How can I save a qrimage in flutter?

I solved first adding this library: esys_flutter_share:

And modifing this:

Future<void> _captureAndSharePng() async {
    try {
      RenderRepaintBoundary boundary =
          globalKey.currentContext.findRenderObject();
      var image = await boundary.toImage();
      ByteData byteData = await image.toByteData(format: ImageByteFormat.png);
      Uint8List pngBytes = byteData.buffer.asUint8List();

      final tempDir = await getTemporaryDirectory();
      final file = await new File('${tempDir.path}/image.png').create();
      await file.writeAsBytes(pngBytes);

      await Share.file(_dataString, '$_dataString.png', pngBytes, 'image/png');
    } catch (e) {
      print(e.toString());
    }
  }

QrImage(
                  backgroundColor: Colors.white,
                        foregroundColor: Colors.black,
                  data: _dataString,
                  size: 0.5 * bodyHeight,
                  onError: (ex) {
                    print("[QR] ERROR - $ex");
                    setState((){
                      _inputErrorText = "Error! Maybe your input value is too long?";
                    });
                  },
                ),

Hope you find this helpful

import 'package:flutter/material.dart';
import 'package:qr_flutter/qr_flutter.dart';
import 'package:flutter/services.dart';
import 'dart:async';
import 'dart:typed_data';
import 'dart:ui';
import 'dart:io';
import 'package:flutter/rendering.dart';
import 'package:path_provider/path_provider.dart';

class GenerateScreen extends StatefulWidget {

  @override
  State<StatefulWidget> createState() => GenerateScreenState();
}

class GenerateScreenState extends State<GenerateScreen> {

  static const double _topSectionTopPadding = 50.0;
  static const double _topSectionBottomPadding = 20.0;
  static const double _topSectionHeight = 50.0;

  GlobalKey globalKey = new GlobalKey();
  String _dataString = "Hello from this QR";
  String _inputErrorText;
  final TextEditingController _textController =  TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('QR Code Generator'),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.share),
            onPressed: _captureAndSharePng,
          )
        ],
      ),
      body: _contentWidget(),
    );
  }

  Future<void> _captureAndSharePng() async {
    try {
      RenderRepaintBoundary boundary = globalKey.currentContext.findRenderObject();
      var image = await boundary.toImage();
      ByteData byteData = await image.toByteData(format: ImageByteFormat.png);
      Uint8List pngBytes = byteData.buffer.asUint8List();

      final tempDir = await getTemporaryDirectory();
      final file = await new File('${tempDir.path}/image.png').create();
      await file.writeAsBytes(pngBytes);

      final channel = const MethodChannel('channel:me.alfian.share/share');
      channel.invokeMethod('shareFile', 'image.png');

    } catch(e) {
      print(e.toString());
    }
  }

  _contentWidget() {
    final bodyHeight = MediaQuery.of(context).size.height - MediaQuery.of(context).viewInsets.bottom;
    return  Container(
      color: const Color(0xFFFFFFFF),
      child:  Column(
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.only(
              top: _topSectionTopPadding,
              left: 20.0,
              right: 10.0,
              bottom: _topSectionBottomPadding,
            ),
            child:  Container(
              height: _topSectionHeight,
              child:  Row(
                mainAxisSize: MainAxisSize.max,
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: <Widget>[
                  Expanded(
                    child:  TextField(
                      controller: _textController,
                      decoration:  InputDecoration(
                        hintText: "Enter a custom message",
                        errorText: _inputErrorText,
                      ),
                    ),
                  ),
                  Padding(
                    padding: const EdgeInsets.only(left: 10.0),
                    child:  FlatButton(
                      child:  Text("SUBMIT"),
                      onPressed: () {
                        setState((){
                          _dataString = _textController.text;
                          _inputErrorText = null;
                        });
                      },
                    ),
                  )
                ],
              ),
            ),
          ),
          Expanded(
            child:  Center(
              child: RepaintBoundary(
                key: globalKey,
                child: QrImage(
                  data: _dataString,
                  size: 0.5 * bodyHeight,
                  onError: (ex) {
                    print("[QR] ERROR - $ex");
                    setState((){
                      _inputErrorText = "Error! Maybe your input value is too long?";
                    });
                  },
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

This code is part of this post Building Flutter QR Code Generator, Scanner, and Sharing App

I solved this issue by using the screenshot package:

Wrap the widget that you want to save with the Screnshot() widget:

Screenshot(
controller: _screenshotController,
child: _buildQRImage('123456')
)

Then I created a builder method:

Widget _buildQRImage(String data) {
    return QrImage(
      data: data,
      size: 250.0,
      version: QrVersions.auto,
      errorCorrectionLevel: QrErrorCorrectLevel.H,
      gapless: false,
      foregroundColor: Colors.black,
      backgroundColor: Colors.white,
    );
  }

Then a _captureAndSaveQRCode() function with a TextButton() to trigger it:

TextButton(
            onPressed: () async {
              await _captureAndSaveQRCode();
            },
            child: Text("capture qr code")),

. To save the image call the captureAndSave function:

Future<void> _captureAndSaveQRCode() async {
    final imageDirectory = await imagePath;

    _screenshotController.captureAndSave(imageDirectory);
    setState(() {
      image = _loadImage();
    });
  }

Here is a complete working example:

import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
import 'package:qr_flutter/qr_flutter.dart';
import 'package:screenshot/screenshot.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const Scaffold(
        body: QrCode(),
      ),
    );
  }
}

class QrCode extends StatefulWidget {
  const QrCode({Key? key}) : super(key: key);

  @override
  State<QrCode> createState() => _QrCodeState();
}

class _QrCodeState extends State<QrCode> {
  final _screenshotController = ScreenshotController();
  Future<Image>? image;

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        TextButton(
            onPressed: () async {
              await _captureAndSaveQRCode();
            },
            child: const Text("capture qr code")),
        if (image != null)
          Center(
              child: Screenshot(
                  controller: _screenshotController,
                  child: _buildQRImage('123456'))),
      ],
    );
  }

  Widget _buildQRImage(String data) {
    return QrImage(
      data: data,
      size: 250.0,
      version: QrVersions.auto,
      errorCorrectionLevel: QrErrorCorrectLevel.H,
      gapless: false,
      foregroundColor: Colors.black,
      backgroundColor: Colors.white,
    );
  }

  Future<String> get imagePath async {
    final directory = (await getApplicationDocumentsDirectory()).path;
    return '$directory/qr.png';
  }

  Future<Image> _loadImage() async {
    return await imagePath.then((imagePath) => Image.asset(imagePath));
  }

  Future<void> _captureAndSaveQRCode() async {
    final imageDirectory = await imagePath;

    _screenshotController.captureAndSave(imageDirectory);
    setState(() {
      image = _loadImage();
    });
  }

  @override
  void initState() {
    image = _loadImage();
    super.initState();
  }
}