QUESTION: Reading previous write Tag Data

[alexlovar]

Hi, i like to learn about nfc, i use write tag with your sample and get this response:
Success to "Ndef Write"

but when i read the tag i get this:

{id: [249, 109, 254, 163], nfca: {atqa: [4, 0], maxTransceiveLength: 253, sak: 8, timeout: 618}, mifareclassic: {blockCount: 64, maxTransceiveLength: 253, sectorCount: 16, size: 1024, timeout: 618, type: 0}, ndef: {cachedMessage: {records: [{typeNameFormat: 1, type: [84], identifier: [], payload: [2, 101, 110, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33]}, {typeNameFormat: 1, type: [85], identifier: [], payload: [4, 102, 108, 117, 116, 116, 101, 114, 46, 100, 101, 118]}, {typeNameFormat: 2, type: [116, 101, 120, 116, 47, 112, 108, 97, 105, 110], identifier: [], payload: [72, 101, 108, 108, 111]}, {typeNameFormat: 4, type: [99, 111, 109, 46, 101, 120, 97, 109, 112, 108, 101, 58, 109, 121, 116, 121, 112, 101], identifier: [], payload: [109, 121, 100, 97, 116, 97]}]}, canMakeReadOnly: false, isWritable: true, maxSize: 716, type: com.nxp.ndef.mifareclassic}}

how i do to process the payload to see the info as was wrote?

thanks for your help!

[ibcoleman]

Anyone? The API seems to support getting NdefRecords but where's the support for getting useful values out of that record?

[BrayFlex]

Map tagData = tag.data;
Map tagNdef = tagData['ndef'];
Map cachedMessage = tagNdef['cachedMessage'];
Map records = cachedMessage['records'][0];
Uint8List payload = records['payload'];
String payloadAsString = String.fromCharCodes(payload);
// or with the dart:convert library String payloadAsString = utf8.decode(payload)

[BrayFlex]

After trying 4 different libraries I would recommend anyone who is targeting sdk 28+ to fork and use this repo: https://github.com/vilvic/nfc_in_flutter save yourself the headaches

[okadan]

@BrayFlex You should not access the tag.data property directly. Use Ndef.from to obtain an Ndef instance.

final ndef = Ndef.from(tag);
if (ndef == null) throw('Tag is not ndef.');
print(String.fromCharCodes(ndef.cachedMessage.records[0].payload));

@ibcoleman

The API seems to support getting NdefRecords but where's the support for getting useful values out of that record?

This plugin is just a bridge to the Android/iOS APIs, and does not do what these APIs do not.

So you need to decode Uint8List based on the NDEF specification (defined by NFC Forum). For example:

final record = ndef.cachedMessage.records[0];

if (record.typeNameFormat == NdefTypeNameFormat.nfcWellknown && record.type.length == 1 && record.type.first == 0x54) {
  // record type: NFC Wellknown Text
  final languageCodeLength = record.payload.first;
  final languageCode = ascii.decode(record.payload.sublist(1, 1 + languageCodeLength));
  final text = utf8.decode(record.payload.sublist(1 + languageCodeLength));
} else if (...) // For more record types...