Timer in Background in Swift 2
<p>I want to count costumer time(like this 00:20:04). But background app can work only 3 min. 
I have timer for it, and I can count with NSDate how many seconds app be close. But don't know how call this func.</p>

<p>So my question is: How app can understand when app in background and stop work, and how understand when return to app?  </p>