title: why is UIWindow a child and also a parent of UView?
body: <p>The UIKit reference says UIView is a superclass of UIWindow, but despite of this parentage, a UIWindow actually manages UIViews. This sounds so unusual to me.</p>
  
  <p>Does anyone know what the significants of this are in terms of software design?</p>
  
  <p>Many thanks.</p>
  
  <p>EDIT:<br>
  I read the related paragraph in the iPhone programming guide. Yet I couldn't see why they do the reverse: let UIWindow be the parent of UIView. There must have been some thing that forced Apple to design the class hierarchy this way.</p>