Apple Vision (which was introduced in v7) makes impossible to scan binary data from QR-code on iOS

[pirrest]

Hello!

My users generate QR-codes from within my app. I'm putting binary data there - encrypted key and some additional data encoded by ASN.1.
With v6 of the lib (which used MLKit from Google on iOS) it worked as a charm.
But starting from v7 I can't decode my QRs anymore :(

Can we have some setting to allow Vision API to return pure bytes? (it seems that it tries to interpret data as some text and returns not usable data).
Barcode.rawValue is null when trying to decode my QRs, so only rawBytes are there..

I have example of QR-code and resulting binary data that v6 and v7 returns if that might be helpful..

[pirrest]

Found a solution to convert Vision API results to a clean binary that I saved into the QR-Code:

`class _BitReader {
final Uint8List data;
int bitPos = 0; // bit offset into data

_BitReader(this.data);

int _availableBits() => (data.length * 8) - bitPos;

int readBits(int n) {
if (n <= 0 || n > 24 || _availableBits() < n) {
throw StateError('Not enough bits: need $n, have ${_availableBits()}');
}
int acc = 0;
for (int i = 0; i < n; i++) {
final byte = data[(bitPos + i) >> 3];
final bitIdx = 7 - ((bitPos + i) & 7);
final bit = (byte >> bitIdx) & 1;
acc = (acc << 1) | bit;
}
bitPos += n;
return acc;
}
}

/// Attempts to recover the original QR message bytes from Vision's payloadData:
/// [Vision rawBytes] -> strip QR segment header/len/terminator/padding -> [payload]
Uint8List recoverQrMessage(Uint8List rawBytes) {
// If it already looks like DER (ASN.1 SEQUENCE tag = 0x30), just return it.
if (rawBytes.isNotEmpty && rawBytes[0] == 0x30) return rawBytes;

final br = _BitReader(rawBytes);
final out = BytesBuilder();

// QR supports multiple segments; loop until terminator or out of bits.
while (br._availableBits() >= 4) {
final mode = br.readBits(4);

if (mode == 0x0) {
  // Terminator
  break;
} else if (mode == 0x7) {
  // ECI mode: read assignment number (per spec: variable-length)
  // Most QRs won't use this. We'll implement the common short form (<= 0x7F).
  // If top bit set, more bytes follow; handle up to 3 bytes.
  int eci = 0;
  int b1 = br.readBits(8);
  if ((b1 & 0x80) == 0) {
    eci = b1;
  } else {
    int b2 = br.readBits(8);
    if ((b1 & 0xC0) == 0x80) {
      eci = ((b1 & 0x3F) << 8) | b2;
    } else {
      int b3 = br.readBits(8);
      eci = ((b1 & 0x1F) << 16) | (b2 << 8) | b3;
    }
  }
  // We ignore ECI; we only need raw bytes.
  continue;
} else if (mode == 0x4) {
  // BYTE mode
  // Character count width depends on QR version. We don't have the version here,
  // but nearly all small QRs are version 1–9, so count is 8 bits.
  // If parsing fails with 8, you can try 16 bits as a fallback.
  int countBits = 8;
  final startBitPos = br.bitPos;
  int count = br.readBits(countBits);

  // If remaining bits are obviously too few/many, retry with 16.
  final needBits = count * 8;
  if (br._availableBits() < needBits) {
    // revert and retry with 16-bit count (versions 10–40)
    br.bitPos = startBitPos;
    countBits = 16;
    count = br.readBits(countBits);
  }

  if (count < 0 || br._availableBits() < count * 8) {
    throw StateError('Invalid BYTE segment length: $count');
  }

  // Read exactly 'count' bytes
  final buf = Uint8List(count);
  for (int i = 0; i < count; i++) {
    buf[i] = br.readBits(8);
  }
  out.add(buf);
  // Continue to see if more segments follow (rare in your use case).
} else if (mode == 0x2 || mode == 0x1 || mode == 0x8) {
  // ALPHANUMERIC / NUMERIC / KANJI — not expected for your data,
  // but we can skip them generically or throw to surface unexpected encodings.
  throw UnsupportedError('Unexpected QR mode $mode in payloadData');
} else {
  // Unknown mode
  throw UnsupportedError('Unknown QR mode $mode');
}

}

// The Vision payload often still has zero or more padding bytes after segments.
// We already stopped at the terminator, but if anything slipped through,
// strip trailing 0xEC, 0x11 padding.
Uint8List result = out.toBytes();
int end = result.length;
while (end > 0 && (result[end - 1] == 0xEC || result[end - 1] == 0x11)) {
end--;
}
if (end != result.length) {
result = Uint8List.sublistView(result, 0, end);
}

return result;
}`