Prerequisites:

1. Apple Developers Account
2. Setup GameCenter Leaderboards with iTunesConnect

Setting up GameCenter Leaderboards:

1. Sign in to iTunesConnect
2. Go to My Apps. Create an app for your project then go to Features.
3. Click on Game Center
4. Click the plus sign next to Leaderboards.
5. Choose Single Leaderboard for Leaderboard types.
6. Create a Leaderboard Reference Name for your reference.
7. Create a Leaderboard ID for your app to refer to when reporting scores.
8. Set score format to Integer
9. Score Submission will be Best Score
10. Click Add language and fill the entries.

Copy your LeaderboardID that you made and lets head over to Xcode.

Working with Xcode

There are 4 functions that we will be working with.

1. Importing the framework and setting up the protocols

2. Checking if the user is signed in to GameCenter

3. Reporting the scores to GameCenter

4. Viewing leaderboards

5. Import GameKit import GameKit Protocols GKGameCenterControllerDelegate

6. Now we want to check if the user is signed in to GameCenter

func authenticateLocalPlayer() {
        
        let localPlayer = GKLocalPlayer.localPlayer()
        localPlayer.authenticateHandler = { (viewController, error) -> Void in
            
            if viewController != nil {
                //If the user is not signed in to GameCenter, we make them sign in
                let vc:UIViewController = self.view!.window!.rootViewController!
                vc.presentViewController(viewController!, animated: true, completion: nil)
                
            } else {
                
                //Do something here if you want
            }
        }
    }

3. Now the user is using the app and suddenly the user has a new high score, we report the high score by calling the function below.

The function below hols 2 parameters.

Identifier which is defined as a string and used to enter your leaderboardID that you made in iTunesConnect.

score which is defined as an Int which will be the users score to submit to iTunesConnect

func saveHighScore(identifier:String, score:Int) {
        
        if GKLocalPlayer.localPlayer().authenticated {
            
            let scoreReporter = GKScore(leaderboardIdentifier: identifier)
            
            scoreReporter.value = Int64(score)
            
            let scoreArray:[GKScore] = [scoreReporter]
            
            GKScore.reportScores(scoreArray, withCompletionHandler: {
                error -> Void in
                
                if error != nil {
                    print("Error")
                } else {
                    
                    
                }
            })
        }
    }

4. Now if the user wants to view leaderboards, call the function below

//This function will show GameCenter leaderboards and Achievements if you call this function.
    func showGameCenter() {
        
        let gameCenterViewController = GKGameCenterViewController()
        gameCenterViewController.gameCenterDelegate = self
        
        let vc:UIViewController = self.view!.window!.rootViewController!
        vc.presentViewController(gameCenterViewController, animated: true, completion:nil)
        
    }
    
    //This function closes gameCenter after showing.
    func gameCenterViewControllerDidFinish(gameCenterViewController: GKGameCenterViewController) {
        
        gameCenterViewController.dismissViewControllerAnimated(true, completion: nil)
        self.gameCenterAchievements.removeAll()
        
    }