title: Use of background/foreground methods in AppDelegate
body: <p>I'm planning to implement multi-task in my app.
  I can see many methods here to do that in the <code>AppDelegate</code> like <code>applicationWillResignActive</code>, <code>applicationDidEnterBackground</code>, <code>applicationWillEnterForeground</code>, ...</p>
  
  <p>But.... I don't see the way they should be used, nor why they are not in the ViewControllers... Nor what they are here for.</p>
  
  <p>I mean : when the app enter in background, i don't know on which view my user is.
  And back, when the app comes into foreground, how would I know what to do and what I may call, to update the view for example ?</p>
  
  <p>I would have understood if those methods where in each view controller, but here, I don't see what they can be used for in a concrete way...</p>
  
  <p>Can you help me to understand the way to implement things into those methods ?</p>