If the user is authenticated, we will add a button to the
MenuScene class so that they can open the leaderboard and view achievements from within our game. Alternatively, players can always use the Game Center app in iOS to view their progress.
Follow these steps to create a leaderboard button in the menu scene:
OverlaySKSceneclass so that our class adopts the
GKGameCenterControllerDelegateprotocol. This allows the Game Center screen to inform our scene when the player closes the Game Center:
class OverlaySKScene: SKScene, SKPaymentTransactionObserver, ...