Writing Game Center Apps in iOS

Retrieving Leaderboards Information Programmatically

Problem

After reporting scores to a leaderboard, you are curious as to how you can retrieve this information from the leaderboard programmatically.

Solution

Use the loadScoresWithCompletionHandler: instance method of the GKLeaderBoard class, as shown in the Discussion section.

Discussion

To retrieve a leaderboard’s scores in your app, follow these steps:

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

2. Instantiate an object of type GKLeaderBoard.

3. Set the category of this object to the Reference ID of the leaderboard whose data you want to read (see Creating Leaderboards in iTunes Connect).

4. Call the loadScoresWithCompletionHandler: instance method of GKLeaderBoard class and pass a block that returns void and accepts two parameters. The first parameter to this block is an instance of NSArray, which will contain the scores that were loaded from the given leaderboard. The second parameter is of type NSError, which will contain an error (if any).

   Note

   Game Center might, under certain circumstances, return a valid array of scores to you and, at the same time, an error. This means that, although some of the scores were retrieved successfully, an error occurred while the scores were being fetched from Game Center. In this case, Game Center stops as soon as it receives the error and you will get some of the scores, not all of them.

Each leaderboard score in Game Center is encapsulated into an instance of GKScore ...