title: How to detect back button is pressed in the next viewController in a navigationController
body: <p>I want to detect if back button is pressed in the next viewController in a navigationController. 
  Let's say I have VC_A and VC_B viewControllers in a navigationController. I know how to detect if back button is pressed in a current view controller but I do not know how to detect it in a previous viewController.</p>
  
  <p>Edit: </p>
  
  <p>I go from VC_A to VC_B and when I press back button in VC_B then I want to call a function in VC_A.</p>