Issue : callback injection fails at second NFC transceive in NFCV

[nicolasgarnet]

Hi,

As mentionned in issue 25, when having particular callback implementation, a second transceive command in NfcV fails, independently from the command content.

This minimal code demonstrates the issue :

 final Uint8List command = Uint8List.fromList([0x02, 0x2B]);

  Future<void> launchNfcSequence1() async {
    await NfcManager.instance.startSession(onDiscovered: (tag) async {
      final nfcV = NfcV.from(tag);
      await sequence(nfcV!);
    });
  }

  Future<void> launchNfcSequence2(
      Future<void> Function(NfcV nfcV) sequence) async {
    await NfcManager.instance.startSession(onDiscovered: (tag) async {
      final nfcV = NfcV.from(tag);
      await sequence(nfcV!);
    });
  }

  Future<void> launchNfcSequence3(
      Future<void> Function(NfcV nfcV) sequence) async {
    await NfcManager.instance.startSession(onDiscovered: (tag) async {
      _onDiscovered(sequence, tag);
    });
  }

  // Transceive command twice
  Future<void> sequence(NfcV nfcV) async {
    try {
      final answer1 = await nfcV.transceive(data: command);
      print(answer1.toString());
      final answer2 = await nfcV.transceive(data: command);
      print(answer2.toString());
    } catch (e) {
      print(e);
      rethrow;
    }

  Future<void> _onDiscovered(
      Future<void> Function(NfcV nfcV) sequence, NfcTag tag) async {
    final nfcV = NfcV.from(tag);
    await sequence(nfcV!);
  }
  }

This code implements 3 NFC implementations called launchSequence1-2-3.

• 1 : onDiscovered calls double commands directly after instantiating NfcVTag => Works
• 2 : a callback is passed to onDiscovered after instanting NfcVTag => Works
• 3 : onDiscovered calls another function to instantiate NfcVTag with a callback passed to it => Fails

It seems that instantiating NfcVTag.from(tag) outside the onDiscovered callback is what causes the issue. I don't know the library enough to explain why the first command works and not the second.

Resolving the issue would make this library better suited for dependency injection.