title: In Meteor, how do you detect when a new user has logged in?
body: <p>The documentation:</p>
  
  <p><a href="http://docs.meteor.com/#publish_userId">http://docs.meteor.com/#publish_userId</a></p>
  
  <p>says</p>
  
  <blockquote>
    <p>This is constant. However, if the logged-in user changes, the publish
    function is rerun with the new value.</p>
  </blockquote>
  
  <p>Is there some way of running some event driven code which fires only on the event that the logged in user changes?</p>