title: How do I call a function when UINavigationController UINavigationTabBar Item is tapped?
body: <p>I want to be able to present a UIAlertAction when the user taps a UINavigationController UINavigationTabBar Item to remind them they need to enter some information before they can proceed to the next UIViewController that would otherwise be called from such a tap. The code that is called from the tap would check if an array is empty, if so, then display the UIAlertAction.</p>
  
  <p>I've tried to create an instance of the UINavigationController's UINavigationTabBar item, to try and detect it being pushed, but this is messy as the UIViewController to be called has already been setup as a segue when the UINavigationTabBar item is tapped.</p>
  
  <p>Do I need to backtrack and remove the existing segue and create the transition in a different way that allows my desired array checking code to be callable on the user's tap?</p>