title: Advantages, problems, examples of adding another UIWindow to an iOS app?
body: <p>Recently I've been wondering about the fact that that an iOS app only has one <code>UIWindow</code>.
  It does not seem to be an issue to create another <code>UIWindow</code> and place it on screen.</p>
  
  <p>My question is kind of vague, but I'm interested in:</p>
  
  <ul>
  <li>What could I potentially achieve with a second <code>UIWindow</code> that cannot be done in other ways?</li>
  <li>What can go wrong when using multiple <code>UIWindow</code> instances?</li>
  <li>I have seen that people use a 2nd <code>UIWindow</code> to display popover like views on iPhone. Is this a good way of doing it? Why? Why not?</li>
  <li>Are there other examples where it is making perfectly sense to have another <code>UIWindow</code>?</li>
  </ul>
  
  <p>It's not that I'm missing something. I have never felt the need to create another <code>UIWindow</code> instance but maybe it would allow doing amazing things I'm not aware of! :-)</p>
  
  <p>I'm hoping that it might help me solve this problem:
  I need to add a "cover view" over whatever is currently displayed. It should also work if there are already one or more modal controllers presented. If I add a <code>UIView</code> to the root controller's view, the modal controllers sit on top, so do the popover controllers.
  If I present the cover view modally and there is already a modal controller, only part of the screen is covered.</p>