Some Game Resources Are Provided at Runtime by the Game Center Service

All apps include images and localized text inside of its bundle that are used to display the app’s user interface. The app loads these resources from the bundle as needed. When you design a Game Center-aware game, some of the resources you create are not stored in the bundle. Instead, those resources are uploaded to the Game Center service during development of your game. At runtime, your game downloads the resources from Game Center. The main reason for storing these resources on Game Center is that those resources are also used by the Game Center app. For example, when the Game Center app displays one of your game’s leaderboards, it downloads the resources you provided so that it displays the score data the same way as your game.

The requirement that some of your resources be provided to Game Center affects how you design, develop, and test your game.

Your Game Displays Game Center’s User Interface Elements

Game Kit provides many classes that present full-screen user interfaces to the player. Standard classes are provided to display leaderboards, achievements, and matchmaking screens. For example, the GKGameCenterViewController class provides the simplest way to display Game Center content in your game.

In iOS, these are provided as view controllers. A view controller in your game presents one of these view controllers when necessary. On OS X, the same classes are used, but Game Center provides the infrastructure required to display them in a window.

Game Kit also provides support for banners. A banner appears for a short time to display a message to the player. Game Kit automatically presents some banner messages to the player when certain events occur, but your game can use the GKNotificationBanner class to display your own messages.

Game Center Features Require an Authenticated Player

All of Game Center’s features require an authenticated player on the device, known as the local player. Before your game uses any Game Center features, it must successfully authenticate the local player. Most Game Center classes function only if there is an authenticated player, and those classes implicitly reference the local player. For example, when your game reports scores to a leaderboard, it always reports scores for the local player.

Your game must disable all Game Center features when there is not an authenticated player.

Leaderboards Require Your Game to Have a Scoring Mechanism

Leaderboards allow your game to post scores to the Game Center service. Players can view these scores by viewing a leaderboard in the Game Center app, but your game can also display the standard leaderboard interface with just a few lines of code. Or, if you'd rather customize the appearance of a leaderboard, your game can download the raw score data. You can create multiple leaderboards for your game and customize each with your game’s scoring mechanisms.

Leaderboard Sets Allow You to Manage Your Leaderboards

Combine leaderboards into leaderboard sets to logically group leaderboards for your game. Implementing leaderboard sets raises the number of leaderboards that your game is allowed to contain. Combine all of the leaderboards for a single level into a set or combine the high score leaderboard from each level into a set, the decision on how to combine your leaderboards is up to you.

Achievements Require Your Game to Measure Player’s Progress

An achievement is a specific goal that the player accomplishes within your game, such as “Find 10 gold coins” or “Capture the flag in less than 30 seconds”. As with leaderboards, a player views achievements within the Game Center app or in your game. Within the Game Center app, players can compare earned achievements with those earned by friends. Within your game, you can choose to display the standard user interface or you can download the raw data to create your own custom interface.

Challenges Allow Players to Challenge Each Other

A challenge is sent from one player to another. Each challenge is a specific goal that the challenged player must complete. When a challenge is completed, both the challenger and the challenged player are notified. Challenges are automatically provided in any game that supports either leaderboards or achievements. However, you can also take the extra step of implementing customized support for challenges directly in your game.

Matchmaking Requires Your Game Design to Incorporate Multiplayer Gaming

Matchmaking allows players interested in playing online multiplayer games to discover each other and be connected into a match. Game Center supports three kinds of matchmaking:

• A real-time match requires all of the players to be connected to Game Center simultaneously. Game Kit provides classes that implement the low-level networking infrastructure to allow the devices to exchange data in real time.

• A hosted match is similar to a real-time match, but involves your own server in the match. In this model, you use Game Center to perform matchmaking but design and implement your own low-level networking code.

• A turn-based match uses a store-and-forward model. Your game stores a snapshot of the match data on Game Center’s servers where it can later be downloaded by any players in the match. At any given time, one of the players is designated as the person who can take a turn in the match. Your game downloads the match data, the player takes a turn, then your game uploads the modified match data to Game Center. When a player’s turn ends, your game designates the next player to act and that player receives a push notification.

  • Exchanges allow players who are not the current player to take actions within your game. A player initiates an exchange by sending an exchange request to one or more other players. These players are then able to respond to the exchange or let it time out. During the exchange, updates are also sent to the current player so that match data can be updated.