title: Detect when a view controller goes to background and gets resumed
body: <p>I'm implementing a ViewController with the following requirement: If the user returns to the app after 15 minutes, the view should reload the data.</p>
  
  <p>I was thinking on using <code>viewDidDisappear</code> to save the timestamp when the app went to background and <code>viewDidAppear</code> for checking previously saved values and refresh if needed, but this methods are not getting called when switching between apps.</p>
  
  <p>How can I solve this in a easy way?</p>