iOS Development: How can I encapsulate a string in an NSData object?

Question

I'm building a multiplayer game on the iPhone and I need to send string data to the other players in the game. To do that, I need to encapsulate my NSString* string data in an NSData object somehow. Here's an example of how my code is structured...

typedef struct 
{
   PACKETTYPE packetType;
   ??? stringToSend;  //<---not sure how to store this
} StringPacket;    

StringPacket msg;
msg.packetType = STRING_PACKET;
msg.stringToSend = ...  //  <---not sure what to do here
NSData *packet = [NSData dataWithBytes:&msg length:sizeof(StringPacket)];

So my question is, if StringPacket is a struct defined in my header, what type should the stringToSend property be so that I can easily call the dataWithBytes method of NSData to encapsulate the packet data in an NSData object?

Thanks for your wisdom!

Answer 1

At first, you should convert your NSString to UTF8 representation via [NSString UTF8String]. After that, i'd recommend to store in packet string length, and after that - the string characters themself. All that can be done via appending NSData, created from char* via [NSData dataWithBytes:]

NSMutableData packet = [[NSMutableData alloc] init];
[packet appendBytes:&msg.packetType, sizeof(msg.packetType)];
char *str = [yourString UTF8String];
int len = strlen(str);
[packet appendBytes:(void*)&len, sizeof(len)];
[packet appendBytes:(void*)str, len];

To parse packet back, you should do:

NSData packet; // your packet
[packet getBytes:(void*)&packet.msg range:NSMakeRange(0, sizeof(packet.msg))];
int len;
[packet getBytes:(void*)&len range:NSMakeRange(sizeof(packet.msg), sizeof(len)];
NSData *strData = [packet subdataWithRange:NSMakeRange(sizeof(packet.msg) + sizeof(len)), packet.length];
NSString *str = [[NSString alloc] initWithData:strData encoding:UTF8Encoding];

There can be some mistakes since i'm writing from memory, but I think you'll get the idea.

Answer 2

If your strings have a maximum length, it's rather easy and can be done efficiently. So, assuming your strings max length for these packets is 255 and you've decided to use UTF-8 to encode your strings (both sides need to agree which encoding they're using), you could do it like this:

typedef struct 
{
   PACKETTYPE packetType;
   uint8_t stringToSend[256];  // UTF8 string with max encoded length of 255 bytes
} StringPacket;    

StringPacket msg;
msg.packetType = STRING_PACKET;
[theString getCString:msg.stringToSend maxLength:256 encoding:NSUTF8StringEncoding];
NSData *packet = [NSData dataWithBytes:&msg length:sizeof(StringPacket)];

Now you will have a proper C string in your packet that is at most 255 bytes of string data and the null terminator. Note, if your string can't be encoded to UTF8 in the size you gave it, the method will return NO, so your real code should actually check for that and handle it.

If you can't have a size limit, you can basically do the same thing, but you have to deal with dynamically allocating the memory, copying the bytes, creating the data and properly freeing the memory at the right time, so it becomes much more involved but it's the same basic idea. See also the method -getBytes:maxLength:usedLength:encoding:options:range:remainingRange: on NSString, it can be very useful in generating these messages where the string size is dynamic and totally unknown.

For the most simple case, however, the code above should get the job done.