Starting with version 3.5, Unity supports GameCenter out of the box! Here’s how to put GameCenter Achievements in your game:

1) Log into iTunes Connect and set up all your achievements. Make sure you have a name, a pre/post description and 512×512 images ready to upload.

2) In Unity you need to start up GameCenter – do this as soon as possible in the game! Code adapted from the docs:

void Start () {
	Debug.Log("SocialHelper: Trying to start up");
	Social.localUser.Authenticate(ProcessAuthentication);
}

// This function gets called when Authenticate completes
// Note that if the operation is successful, Social.localUser will contain data from the server.
void ProcessAuthentication (bool success) {
    if (success) {
        Debug.Log ("Authenticated, checking achievements");
        // Request loaded achievements, and register a callback for processing them
        Social.LoadAchievements (ProcessLoadedAchievements)
    } else
        Debug.Log ("Failed to authenticate");
    }

    // This function gets called when the LoadAchievement call completes
    void ProcessLoadedAchievements (IAchievement[] achievements) {
    if (achievements.Length == 0) Debug.Log ("No achievements found"); //This only means that the user has not earned any yet!
    else Debug.Log ("Got " + achievements.Length + " achievements");
}

3) Each time the player earns an achievement, do something like this:

IAchievement achievement = Social.CreateAchievement();
achievement.id = "AchID001"; //This is the id you had to set up for each achievement in iTunes Connect...
achievement.percentCompleted = 100.0; //progress as double
achievement.ReportProgress(result => {
if (result) {
    Debug.Log ("Successfully reported progress");
} else {
    Debug.Log ("Failed to report progress");
}});

4) Log out of GameCenter – then build it, run it on your device. The GameCenter-Login window will pop up – make a new account. It should say “Sandbox” on the top left of the screen because this will be your test-account. Once you’re done it should report how many achievements you have in the console: “Achievements loaded 0, No achievements returned, No achievements found” (This is what it should read the very first time before you’ve earned any achievements) NOTE: If you’re not getting the Sandbox watermark and achievements can’t be loaded, make sure you’re not building with a wildcard-provisioning-profile (com.yoursite.*) – those won’t work with GameCenter! Best delete the app and build again with the correct Provisioning Profile.

5) Go earn an achievement and have a look at the console: “Looking for AchID001, cache count is 0, Reporting achievement AchID001, with 100.000000, Successfully reported progress” – Yay!

6) Start the game again through XCode and once it’s logged in it will now report the earned achievement in the console! “Achievements loaded 1, 2012-02-14 18:04:56.543 yourgame[20000:707] Nr 0: AchID001 – 1 – 2012-02-14 16:41:16 +0000, Got 1 achievements”

7) Now you just need to add a way for the player to get to the Achievements screen in GameCenter and you have the most basic stuff working!

Social.ShowAchievementsUI();

And that’s it! But beware: I haven’t actually submitted the update to Apple yet.

Anyway, here’s a sneak-peak of NEW ORBIT‘s Achievements in GameCenter!