title: How to detect if SpriteKit game moves to background(the person exits the app)
body: <p>I am creating a SpriteKit game, and in my game scene I don't want them to be able to close the app and then go back into it later and keep playing where they left off.</p>
  <p>After some research I figured out that Notification Center lets the app know when it is moved to the background, but I don't know how to implement that into my game? Is it some sort of override of a function, or something I add into my &quot;didMove to view&quot; function.</p>
  <p>Im not sure where to start with this because I am using SpriteKit and I haven't been able to find anything online describing how to do this.</p>
  <p>Thanks for your help!</p>