Makes a snhapshot of widget

make_snapshot.dart

import 'dart:typed_data';
import 'dart:ui';

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

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

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

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Web Snapshot Bug',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

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

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final repaintBoundaryKey = GlobalKey();

  final snapshot = ValueNotifier(Uint8List(0));

  @override
  void dispose() {
    snapshot.dispose();
    super.dispose();
  }

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) async {
      snapshot.value = await _makeSnapshot();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: RepaintBoundary(
        key: repaintBoundaryKey,
        child: Center(
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              const ElevatedButton(
                onPressed: null,
                child: Text('Press to make a snapshot'),
              ),
              const SizedBox(height: 50),
              ValueListenableBuilder<Uint8List>(
                valueListenable: snapshot,
                builder: (context, data, child) {
                  if (data.isNotEmpty) {
                    return Image.memory(
                      data,
                      width: 500,
                      height: 500,
                    );
                  }
                  return const Center(child: CircularProgressIndicator());
                },
              ),
            ],
          ),
        ),
      ),
    );
  }

  Future<Uint8List> _makeSnapshot() async {
    final repaintBoundary = repaintBoundaryKey.currentContext!
        .findRenderObject() as RenderRepaintBoundary;
    final image = await repaintBoundary.toImage();
    final byteData = await image.toByteData(format: ImageByteFormat.png);
    return byteData!.buffer.asUint8List();
  }
}