageVisible example

protected $defaultTheme = 'stark';

  /** * Tests AJAX MessageCommand use in a form. */
  public function testMessageCommand() {
    $page = $this->getSession()->getPage();
    $assert_session = $this->assertSession();

    $this->drupalGet('ajax-test/message');
    $page->pressButton('Make Message In Default Location');
    $this->waitForMessageVisible('I am a message in the default location.');
    $this->assertAnnounceContains('I am a message in the default location.');
    $assert_session->elementsCount('css', '.messages__wrapper .messages', 1);

    $page->pressButton('Make Message In Alternate Location');
    $this->waitForMessageVisible('I am a message in an alternate location.', '#alternate-message-container');
    $assert_session->pageTextContains('I am a message in the default location.');
    $this->assertAnnounceContains('I am a message in an alternate location.');
    $assert_session->elementsCount('css', '.messages__wrapper .messages', 1);
    $assert_session->elementsCount('css', '#alternate-message-container .messages', 1);

    $page->pressButton('Make Warning Message');