Code:
$user_id =XenForo_Visitor::getInstance()->getUserId();
Code:
$visitor = \XF::visitor();
$user_id = $visitor['user_id'];
Once the user_id is known then the $user information is available.
Code:
$finder = \XF::finder('XF:User');
$user = $finder->where('user_id', 1)->fetchOne();
\XF::dump($user);