title: Detect when app is minimized (iOS)
body: <p>I am building a mobile game and I have a NSTimer for my stopwatch.  Everything works fine, except when I minimize the app (press home screen), or when it is interrupted by, say a phone call, the NSTimer continues running in the background, when the application is not in use.</p>
  
  <p>I need to invalidate this timer when the app is minimized/interrupted and create a new timer when the app is resumed.  What methods handle when the app is minimized and resumed?  </p>