iOS中生成随机的UUID

+ (NSString *)uuidString
{
    CFUUIDRef uuid_ref = CFUUIDCreate(NULL);
    CFStringRef uuid_string_ref= CFUUIDCreateString(NULL, uuid_ref);
    NSString *uuid = [NSString stringWithString:(__bridge NSString *)uuid_string_ref];
    CFRelease(uuid_ref);
    CFRelease(uuid_string_ref);
    return [uuid lowercaseString];
}