Signature to image transformation

main.dart

import 'dart:typed_data';

import 'package:flutter/material.dart';
import 'dart:ui' as ui;

void main() => runApp(new MaterialApp(
      home: new HomePage(),
      debugShowCheckedModeBanner: false,
    ));

class HomePage extends StatefulWidget {
  @override
  HomePageState createState() => new HomePageState();
}

class HomePageState extends State<HomePage> {
  List<Offset> _points = <Offset>[];
  ByteData imgBytes= new ByteData(1024);

  void saveToImage(List<Offset> points) async {
    final recorder = new ui.PictureRecorder();
    final canvas = new Canvas(recorder,
        new Rect.fromPoints(new Offset(0.0, 0.0), new Offset(200.0, 200.0)));
    Paint paint = new Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5.0;

    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null) {
        canvas.drawLine(points[i], points[i + 1], paint);
      }
    }

    final picture = recorder.endRecording();
    final img = picture.toImage(200, 200);
    final pngBytes = await img.toByteData(format: ui.ImageByteFormat.png);
    setState(() {
      imgBytes = pngBytes;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: <Widget>[
          new Container(
            child: new GestureDetector(
              onPanUpdate: (DragUpdateDetails details) {
                setState(() {
                  RenderBox object = context.findRenderObject();
                  Offset _localPosition =
                      object.globalToLocal(details.globalPosition);
                  _points = new List.from(_points)..add(_localPosition);
                });
              },
              onPanEnd: (DragEndDetails details) => _points.add(null),
              child: new CustomPaint(
                painter: new Signature(points: _points),
                size: new Size(200.0, 200.0),
              ),
            ),
          ),
          new Image.memory(new Uint8List.view(imgBytes.buffer)),
        ],
      ),
      floatingActionButton: new FloatingActionButton(
        child: new Icon(Icons.edit),
        onPressed: () => saveToImage(_points),
      ),
    );
  }
}

class Signature extends CustomPainter {
  List<Offset> points; //offset is object with x,y

  Signature({this.points});

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = new Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5.0;

    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null) {
        canvas.drawLine(points[i], points[i + 1], paint);
      }
    }
  }

  @override
  bool shouldRepaint(Signature oldDelegate) => oldDelegate.points != points;
}