class LoginLogoutMenuLink

class LoginLogoutMenuLink extends MenuLinkDefault {
  
  /**
   * The current user.
   *
   * @var \Drupal\Core\Session\AccountInterface
   */
  protected $currentUser;
  
  /**
   * Constructs a new LoginLogoutMenuLink.
   *
   * @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\Menu\StaticMenuLinkOverridesInterface $static_override
   *   The static override storage.
   * @param \Drupal\Core\Session\AccountInterface $current_user
   *   The current user.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, StaticMenuLinkOverridesInterface $static_override, AccountInterface $current_user) {
    parent::__construct($configuration, $plugin_id, $plugin_definition, $static_override);
    $this->currentUser = $current_user;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($configuration, $plugin_id, $plugin_definition, $container->get('menu_link.static.overrides'), $container->get('current_user'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function getTitle() {
    if ($this->currentUser
      ->isAuthenticated()) {
      return $this->t('Log out');
    }
    else {
      return $this->t('Log in');
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function getRouteName() {
    if ($this->currentUser
      ->isAuthenticated()) {
      return 'user.logout';
    }
    else {
      return 'user.login';
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function getCacheContexts() {
    return [
      'user.roles:authenticated',
    ];
  }

}