Game center cocos2d questions - iphone

Question

Do I need iPhone developer certificate and the app available on the app store in order to implement game center.
Is it easy to implement game center leader boards into your game and if so any tutorials on doing so. I have a score counter already ,but not sure how I would add it into game center.
Any good tutorials on implementing game center into cocos2d and if so may I see a link (yes I have seen some tutorials ,but I want to be recommended a good tutorial).

Answer 1

You need an iPhone developer account to test and implement game center. You don't need to have an app on the App Store you can just test it on an unreleased project. I suggest you read the following tutorial, which explains everything about leaderboards pretty clearly:
Leaderboards Tutorial

Answer 2

Yes - but you would anyway to test on a device. And no, you have a 'sandboxed' version of Game Centre where games not on the app store/ development builds are tested.
Implementing leader-boards difficulty depends on what version of cocos2D you are using. From experience version 1 is a little more challenging then 2. Here is some code on how I have implemented it;
- (void)showLeaderboardForCategory:(NSString *)category
{
// Create leaderboard view w/ default Game Center style
GKLeaderboardViewController *leaderboardController = [[GKLeaderboardViewController alloc] init];
// If view controller was successfully created...
if (leaderboardController != nil)
{
// Leaderboard config
leaderboardController.leaderboardDelegate = self; // The leaderboard view controller will send messages to this object
leaderboardController.category = category; // Set category here
leaderboardController.timeScope = GKLeaderboardTimeScopeAllTime;
// Create an additional UIViewController to attach the GKLeaderboardViewController to
myViewController = [[UIViewController alloc] init];
[[[CCDirector sharedDirector] openGLView] addSubview:myViewController.view];
// Tell UIViewController to present the leaderboard
[myViewController presentModalViewController:leaderboardController animated:YES];
}
}
- (void)leaderboardViewControllerDidFinish:(GKLeaderboardViewController *)leaderboardController
{
[myViewController dismissModalViewControllerAnimated:YES];
myViewController.view = nil;
[myViewController release];
}
(Credit to the SO post that I originally used though - Leaderboard doesn't add in the screen cocos2d)

Answer 3

The Rod Strougo and Ray Wenderlich book "Learning Cocos2d" (2012) gives some good sample code for implementing game center leaderboards and achievements. Also "Learn cocos2d Game Development with iOS5" by Itterheim and Loew gives good examples too.
I followed the Strougo examples and it worked more-or-less the first time. But something people get stuck on (I did) is how to test. If you don't do it right it will seem like it isn't working when it actually is. It sounds like you haven't gotten started with GC yet so I won't try to explain what to do. Just be aware when the time comes to test that there is a procedure you need to follow that isn't completely obvious.