title: What is self.window in iOS app delegate
body: <p>I have a question regarding window and viewcontroller in iOS. I just have a look at the app delegate of iOS project that I am working on today and found that it is required to have...</p>
  
  <pre><code>UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
  </code></pre>
  
  <p>in my app. </p>
  
  <p>Here are the questions:</p>
  
  <ol>
  <li><p>Does this means that navigationController is the root view controller of my app? NavigationController is a subclass of UIViewController, but its task is only for providing navigation function at the navigation bar, correct?</p></li>
  <li><p>What is self.window? I think I understand the concept of 'view' and 'viewController' but I do not quite understand what a 'window' is... An iPhone has one screen, but MacPro could have 2 monitors; Are these windows in terms of iOS and OS X?</p></li>
  </ol>