How to turn CBUUID into string

Question

I can't find any official way to get a UUID string back out of a CBUUID. These UUIDs can be 2 or 16 bytes long.

The goal is to store CBUUIDs in a file somewhere as a string, and then resurrect with [CBUUID UUIDWithString:] etc. Here is what I have so far.

// returns a simple 4 byte string for 16bit uuids, 128 bit uuids are in standard 8-4-4-4-12 format
// the resulting string can be passed into [CBUUID UUIDWithString:]
+(NSString*)CBUUIDToString:(CBUUID*)cbuuid;
{
    NSData* data = cbuuid.data;
    if ([data length] == 2)
    {
        const unsigned char *tokenBytes = [data bytes];
        return [NSString stringWithFormat:@"%02x%02x", tokenBytes[0], tokenBytes[1]];
    }
    else if ([data length] == 16)
    {
        NSUUID* nsuuid = [[NSUUID alloc] initWithUUIDBytes:[data bytes]];
        return [nsuuid UUIDString];
    }

    return [cbuuid description]; // an error?
}

Answer 1

I rigged up the following category to do this for CBUUID:

@interface CBUUID (StringExtraction)

- (NSString *)representativeString;

@end

@implementation CBUUID (StringExtraction)

- (NSString *)representativeString;
{
    NSData *data = [self data];

    NSUInteger bytesToConvert = [data length];
    const unsigned char *uuidBytes = [data bytes];
    NSMutableString *outputString = [NSMutableString stringWithCapacity:16];

    for (NSUInteger currentByteIndex = 0; currentByteIndex < bytesToConvert; currentByteIndex++)
    {
        switch (currentByteIndex)
        {
            case 3:
            case 5:
            case 7:
            case 9:[outputString appendFormat:@"%02x-", uuidBytes[currentByteIndex]]; break;
            default:[outputString appendFormat:@"%02x", uuidBytes[currentByteIndex]];
        }

    }

    return outputString;
}

@end

For this input:

NSLog(@"UUID string: %@", [[CBUUID UUIDWithString:@"0bd51666-e7cb-469b-8e4d-2742f1ba77cc"] representativeString]);
NSLog(@"UUID string2: %@", [[CBUUID UUIDWithString:@"1800"] representativeString]);

it produces the following output:

UUID string: 0bd51666-e7cb-469b-8e4d-2742f1ba77cc
UUID string2: 1800

and preserves the appropriate hyphenation for the 16 byte UUIDs, while supporting the simple 2-byte UUIDs.

Answer 2

To all those saying that CBUUID is toll-free bridged with CFUUIDRef, it's not.

CBUUID * foo = [CBUUID UUIDWithString:CBUUIDCharacteristicExtendedPropertiesString];
CFStringRef fooBar = CFUUIDCreateString(NULL, (__bridge CFUUIDRef)foo);
if (![CBUUIDCharacteristicExtendedPropertiesString isEqualToString:(__bridge NSString *)fooBar])
    NSLog(@"fubar!");

It's not crashing but you're getting garbage out. It's probably uniquely identifying garbage, but it can't be round-tripped.

PS: This didn't work as a comment because SO comments oddly don't allow code formatting.

Answer 3

iOS 7.1 (beta released yesterday, 11/18/13) introduced the following property on CBUUID:

@property(nonatomic, readonly) NSString *UUIDString

The UUID represented as a string. (read-only)

From CBUUID Class Reference.

It's also worth noting that for comparing a UUID string with a CBUUID, this works:

if ([cbuuidInQuestion isEqual:[CBUUID UUIDWithString:@"1234-5678-9012-1234"]]) {
    // isEqual tests for "the same UUID"
    // == tests for "the same CBUUID object"
}

Answer 4

I know it's been 7 month since it was asked and answered, but... CBUUID is “toll-free bridged” to CFUUID and the easiest way to convert is

 //CBUUID* uuid = descr.UUID;
 NSString* str = CFUUIDCreateString(nil, uuid);