iOS - GameKit

Gamekit is a framework that provides leader board, achievements, and more features to an iOS application. In this tutorial, we will be explaining the steps involved in adding a leader board and updating the score.
Step 1 − In iTunes connect, ensure that you have a unique App ID and when we create the application update with the bundle ID and code signing in Xcode with corresponding provisioning profile.
Step 2 − Create a new application and update application information. You can know more about this in apple-add new apps documentation.
Step 3 − Setup a leader board in Manage Game Center of your application's page where add a single leaderboard and give leaderboard ID and score Type. Here we give leader board ID as tutorialsPoint.
Step 4 − The next steps are related to handling code and creating UI for our application.
Step 5 − Create a single view application and enter the bundle identifier is the identifier specified in iTunes connect.
Step 6 − Update the ViewController.xib as shown below −
Step 7 − Select your project file, then select targets and then add GameKit.framework.
Step 8 − Create IBActions for the buttons we have added. 
Step 9 − Update the ViewController.h file as follows −
#import <UIKit/UIKit.h>
#import <GameKit/GameKit.h>

@interface ViewController : UIViewController
<GKLeaderboardViewControllerDelegate>

-(IBAction)updateScore:(id)sender;
-(IBAction)showLeaderBoard:(id)sender;

@end
Step 10 − Update ViewController.m as follows −
#import "ViewController.h"

@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad {
   [super viewDidLoad];
   if([GKLocalPlayer localPlayer].authenticated == NO) {
      [[GKLocalPlayer localPlayer] 
      authenticateWithCompletionHandler:^(NSError *error) {
         NSLog(@"Error%@",error);
      }];
   }    
}

- (void)didReceiveMemoryWarning {
   [super didReceiveMemoryWarning];
   // Dispose of any resources that can be recreated.
}

- (void) updateScore: (int64_t) score 
   forLeaderboardID: (NSString*) category {
   GKScore *scoreObj = [[GKScore alloc]
   initWithCategory:category];
   scoreObj.value = score;
   scoreObj.context = 0;
   
   [scoreObj reportScoreWithCompletionHandler:^(NSError *error) {
      // Completion code can be added here
      UIAlertView *alert = [[UIAlertView alloc]
      initWithTitle:nil message:@"Score Updated Succesfully" 
      delegate:self cancelButtonTitle:@"Ok" otherButtonTitles: nil];
      [alert show];
   }];
}

-(IBAction)updateScore:(id)sender {
   [self updateScore:200 forLeaderboardID:@"tutorialsPoint"];
}

-(IBAction)showLeaderBoard:(id)sender {
   GKLeaderboardViewController *leaderboardViewController =
   [[GKLeaderboardViewController alloc] init];
   leaderboardViewController.leaderboardDelegate = self;
   [self presentModalViewController:
   leaderboardViewController animated:YES];
}

#pragma mark - Gamekit delegates
- (void)leaderboardViewControllerDidFinish:
(GKLeaderboardViewController *)viewController {
   [self dismissModalViewControllerAnimated:YES];
}
@end
When we run the application, we'll get the following output −
When we click "show leader board", we would get a screen similar to the following −
When we click "update score", the score will be updated to our leader board and we will get an alert as shown below −

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