Tag Archives: Game Design

Handling Game Center With iOS Multitasking

Apple’s Game Kit Programming Guide contains the following passage:

A game that supports multitasking and Game Center must take additional steps when authenticating the local player. When your game is in the background, the status of the authenticated player may change. A player can use the Game Center app to sign out. Another player might sign in before your game returns to the foreground.

Multitasking is available from iOS 4.0 onwards, though not all devices running 4.0 support multitasking. Game Center was added in iOS 4.1.

So what exactly should my game do when the Game Center player changes behind it’s back? (Note: I’m concentrating on achievements here, as Pawns does not have leaderboards.)

The simplest method that also allows for offline play is:

  • The iOS game tracks achievements that were earned on that device, and stores that information locally.
  • If Game Center is available, achievements are also pushed to the server.
  • All achievements are pushed to the server, not just as they are earned, but whenever a Game Center player is authenticated. This handles the case where the player may have earned some achievements offline.

In short: make your game work offline, then push achievements to Game Center (one-way) whenever it is available without worrying about which player is now logged in.

This method is straightforward but will lead to two glaring side effects:

  1. If a player logs into a second Game Center profile, the new profile will automatically get some or all of the achievements that were earned on the first profile.
  2. Since this is a one-way synchronization (i.e. from game to Game Center) a player with two devices may see some achievements on Game Center but not on one of the devices.

We can try to fix these issues but it quickly gets complicated. (If you aren’t interested in a discussion of the complications, feel free to scroll down to the next section: The Experiment.)

For example, fixing the first requires you to be strict about which achievements are earned by which player. You could read the achievements back from Game Center after every profile switch, but then you have to handle the case where the player earned some achievements offline- this requires merging the online and offline lists. If you do that you’ve also solved the second problem– a nice feature, but I personally think it’s pointless unless the player’s entire saved progress (saved games, list of finished levels, etc.) is also stored in the cloud using iCloud, Dropbox, or some other service.

The situation is even more complicated if the game itself supports multiple local user profiles (as opposed to games that treat everyone on the device as the same player.) Note that when the user switches to a new local profile there’s no way to switch their Game Center profile for them automatically. We can only do the reverse: switch local profiles when the Game Center player has changed. This approach would work especially well for an online-only game.

In short, there are a lot of decisions to make unless we opt for the simple, “stupid” approach. Being naturally lazy, my inclination was to do this and hope that players (and Apple) would put up with the side-effects. But try as I might I couldn’t find any blogs or forum posts describing how other developers handle this, and that made me very nervous.

The Experiment

I decided to see how other game developers handle this. I spent a happy evening playing six iOS games that I’d already earned achievements in, but as a different Game Center player. I also switched to a second device to see if any of them read the player’s Game Center achievements back from Apple server.

It was a tough job, but someone had to do it.

The results? Judging from the side effects, the following five games use the simple achievements model I described above:

All of them had the side effect where the original profile’s achievements get copied to the new profile. This was even true for Plants vs Zombies, which lets you switch local profiles. The local profiles are completely ignorant of which Game Center profile you are logged into.

The one game which didn’t take the easy way out was Space Miner. After switching to the second Game Center profile, Space Miner warned me whenever I tried to load a saved game that had been saved by a different Game Center player. (It was my choice whether to go ahead and load the saved game or cancel.) In addition, it seemed that some but not all of the old achievements were copied to the new Game Center profile! I’m guessing that the achievements specific to a single campaign of Space Miner were NOT copied to the new profile, but achievements that you’d earn over multiple games were copied. I don’t claim to understand the nuances of Space Miner’s design just from this test, but clearly the developers went above and beyond to try to intelligently reconcile local saved games and achievements with Game Center’s achievements.

None of the 6 games ever seemed to read achievements back from Game Center. When I played on the second device, only my Game Center account showed the achievements that had not been earned on that device. (This looked especially odd in Pit Droids, where the achievement “Hutt Flats Unlocked” was shown in Game Center, even though the Hutt Flats puzzles were still locked on that device.)

Note that none of the six games I tested store their saved games in the cloud for access on more than one device. Such a game would not take the simple approach I described here. I believe the easiest approach there would be to force the user to select local or cloud storage for each saved game to avoid having to merge them, but with effort it should be possible to synchronize the two more seamlessly.


It appears that most games simply take the straightforward approach: make and display achievements as if the game were offline, and push achievements to Game Center (one-way) whenever it is available. This wasn’t too surprising for games that have no local player profiles to switch between. I wasn’t so sure what to expect for games that do have them, but in retrospect it makes sense. Developers are simply counting on the fact that players just aren’t likely to switch to a different Game Center profile.

Black Cube Mention

I recently found a blog mention of Black Cube. A fan of Black Box-type logic games blogged about the versions that he’s played.

“I do not think [Black Cube] is a better version,” he wrote, referring to the new 3D rules, and I have to agree: they are not simple and accessible like the original rules. Having said that, it would be an interesting game design challenge to make Black Cube more accessible than I was able to manage during the contest. Perhaps one day I’ll take a crack at it.

Supporting iPhone and iPad Resolutions

I recently added a profile of my forthcoming game, Pawns for iPhone, to the website IndieDB. As I was generating screenshots I found to my dismay that I could not generate high-resolution screenshots of games in progress. The iPhone version of Pawns is currently hard-wired for iPhones in landscape mode.

I had planned all along to look into iPad and “retina display” support later on, possibly after the first release for iPhone. It never occurred to me that I would find those higher resolutions useful myself. Oops.

Even more embarrassing, the original version of Pawns supported many screen resolutions. It’s something I lost as I tuned the game for iPhone.

The original version used Unity’s GUI objects, which are always sized proportionally to the screen. To make the graphics crisper on the iPhone I had converted much of it over to UnityGUI. But pixel-correct textures come at the cost of needing to draw gui elements at every resolution that you will need them. The plan is to rebuild my gui elements at a much higher resolution and let them be scaled pretty much wherever they are used. Hopefully the result won’t be too blurry.

Isometric vs Perspective Camera

Recently I’ve been experimenting with using an isometric camera for Pawns for the iPhone.

Isometric views have always felt a little cheesy to me, for historical reasons. In the old days it was often used to fake 3D. Since things don’t get smaller as they move further away from an isometric viewpoint, you could use 2D sprites instead of drawing true 3D objects. This was useful when the graphics hardware was only good at drawing sprites. (For example, old arcade games like Q*Bert or Zaxxon painted their 3D graphics this way.)

The iPhone is of course perfectly capable of rendering true 3D. And with Unity I don’t gain any speed just by switching to a isometric view. Even so, the isometric view has a few advantages.

Firstly, the squares near the rear of the board are the same size as those in front, making it easier for the player to place arrows. Pieces in the back are also more visible this way.

I can also optimize the appearance of the 3D pieces. Under perspective it’s hard to make out the crosspiece on top of the king, for example, depending on the angle. In an isometric view the king’s rotation relative to the camera is always the same, whether it is on the left side of the board or the right. So each model only needs to look good from one angle.

This isn’t just a matter of aesthetics, or covering up my mediocre artistic skills. Pawns will not be fun on the iPhone unless each type of piece is instantly recognizable despite the small screen.

For that matter, with care I could improve the graphics by prerendering high-resolution chess models to make 2D sprites. This might improve their appearance- I’ve been having wrestling with lowering the polygon counts without having the pieces look too crappy. (A topic for a future blog entry, perhaps.)

Of course, if I go to billboarded sprites I can’t simply reposition the camera or lights whenever I feel like. And after seeing Pawns in perspective for so many years, I’m having a hard time getting used to the isometric look. Still, I’ll leave it this way for a while longer and see if it grows on me.