Writing Game Center Apps in iOS

Retrieving Achievements Information Programmatically

Problem

You want to retrieve the progress of achievements that have been reported to Game Center for the local authenticated player.

Solution

You need to invoke the loadAchievementsWithCompletionHandler: class method of the GKAchievement class.

Discussion

Just as we can retrieve leaderboard information programmatically (see Retrieving Leaderboards Information Programmatically), we can ask Game Center to provide us with the latest information about the progress the authenticated local player has made on each one of the achievements that we have enabled on our app. To do this, simply follow these steps:

1. Authenticate the local player (see Authenticating the Local Player in Game Center).

2. Next, invoke the loadAchievementsWithCompletionHandler: class method of the GKAchievement class. This method accepts one parameter, which must be a block object that returns void and accepts two parameters. The first parameter is an array of type NSArray, which will contain the achievements retrieved from Game Center. The second parameter is an error of type NSError, which will contain any error that might happen during this process.

3. You can then enumerate the objects in the array of achievements retrieved from the aforementioned method. Each achievement object will be of type GKAchievement.

Here is an example of how we can retrieve achievements programmatically:

- (void) authenticateAndGetAchievements{ GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer]; ...