title: How to display navigation controller (set of view controllers) such as login or register before adding UITabBarController
body: <p>I had an application in which I need to have a login or register system before the tab bar controller is added to the window, such as in Instagram.</p>
  
  <p>I have added 5 navigation controllers (with view controller as its root) to the <code>UITabBarController</code> and then set it as the root of the window. Before that, I need to have another <code>UINavigationController</code> for the login system.</p>
  
  <p>If I add that, how do I remove it before adding tab? </p>
  
  <p>Another problem is that I also have to handle logging out, so I need to come back to it.</p>
  
  <p>Can anybody help me with me how to do this?</p>