Flutter widget to Google Maps Marker icon

create_symbol_img.dart

import 'dart:io';

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:example_flutter/utils/widget_utils.dart';

void main() {
  testWidgets('create image from widget', (WidgetTester tester) async {
    final globalKey = GlobalKey();
    final widget = MaterialApp(
      home: Container(
        color: Colors.red,
        child: RepaintBoundary(
          key: globalKey,
          child: Center(
            child: Container(
              width: 100,
              height: 100,
              color: Colors.grey,
              child: Center(child: FlutterLogo(size: 100)),
            ),
          ),
        ),
      ),
    );

    await tester.pumpWidget(widget);

    
    final Uint8List markerIcon = await widgetToUint8List(key: globalKey, pixelRatio: 1.0);
    final Marker marker = Marker(icon: BitmapDescriptor.fromBytes(markerIcon));
  });
}

widget_utils.dart

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

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

// From:
// https://api.flutter.dev/flutter/rendering/RenderRepaintBoundary/toImage.html
Future<Uint8List> widgetToUint8List({GlobalKey key, double pixelRatio}) async {
  RenderRepaintBoundary boundary = key.currentContext.findRenderObject();
  ui.Image image = await boundary.toImage(pixelRatio: pixelRatio);
  ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
  return byteData.buffer.asUint8List();
}