title: Detect backward/pop navigation in destination view controller viewWillAppear
body: <p>There are a number of well known solutions for detecting when a view controller is in the process of disappearing off the screen in the process of a backwards navigation, managed by a <code>UINavigationController</code> (i.e. when a view controller is being popped off the navigation stack). In other words detecting a backwards navigation within the source view controller.</p>
  
  <p>In contrast, I need to detect a backwards navigation within <code>viewWillAppear</code> for the <em>destination</em> view controller. Or to put it another way, detect when a view controller is about to be displayed as a result of a <code>UINavigationController</code> pop.</p>
  
  <p>What is the cleanest/simplest way to achieve this? Implementing <code>UINavigationControllerDelegate</code> and setting a flag might be one approach, but would rather avoid mutable state if possible.</p>
  
  <p>I am not using Storyboards to manage the transition between these two screens.</p>