Opening Game Center in our game

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:

1. Open OverlaySKScene.swift in Xcode.
2. Add a new import statement at the top of the file so we can use the GameKit framework:

           import GameKit 

3. Update the line that declares the OverlaySKScene class so that our class adopts the GKGameCenterControllerDelegate protocol. This allows the Game Center screen to inform our scene when the player closes the Game Center:

           class OverlaySKScene: SKScene, SKPaymentTransactionObserver, SKProductsRequestDelegate, GKGameCenterControllerDelegate  {

   Add two nw variables at the top of the OverlauSKScene to add the leaderboard text and to check if the player was authenticated:

       var leaderboardText: SKLabelNode!...