Google Maps Marker with Flutter Widgets

main.dart

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

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final markerKey = GlobalKey();
  final markers = Set<Marker>();

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((_) async {
      final markerUint = await getUint8List(markerKey);

      markers.add(Marker(
          markerId: MarkerId('one'),
          position: LatLng(48.1548895, 11.4717969),
          icon: BitmapDescriptor.fromBytes(markerUint)));
      setState(() {});
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Stack(
        children: <Widget>[
          Positioned(
            left: 10000,
            top: 10000,
            child: RepaintBoundary(
              key: markerKey,
              child: CustomMarkerWidget(),
            ),
          ),
          GoogleMap(
            initialCameraPosition: CameraPosition(
                target: LatLng(48.1548895, 11.4717969), zoom: 10.0),
            markers: markers,
          ),
        ],
      ),
    );
  }
}

class CustomMarkerWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 50.0,
      height: 50.0,
      color: Colors.red,
      child: Center(child: Text('Market')),
    );
  }
}

Future<Uint8List> getUint8List(GlobalKey markerKey) async {
  RenderRepaintBoundary boundary = markerKey.currentContext.findRenderObject();
  var image = await boundary.toImage(pixelRatio: 2.0);
  ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
  return byteData.buffer.asUint8List();
}