An ongoing discussion about how to correctly send/receive data with Game Kit on Cocos2D Central prompted me to write a demo project to complement the Learn Cocos2D book’s Game Center chapter. The resulting working code is in this post, and also reprinted below.
The Game Kit Data Send/Receive Demo Project (download here) is based on the Learn Cocos2D book code made for the Game Center Chapter. It’s called Tilemap16 just to stay in line with the naming scheme.
You control the little Ninja as usual, but this time anytime you move it, it will also move on all other connected devices because the tile coordinate is sent via Game Kit to all connected players but only when it actually changes. The demo even allows you to move the Ninja on any device, and all others will follow, so it truly works in every direction. In addition, a score variable (int) is transmitted every frame just to show how to send different types of data at different times.
I wish I could have made a movie to show how cool this is but alas, my iPod was busy running the demo, so all I can show as proof is my iPod and iPad running the game with both Ninjas at the same position (mushy image made with my already-ancient iPhone 3G under perfect programming but terrible lighting conditions):
Source Code Example
In summary this is the code that’s used to send/receive data with the help of the GameKitHelper class I wrote for the book (also included in the download and free to use for anyone and any purpose):
[cc lang=”ObjC”]// add this to the header
typedef enum
{
kPacketTypePoint,
} EPacketTypes;
typedef struct
{
EPacketTypes type;
} SPacketInfo;
typedef struct
{
EPacketTypes type;
CGPoint point;
} SPointPacket;
[/cc]
Sending a packet:
[cc lang=”ObjC”]
-(BOOL)ccTouchBegan:(UITouch *)touch withEvent:(UIEvent *)event
{
CGPoint location = [touch locationInView: [touch view]];
SPointPacket packet;
packet.type = kPacketTypePoint;
packet.point = location;
gkHelper = [GameKitHelper sharedGameKitHelper];
[gkHelper sendDataToAllPlayers:&packet length:sizeof(packet)];
}
[/cc]
Receiving the packet:
[cc lang=”ObjC”]
-(void) onReceivedData:(NSData*)data fromPlayer:(NSString*)playerID
{
// first, assume it’s the general SPacketInfo, that way we can access type
SPacketInfo* packet = (SPacketInfo*)[data bytes];
switch (packet->type)
{
case kPacketTypePoint:
{
SPointPacket* pointPacket = (SPointPacket*)packet;
CCLOG(@”received point: %.1f, %.1f”, pointPacket->point.x, pointPacket->point.y);
break;
}
default:
CCLOG(@”received unknown packet type %i”, packet->type);
break;
}
}
[/cc]
Requirements & Setup
You need:
- 2 devices which are Game Center capable, eg they must have the Game Center App installed. If it isn’t, that device is not ready for Game Center. 1st & 2nd Generation devices up to iPhone 3G do not support Game Center. In addition Simulator will not work due to Matchmaking/Invites not being supported on the Simulator.
- 2 Game Center accounts (you can create dummy accounts via the Game Center App)
What you have to do:
- Create a new App on iTunes Connect, enable Game Center for that App, and replace the Bundle ID of your App with the one in the project’s info.plist. Please refer to the Game Center section in the iTunes Connect documentation.
- Make sure Push Notifications are enabled on all devices via Settings App.
- Run the Game Center App on both devices, login and make sure each device is logged in with a unique account. Then invite the other device’s account as friend and accept the friend request, this makes it easier to join each other’s matches.
- Build the Tilemap16 App and deploy it to both devices. Whenever you make a change to the code, you must deploy the App to both devices again, to make sure they run the same code.
- Run the App on one device, wait for the matchmaking screen to appear. Invite your other device’s friend account to join the match.
- Wait on the other device for the match invite to pop up. Tap Accept. The Tilemap16 game will launch.
- Tap Play Now on the first device once the other player has successfully connected and is ready.
- Move the Ninja on either device and watch it move on the other.
Enjoy!
Chapter 15 - The Final Chapter
This is it. The last one. I don’t know what’s going to be in it. It’s supposed to give the reader a “where to go next” kind of outlook. I hope that one of the places will be here, but obviously there’s tons of places to go and tons of things for cocos2d developers to learn.
If you have a particular idea what should be included in the greater cocos2d developer outlook kind of sense, please let me know!
Summary of working on Chapter 14 - Game Center
Amazing! Simply amazing. I think I fell in love with Game Center in the process, I haven’t worked with a networking API that’s so smooth, straightforward and easy to comprehend. No thanks to Apple’s already excellent step-by-step documentation. Still, there were a couple pitfalls and things that one could forget, and I did, that I’ve obviously included in the chapter.
On the other side, with almost a week late for this chapter, I realized how time consuming network programming is. There’s a lot of testing going on, and especially if you’re testing on two devices the process is incredibly inefficient compared to a simple single-player game. You always have to deploy to two devices for every test, and every time the delay between request and response adds to the time spent on testing. Add to that common network errors such as a drop in connection or something blocking the line with a download, and you’re up for a fun ride.
But that alone didn’t account for the one week delay. In the past two weeks I’ve been helping to renovate someone’s apartment, I got a cold, I held a presentation at the Macoun Mac OS X conference (about cocos2d obviously) while still recovering from the cold. Then our pet cat Yoshi had to see the vet and almost died during anesthesia because of a pulmonary edema. On the brighter side I’ve also attended a wedding and wrote a GameKitHelper class for this chapter which contains more stuff than I could describe in the book, including storing achievements which failed transmission, as is recommended but not implemented by Apple’s Game Center Programming Guide.
That’s also why I haven’t been answering emails timely recently. Please be patient, I’ll get to yours soon!
What’s left?
Now that I’m almost finished writing the book, what’s left? Obviously I have to review technical and other edits done by Apress. Current chapter 13 is in technical review while chapter 6 is in editorial review, after which it’s ready for production. So I’ll still be busy reviewing and making changes and additions to chapters during October and probably even November.
Then there’s the issue of the example projects having used three different versions of cocos2d, starting with 0.99.3. That was actually an oversight on my part because at the time I had 0.99.4 available. Luckily those changes are really insignificant for the first few projects. The DoodleDrop game already uses 0.99.4 and so does most of the book’s code. But for the Game Center chapter, I had to migrate the Tilemap project from 0.99.5 beta 1 to beta 3 and that was a huge step. It was easier to simply create a new project from the latest cocos2d template, then re-adding all game source code and resources to the new project. Still, that’s doable.
The bigger issue I have here is the fact that I can’t change anything in the book anymore, so the code should reflect what’s in the book. If the book mentions CCLabel then the code should use CCLabel and not CCLabelTTF. What I think is probably going to be a good compromise is to update the important (final) versions of each chapter’s example projects to cocos2d v1.0 once that is released. Obviously the code supplied with the book will remain as it is described in the book, so the upgraded code would be for reference only and a separate download. Whether I wait for v1.0 depends on how progress towards v1.0 is coming along around the book’s release date some time in December 2010.
In hindsight, I really wished I had used my Xcode template project and used that throughout the book. Back then I decided against it because it was important for me to write the code like almost all cocos2d developers would do. Now I regret the decision because I could have changed the way cocos2d developers start new cocos2d projects for the better. The whole updating process for cocos2d is a major PITA and then some, so I think I need to bring this issue to the table more frequently, more actively. At least until the cocos2d template installation procedure is changed to not copy all of the source code into each new project and then leaving it up to the developer to deal with upgrades.
Chapter 14 - Game Center
This chapter will revolve all around Apple’s Game Center technology. Especially peer to peer networking is very interesting. The idea is to create a small multiplayer room using the Isometric game built in Chapter 11.
I actually had to buy a new iPod Touch 4 to be able to test all the new features. I was blissfully unaware that the more exciting features aren’t available on my iPhone 3G. Darn, technology moves so fast.
Summary of working on Chapter 13 - Pinball Game
I think it’s an amazing game, and I’m happy that several commenters, Twitterers and people I know in real life (you know, those you can actually touch while you’re talking to them) mentioned a pinball game as a possible example. Come to think of it, a pinball game is just great! It’s almost a sandbox that you can add more stuff to it and experiment. And it allowed me to use VertexHelper, which I haven’t been able to mention in the previous chapter.
Also, I’m a big pinball fan. Well, I used to be a computer pinball game crack. Have I ever mentioned that I was a beta tester for Balls of Steels, the pinball game from Apogee/3D Realms? When the shareware table was officially released, the highscore I achieved held the #1 spot for roughly 6 months in the worldwide leaderboard. And I basically stopped playing this game session after about 12 consecutive hours, just because I didn’t see the end of it. I always had the extra balls maxed out and I was still on my first ball. I just got bored.
Anyhow, it was great fun to actually build a pinball table myself. I can now appreciate more than ever how complex it is to create these beasts, be it for real or as a computer simulation. My respect to all game developers who ever built a pinball game, most importantly: the developers of the Pinball Dreams/Fantasies/Illusions series created by DICE, the developer of Epic Pinball James Schmalz and of course the creators of the Balls of Steel game Wildfire Studios.
Of course, when you read this chapter you’ll learn more about the prismatic and revolute joints of Box2d as well as joint limits and motors, among many other things. I’m also excited to see some of the readers turn this baby into a really good pinball game. I admit, this version, it don’t look too good. But as it is, it’s fully functional and incredibly cool for the short time frame I had to develop it in.
