The Flutter plugin for iOS and Android to decoding QR codes.
First, add qr_code_tools as a dependency in your pubspec.yaml file.
Add one rows to the ios/Runner/Info.plist:
- one with the key 
Privacy - Photo Library Usage Descriptionand a usage description. 
Or in text format add the key:
<key>NSPhotoLibraryUsageDescription</key>
<string>Can I use the photo library please?</string>Nothing to do
import 'package:qr_code_tools/qr_code_tools.dart';
String _data;
/// decode from local file
Future decode(String file) async {
  String data = await QrCodeToolsPlugin.decodeFrom(file);
  setState(() {
    _data = data;
  });
}For a more elaborate usage example see here.