iOS中生成随机的UUID

1 + (NSString *)uuidString
2 {
3     CFUUIDRef uuid_ref = CFUUIDCreate(NULL);
4     CFStringRef uuid_string_ref= CFUUIDCreateString(NULL, uuid_ref);
5     NSString *uuid = [NSString stringWithString:(__bridge NSString *)uuid_string_ref];
6     CFRelease(uuid_ref);
7     CFRelease(uuid_string_ref);
8     return [uuid lowercaseString];
9 }