title: UINavigationController and back button action
body: <p>I have an two <code>controllers</code> 1st is <code>self</code> and 2nd is <code>maincontroller</code>, where I'm pushing <code>maincontroller</code> in <strong>stack</strong>, so the back button is automatically coming.</p>
  
  <p>Here I need to make an alert when the user presses the back button.</p>
  
  <p>How can I do this?</p>