Capturing a Canvas as an Image in Flutter Web

Question

I am trying to capture the content of a Canvas as an Image using PictureRecorder in Flutter Web, but have trouble in doing so

While experimenting I drew a simple circle and tried recording it, but I keep getting 'null' for the Picture recorded

import 'package:flutter_web/material.dart';
import 'package:flutter_web_ui/ui.dart' as ui;

import 'dart:typed_data';

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

class App extends StatelessWidget {
  void generateImage() async {
    final ui.Paint paint = ui.Paint()
      ..style = ui.PaintingStyle.stroke
      ..strokeWidth = 1.0;
    final ui.PictureRecorder recorder = ui.PictureRecorder();
    final ui.Canvas pictureCanvas = ui.Canvas(recorder);
    pictureCanvas.drawCircle(Offset.zero, 20.0, paint);
    final ui.Picture picture = recorder.endRecording();
    ui.Image referenceImage = picture.toImage(50, 50);
    ByteData img =
        await referenceImage.toByteData(format: ui.ImageByteFormat.png);
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Column(
          children: <Widget>[
            Padding(
              padding: const EdgeInsets.all(12.0),
              child: RaisedButton(
                child: Text('Generate image'),
                onPressed: generateImage,
              ),
            ),
          ],
        ),
      ),
    );
  }
}

This is the error I get in chrome console,

main.dart:24 Uncaught (in promise) TypeError: Cannot read property 'toByteData' of null
    at generateImage (main.dart:24)
    at generateImage.next (<anonymous>)
    at runBody (dart_sdk.js:22330)
    at Object.async.async (dart_sdk.js:22358)
    at main.App.new.generateImage (main.dart:15)
    at src__material__ink_well.InkWell.new.<anonymous> (main.dart:38)
    at _InkResponseState.new.[_handleTap] (ink_well.dart:511)
    at ink_well.dart:565
    at src__gestures__tap.TapGestureRecognizer.new.invokeCallback (recognizer.dart:169)
    at src__gestures__tap.TapGestureRecognizer.new.[_checkUp] (tap.dart:251)
    at src__gestures__tap.TapGestureRecognizer.new.handlePrimaryPointer (tap.dart:176)
    at src__gestures__tap.TapGestureRecognizer.new.handleEvent (recognizer.dart:439)
    at src__gestures__pointer_router.PointerRouter.new.[_dispatch] (pointer_router.dart:73)
    at src__gestures__pointer_router.PointerRouter.new.route (pointer_router.dart:100)
    at src__widgets__binding.WidgetsFlutterBinding.new.handleEvent (binding.dart:223)
    at src__widgets__binding.WidgetsFlutterBinding.new.dispatchEvent (binding.dart:201)
    at src__widgets__binding.WidgetsFlutterBinding.new.[_handlePointerEvent] (binding.dart:156)
    at src__widgets__binding.WidgetsFlutterBinding.new.[_flushPointerEventQueue] (binding.dart:103)
    at src__widgets__binding.WidgetsFlutterBinding.new.[_handlePointerDataPacket] (binding.dart:87)
    at src__engine.PointerBinding.new.[_onPointerData] (pointer_binding.dart:80)
    at pointer_binding.dart:194
    at HTMLElement.<anonymous> (pointer_binding.dart:135)

Solution

I reported it as a bug. It seems that the only way to do it, at least until they fix it (it's end of May 2020, for future reference), is by doing it using the HTML5 canvas:

import 'dart:html' as html;

final canvas = html.CanvasElement(width: width, height: height);
var context = canvas.context2D;
context.draw...(); // do whatever drawing you need
final blob = await canvas.toBlob('image/jpeg', 0.8);

Converting from blob to byte array can be done with this:

Future<Uint8List> _getBlobData(html.Blob blob) {
  final completer = Completer<Uint8List>();
  final reader = html.FileReader();
  reader.readAsArrayBuffer(blob);
  reader.onLoad.listen((_) => completer.complete(reader.result));
  return completer.future;
}