Warp/apps/mobile/lib/widgets/qr_scanner.dart

import 'dart:convert';
import 'dart:typed_data';

import 'package:flutter/material.dart';
import 'package:mobile_scanner/mobile_scanner.dart';

/// Scans the other side's verification QR code with the camera and returns
/// its bytes.
///
/// A verification code is binary, not text: it starts with "MATRIX", then
/// the version, the mode, the transaction and the keys. Codes that don't
/// start that way are ignored, so pointing the camera at something else
/// doesn't end the scan.
Future<Uint8List?> scanVerificationCode(BuildContext context) =>
    Navigator.of(context).push<Uint8List>(
      MaterialPageRoute(
        fullscreenDialog: true,
        builder: (_) => const _QrScanScreen(),
      ),
    );

final _prefix = ascii.encode('MATRIX');

bool _isVerificationCode(Uint8List bytes) {
  if (bytes.length < _prefix.length) return false;
  for (var i = 0; i < _prefix.length; i++) {
    if (bytes[i] != _prefix[i]) return false;
  }
  return true;
}

class _QrScanScreen extends StatefulWidget {
  const _QrScanScreen();

  @override
  State<_QrScanScreen> createState() => _QrScanScreenState();
}

class _QrScanScreenState extends State<_QrScanScreen> {
  final _controller = MobileScannerController(
    formats: const [BarcodeFormat.qrCode],
  );
  bool _done = false;

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  void _onDetect(BarcodeCapture capture) {
    if (_done) return;
    for (final barcode in capture.barcodes) {
      final bytes = switch (barcode.rawDecodedBytes) {
        DecodedBarcodeBytes(:final bytes) => bytes,
        DecodedVisionBarcodeBytes(:final bytes) => bytes,
        _ => null,
      };
      if (bytes != null && _isVerificationCode(bytes)) {
        _done = true;
        Navigator.pop(context, bytes);
        return;
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.black,
      appBar: AppBar(
        backgroundColor: Colors.black,
        foregroundColor: Colors.white,
        shape: const Border(),
        title: const Text(
          'Scan their code',
          style: TextStyle(color: Colors.white),
        ),
      ),
      body: Stack(
        children: [
          MobileScanner(
            controller: _controller,
            onDetect: _onDetect,
            errorBuilder: (context, error) => Center(
              child: Padding(
                padding: const EdgeInsets.all(32),
                child: Text(
                  error.errorCode == MobileScannerErrorCode.permissionDenied
                      ? 'Warp needs the camera to scan the code. Allow it in '
                            'the app settings, or compare emoji instead.'
                      : "The camera isn't available.",
                  textAlign: TextAlign.center,
                  style: const TextStyle(color: Colors.white),
                ),
              ),
            ),
          ),
          Center(
            child: Container(
              width: 260,
              height: 260,
              decoration: BoxDecoration(
                border: Border.all(color: Colors.white70, width: 3),
                borderRadius: BorderRadius.circular(18),
              ),
            ),
          ),
          const Positioned(
            left: 24,
            right: 24,
            bottom: 48,
            child: Text(
              "Point the camera at the code on the other device.",
              textAlign: TextAlign.center,
              style: TextStyle(color: Colors.white, fontSize: 15),
            ),
          ),
        ],
      ),
    );
  }
}