image.dart

/*
 * QR.Flutter
 * Copyright (c) 2019 the QR.Flutter authors.
 * See LICENSE for distribution and usage details.
 */
import 'package:flutter/foundation.dart';
import 'package:flutter/widgets.dart';
import 'package:qr/qr.dart';

import 'painter.dart';

typedef ErrorBuilder = Widget Function(BuildContext context, dynamic error);

class QrImage extends StatelessWidget {
  final String data;
  final ErrorBuilder errorBuilder;
  final Color foregroundColor;
  final Color backgroundColor;
  final EdgeInsets padding;
  final double size;
  final bool gapless;

  final int version;
  final int errorCorrectionLevel;

  QrImage({
    @required this.data,
    this.errorBuilder,
    Key key,
    this.size,
    this.padding = const EdgeInsets.all(10.0),
    this.backgroundColor,
    this.foregroundColor = const Color(0xFF000000),
    this.version = 4,
    this.errorCorrectionLevel = QrErrorCorrectLevel.L,
    this.gapless = true,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final qr = QrCode(version, errorCorrectionLevel);

    bool hasError = false;
    dynamic error;

    try {
      qr.addData(data);
      qr.make();
      hasError = false;
    } catch (e) {
      hasError = true;
      error = e;
    }

    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        final double widgetSize = size ?? constraints.biggest.shortestSide;
        return Container(
          width: widgetSize,
          height: widgetSize,
          color: backgroundColor,
          child: Padding(
            padding: padding,
            child: hasError
                ? errorBuilder(context, error)
                : CustomPaint(
                    painter: QrPainter(
                      qr: qr,
                      color: foregroundColor,
                      gapless: gapless,
                    ),
                  ),
          ),
        );
      },
    );
  }
}

main.dart

import 'package:flutter/material.dart';
import 'image.dart' show QrImage;

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sandbox',
      home: Scaffold(
        body: SafeArea(
          child: Center(
            child: _Home(),
          ),
        ),
      ),
    );
  }
}

class _Home extends StatefulWidget {
  @override
  __HomeState createState() => __HomeState();
}

class __HomeState extends State<_Home> {
  bool hasError = false;

  _toggleError() {
    setState(() {
      hasError = !hasError;
    });
  }

  @override
  Widget build(BuildContext context) {
    print(hasError);
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        QrImage(
          data: !hasError
              ? "yo"
              : "yo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79fyo9s87df8s79f",
          size: 128,
          errorBuilder: (_, e) => Text("error! $e"),
        ),
        RaisedButton(
          onPressed: _toggleError,
          child: Text("Toggle error"),
        )
      ],
    );
  }
}

painter.dart

/*
 * QR.Flutter
 * Copyright (c) 2019 the QR.Flutter authors.
 * See LICENSE for distribution and usage details.
 */
import 'dart:async';
import 'dart:ui' as ui;

import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';
import 'package:qr/qr.dart';

typedef QrError = void Function(dynamic error);

class QrPainter extends CustomPainter {
  QrPainter({
    @required this.qr,
    this.color = const Color(0xff000000),
    this.emptyColor,
    this.gapless = false,
  });

  final Color color; // the color of the dark squares
  final Color emptyColor; // the other color
  final bool gapless;

  final QrCode qr; // our qr code data
  final Paint _paint = Paint()..style = PaintingStyle.fill;

  QrCode get _qr => qr;

  @override
  void paint(Canvas canvas, Size size) {
    assert(size.shortestSide != 0,
        "[QR] WARN: width or height is zero. You should set a 'size' value or nest this painter in a Widget that defines a non-zero size");

    if (emptyColor != null) {
      canvas.drawColor(emptyColor, BlendMode.color);
    }

    final double squareSize = size.shortestSide / _qr.moduleCount.toDouble();
    final int pxAdjustValue = gapless ? 1 : 0;
    for (int x = 0; x < _qr.moduleCount; x++) {
      for (int y = 0; y < _qr.moduleCount; y++) {
        if (_qr.isDark(y, x)) {
          final Rect squareRect = Rect.fromLTWH(x * squareSize, y * squareSize,
              squareSize + pxAdjustValue, squareSize + pxAdjustValue);
          canvas.drawRect(squareRect, _paint);
        }
      }
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    if (oldDelegate is QrPainter) {
      return color != oldDelegate.color || _qr != oldDelegate._qr;
    }
    return false;
  }

  ui.Picture toPicture(double size) {
    final ui.PictureRecorder recorder = ui.PictureRecorder();
    final Canvas canvas = Canvas(recorder);
    paint(canvas, Size(size, size));
    return recorder.endRecording();
  }

  Future<ByteData> toImageData(double size,
      {ui.ImageByteFormat format = ui.ImageByteFormat.png}) async {
    final ui.Image uiImage =
        await toPicture(size).toImage(size.toInt(), size.toInt());
    return await uiImage.toByteData(format: format);
  }
}