Is this a valid way to build and send NSData through GameKit?

Question

Hi guys,

Its y first time trying to use NSData and Gamekit. So was wondering am i packing the data properly?

- (void)sendNetworkPacket:(GKSession *)session packetID:(int)packetID  
reliable:(BOOL)howtosend 
{
// the packet we'll send is resued
static unsigned char networkPacket[kMaxTankPacketSize];
const unsigned int packetHeaderSize = 2 * sizeof(int); // we have two "ints" for our 
header

int *pIntData = (int *)&networkPacket[0];
// header info
pIntData[0] = gamePacketNumber++;
pIntData[1] = packetID;
int theLength = 2 * sizeof(int);

for (int i=0; i<([theHands.player1Hand count]); i++)
{
    pIntData[2+i] = [[theHands.player1Hand objectAtIndex:i] intValue];
    theLenght += sizeof(int);
}   

NSData *packet = [NSData dataWithBytes: networkPacket length: theLength];

[session sendData:packet toPeers:[NSArray arrayWithObject:gamePeerId]  
withDataMode:GKSendDataReliable error:nil];
}

Will the data I put into NSData *packet be valid?

Many Thanks, -Code

Answer 1

You create the NSData correctly, and it will contain what you expect. But this is rather more complicated than necessary. The following will do too:

enum { kHeaderLength = 2 };
NSMutableData *packet = [NSMutableData dataWithLength: (kHeaderLength + [theHands.player1Hand count]) * sizeof( int )];
int *pIntData = (int *)[packet mutableBytes];

*pIntData++ = gamePacketNumber++;
*pIntData++ = packetID;

for (id thing in theHands.player1Hand) { 
  *pIntData++ = [thing intValue];
}

[session sendData: packet toPeers: [NSArray arrayWithObject: gamePeerId] withDataMode: GKSendDataReliable error: NULL];

This will have some advantages:

1. The packet data will not be copied to the NSData object, it will be created directly in there.
2. You don’t have to depend on a maximum packet size. You used a constant, which is good since you could change that in one place if bigger packets are required some time. But this is not really needed.
3. Your version is not thread safe since it depends on the static buffer. This might be fine, since not every method has to be thread-safe. But this is something one has to look out for.
4. Using fast enumeration also helps keeping the overhead down and is more readable.