title: How to request AdMob Interstitial before transition to display the ads immediatelly
body: <p>I've been using AdMob integration (interstitial ads) in my project.</p>
  
  <p>My project has the following structure:
  -Main Screen
  -Choose Level
  -Play Screen</p>
  
  <p>Is there anyway to request the interstitial ads in the Main Screen, and once the user enters the category or play screen the interstitial ads would be available to show?</p>
  
  <p>Currently the example provided by AdMob only shows code for a single page, which is to load and cache in the page itself. I would like to create an object or singleton in my App Delegate which would persist across all my View Controllers.</p>