title: How to know when `navController.popBackStack()` was called?
body: <p>I am popping the backstack on my nav controller on some point in my code -</p>
  <pre><code>  navController.popBackStack()
  </code></pre>
  <p>The fragment that added that following fragment to the backstack needs to know exactly when that one was popped in order to trigger code following that.</p>
  <p>How do I make the first fragment know about it?</p>
  <p>I thought about adding a callback as an argument but I doubt it's a good practice.</p>