Sunday, September 30, 2012

Tutorial 19 - Integrating Game Centre (Part 1)

19.1 Game Center

Game Center is Apple’s social gaming network. Integrating Game Center functionality into your Codea App has to be done in Xcode. There are three main areas that we need to concern ourself with to implement Game Center:

  1. Players;
  2. Scores; and
  3. Achievements.

All of your metadata for Game Center functionality is set up and managed in iTunes Connect, allowing you to test your Game Center features before submitting your app to the App Store. You use iTunes Connect to enable your app for Game Center testing, and set up your leaderboards and achievements. Then use the Game Kit framework in your app to add Game Center functionality.
Apple suggests that you should consider scores and achievements as part of your initial game design (as opposed to tacking it on at the end). This is a legitimate observation in most cases, for example passing other players in DoodleJump is arguably why it is so popular.
Be aware that once your Game Center assets are published to the live servers, some assets become more difficult to change because they are already in use by players and on live versions of your game. For example, leaderboard scores are formatted using the leaderboard assets you created. If you change your scoring mechanism and change your leaderboard assets to match, older scores would still be posted on Game Center and would be inconsistent with the newer scores. For this reason, some assets you create cannot be modified after the game ships.

19.2 Why Bother?

Many iOS games use Game Center, but not all of them use every feature. Apps can choose to include any or all of the following features supported by Game Center:
  • Leader-boards – compares scores with the player's friends and with other players from around the world
  • Achievements – shows goals that can be accomplished by the player and also allows the player to compare with friends' achievements
  • Multiplayer – the game can host matches in real time, either between the player's friends or by "auto-matching" with random players from around the world.
Some of the reasons that you may want to include Game Centre are:
  1. Improve the longevity and replayability of your game by adding challenges (aka Achievements) or multiplayer capability.
  2. Being able to see usage of your Apps via leaderboard activity.
  3. Improve the discoverability of your Apps through players challenging their friends and the new facebook "like" button.
  4. Allows players to rate your App from Game Center (if your App is rubbish this may not be a good thing).
To setup Game Centre there is code that we need to add to the App and then metadata which we need to add to iTunes Connect for the App. This tutorial will deal with the App side changes.

19.3 Step 1 - Authenticate the Player

To demonstrate the implementation of Game Center, we will use the MineSweeper App developed in previous tutorials. We are assuming that you are familiar with the Codea runtime and have got your App running in Xcode. If you haven't then read Tutorials 12 and 13 first. 
To start, open up your App in Xcode. The first thing we need to do is to link the GameKit framework. In the navigator area of Xcode 4, select the project name in the top left, it should be “CodeaTemplate”. Next, select the current target (“MineSweeper” in our case), and then select the “Build Phases” tab. Expand the “Link Binary With Libraries” option, and then click the “+” button to add a new framework. Type “g″ into the search box, and select the GameKit.framework framework that appears in the list. Click “Add” to link this framework to  your project. So far so good.

Once again in the Project Navigator pane, Under Classes -> Supporting Files, click on the CodifyAppDelegate.h file and add:
#import <GameKit/GameKit.h>
Your App will now recognise the Game Center methods and variables. If we weren't using Codea, setting up Game Center is pretty simple, to illustrate:
Click on CodifyAppDelegate.m and add the following to the end of the didFinishLaunchingWithOptions method:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
    // Existing code here...

    // Authenticate Player with Game Center
    GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
    // Handle the call back from Game Center Authentication
    [localPlayer authenticateWithCompletionHandler:^(NSError *error)
        if (localPlayer.isAuthenticated)
            // Player was successfully authenticated.
            // Perform additional tasks for the authenticated player.
        else if (error != nil)
            NSLog(@"error : %@", [error description]);

    // ...and back to the existing code.

    Return YES;

This is the code for pre-iOS 6 (since I have an iPad 1) devices. If you are developing for iOS 6, note that authenticateWithCompletionHandler is deprecated (use localPlayer.authenticateHandler instead).
Running this code in the Xcode simulator should present you with the pop up shown in Figure 1. Log in with your usual App ID to test.
Figure 1. Game Center Sign In.
If you haven't enabled your game for Game Center, once you log in you will get the error shown in Figure 2.
Figure 2. Game Center not enabled Error.

The problem is that we need to be able to submit scores and achievements from within our Codea App and Codea doesn't know about Game Center. However, one of the gun coders, @juaxix over on the Codea Forums has built a bridge between Objective C and Lua. We will extend this and use it to enable Game Center for MineSweeper. So delete the authentication code above if you added it to your App and we will show you the correct way to bring Game Center functionality to your Codea App.

In the next tutorial we will show you how to add leader boards and achievements in iTunes Connect. We will need these before we can make the changes required to our Lua and Objective C code.