Objective C - CFUUIDRef 创建UUID：

1.随机生成一串字符.可以用来当做文件名或其他：

+ (NSString*) createUuidString
{
    // create a new UUID which you own
    CFUUIDRef uuid = CFUUIDCreate(kCFAllocatorDefault);
    
    // create a new CFStringRef (toll-free bridged to NSString)
    // that you own
    NSString *uuidString = (__bridge NSString*)CFUUIDCreateString(kCFAllocatorDefault, uuid);
    
    // release the UUID
    CFRelease(uuid);
    
    // transfer ownership of the string
    // to the autorelease pool
    return uuidString;
}

2.根据需要，可以对生成的UUID进行MD5或者SHA1加密（感觉有点脑残，注意可能需要 字符编码转化UTF8）：

- (NSString*)md5Hash {
    unsigned char result[CC_MD5_DIGEST_LENGTH];
    CC_MD5([self bytes], [self length], result);
    
    return [NSString stringWithFormat:
            @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
            result[0], result[1], result[2], result[3], result[4], result[5], result[6], result[7],
            result[8], result[9], result[10], result[11], result[12], result[13], result[14], result[15]
            ];
}

- (NSString*)sha1Hash {
    unsigned char result[CC_SHA1_DIGEST_LENGTH];
    CC_SHA1([self bytes], [self length], result);
    
    return [NSString stringWithFormat:
            @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
            result[0], result[1], result[2], result[3], result[4], result[5], result[6], result[7],
            result[8], result[9], result[10], result[11], result[12], result[13], result[14], result[15],
            result[16], result[17], result[18], result[19]
            ];
}