How to stop overwrite to an NFC tag using ios Swift

Question

I've application for NFC write in swift 5, Write is working properly but I want to update only 1 key from nfc without overwrite full nfc card data. NFC device is ISO 14443-3A NXP MIFARE Ultralight (Ultralight C). Please check code below and anybody can help on this.

 var payloadData = Data([0x02,0x65,0x6E])
  payloadData.append(self.record_to_write.data(using: .utf8)!)
                                    
                                let customTextPayload = NFCNDEFPayload.init(
                                   format: .nfcWellKnown,
                                   type: "T".data(using: .utf8)!,
                                   identifier: Data(),
                                   payload:payloadData
                                )
                                self.record_new_arr.append(customTextPayload)
let messge = NFCNDEFMessage.init(records: self.record_new_arr )
tag.writeNDEF(messge, completionHandler: { (error: Error?) in
            if nil != error{
                session.alertMessage = "Write NDEF message fail: \(error)"
                print("Write NDEF message fail: \(error)")
            }else{
                print("Write NDEF message successfull.")
                session.alertMessage = "Write NDEF message successfull."
            }

        })

Only need to prevent overwrite data into nfc using swift. Thanks in advance

score 1 · Answer 1 · answered Jul 03 '20 at 20:53

NDEF format is not really designed to appended to or updated directly on the card.

Because of the structure of the data using TLV records
T = Terminator (1 Byte)
L = Length (1+ Bytes)
V = Value (0+ Bytes) (this is the data to be stored)

and then another Terminator

Then in a sizeable number of cases the to append/update you would have to re-write the whole card or some large portion of the card anyway because the length would likely to need to be updated.
There are only a limited few cases where and replace existing data would make sense.

Therefore most implementation will only write a complete NDEF message, overwriting what already exists on the Card because to make updates you would have to read the whole message anyway.

The only way to update a NDEF message in read it in to your program, update/append records in memory, write the whole thing back to the card.

So in short what you are asking is not possible (while in a very limited case it might be possible for you to work out the changes yourself and make changes on a Type 2 card, but this would not work on other card types)

Thank you but I've already fetch all record then write entire record but records are not write using tag.writeNDEF in card. my card details is : Tag Type: ISO 14443-3A, technology: Type A, Mifare ultralight. **tag.writeNDEF** works in other card — 99 Degree Info, Jul 04 '20 at 11:49

How to stop overwrite to an NFC tag using ios Swift

1 Answers1