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javaandroidformatnfcndef

Setting and getting NdefRecord ID

I am trying to write and read back a number of NdefRecords on Android and am having problems with retrieving the record IDs. I believe it is because they are not being written to the tag initially.

I am creating my record(s) as such:

    private NdefRecord createRecord(String text, byte ID) throws UnsupportedEncodingException {
        String lang       = "en";
        byte[] textBytes  = text.getBytes();
        byte[] langBytes  = lang.getBytes("US-ASCII");
        int    langLength = langBytes.length;
        int    textLength = textBytes.length;
        byte[] id = new byte[1];
        id[0] = ID;
        int idLength = id.length;
        byte[] payload    = new byte[1 + langLength + textLength + idLength];

        payload[0] = (byte) langLength;
        //set use id flag
        payload[0] |= (1 << 3);

        // copy langbytes and textbytes into payload
        System.arraycopy(langBytes, 0, payload, 1,              langLength);
        System.arraycopy(textBytes, 0, payload, 1 + langLength, textLength);
//        System.arraycopy(id, 0, payload, 1 + langLength + textLength, idLength);

        NdefRecord recordNFC = new NdefRecord(NdefRecord.TNF_WELL_KNOWN,  NdefRecord.RTD_TEXT,  id, payload);

        return recordNFC;
    }


public void addRecord(String record_contents, RECORD_IDS record_id) throws UnsupportedEncodingException {
    this.records.add(createRecord(record_contents, (byte) record_id.getValue()));
}

I thought I was on to something with the 

//        System.arraycopy(id, 0, payload, 1 + langLength + textLength, idLength);

But it hasn't worked for me. This method stores NdefRecords in the class object which are then sent using

public void writeStoredRecords(Tag tag) throws IOException, FormatException {
    NdefRecord[] final_records = (NdefRecord[]) this.records.toArray(new NdefRecord[0]);
    NdefMessage message = new NdefMessage(final_records);
    try {
        Ndef ndef = Ndef.get(tag);
        if (ndef != null) {
            ndef.connect();
            if(!ndef.isWritable())
                return;
            ndef.writeNdefMessage(message);
            ndef.close();
        }
    }catch(Exception e){}
}

The record objects have their IDs populated after calling new NdefRecord but then when reading the tag using the app NXP TagInfo the NDEF record IDs are showing as " ".

Does anyone have any experience with this? As the record IDs are rarely used with NFC the online resources are scarce.

Solution

  • According to the NFC Forum NDEF specification, the ID field of an NDEF record must be a URI. Consequently, NXP TagInfo will treat this value as a URI string and decodes the byte array into a string (I'm not quite sure which encoding they expect though but browsing through the NDEF specification, I would expect US-ASCII encoding).

    Since you use a single byte as ID field, that value probably doesn't decode to a printable character. Therefore, NXP TagInfo simply prints " " (quotation marks surrounding the unprintable string value).