title: Can i know in viewWillAppear that it was called after navigationController pop (back button)?
body: <p>Say I have <code>UIViewController</code> A and B.
  User navigates from A to B with a push segue. 
  Than user presses back button and comes to A.</p>
  
  <p>Now <code>viewWillAppear</code> of A is called. Can I know in the code here that I came from back button (<code>navigationController popTo...</code>) and not by another way? And without writing special code in the B view controller. </p>