Flutter cluster bitmap rendering example

cluster.dart

BitmapDescriptor _stationPin;
ui.Image _clusterBgImage;

Future<BitmapDescriptor> _getClusterBitmap(
  BuildContext context,
  MapCluster cluster,
) async {
  if (!cluster.isCluster && _stationPin != null) {
    return _stationPin;
  }

  final density = MediaQuery.of(context).devicePixelRatio;
  final canvasScale = 1 / 3 * density;

  final pictureRecorder = ui.PictureRecorder();
  final canvas = Canvas(pictureRecorder);
  canvas.scale(canvasScale);

  final text = cluster.isCluster ? cluster.children.length.toString() : null;

  ui.Image bgImage;
  if (text != null && _clusterBgImage != null) {
    bgImage = _clusterBgImage;
  } else {
    final bgImageData = await rootBundle.load(
      text != null
          ? 'assets/map/cluster_icon@3x.png'
          : 'assets/map/gas_station_icon@3x.png',
    );
    bgImage = await decodeImageFromList(bgImageData.buffer.asUint8List());
  }

  paintImage(
    canvas: canvas,
    image: bgImage,
    rect: Rect.fromLTRB(
      0,
      0,
      bgImage.width.toDouble(),
      bgImage.height.toDouble(),
    ),
  );

  if (text != null) {
    final painter = TextPainter(textDirection: TextDirection.ltr);
    painter.text = TextSpan(
      text: text,
      style: TextStyle(
        fontSize: bgImage.width / 4,
        color: Colors.white,
        fontWeight: FontWeight.normal,
      ),
    );
    painter.layout();
    painter.paint(
      canvas,
      Offset(
        bgImage.width / 2 - painter.width / 2 + 3,
        bgImage.height / 5,
      ),
    );
  }

  final img = await pictureRecorder.endRecording().toImage(
        (bgImage.width * canvasScale).toInt(),
        (bgImage.height * canvasScale).toInt(),
      );
  final data = await img.toByteData(format: ui.ImageByteFormat.png);

  final result = BitmapDescriptor.fromBytes(data.buffer.asUint8List());
  if (text == null) {
    _stationPin ??= result;
  } else {
    _clusterBgImage ??= bgImage;
  }

  return result;
}