AddData.dart

// load the data  
void loadCoffeeCountries() async {
    const jsonFilePath = 'assets/pathto.geojson';
    try {
      final contents = await services.rootBundle.loadString(jsonFilePath);
      final coffeeCountries = coffeeCountriesDataFromJson(contents);
      coffeeCountries.features.sort(
        (a, b) => a.properties.admin.compareTo(b.properties.admin));

      coffeeCountries.features
          .map((feature) => _coffeeFeatures.add(feature))
          .toList();
      
      // ...
  }

  // get single part and multipart polygon geometry
  Geometry? _getFeatureGeometry(CoffeeFeature feature) {
    final coffeeFeatureCoordinatesList = feature.geometry.coordinates;
    Geometry? polygon;

    if (feature.geometry.type == CountryGeometryType.polygon) {
      // if country is a single part polygon
      final polygonBuilder =
          PolygonBuilder.fromSpatialReference(SpatialReference.wgs84);
      for (final coordinates in coffeeFeatureCoordinatesList[0]) {
        final lat = coordinates[0];
        final long = coordinates[1];
        polygonBuilder.addPoint(
          ArcGISPoint(
            x: lat,
            y: long,
            spatialReference: SpatialReference.wgs84,
          ),
        );
      }
      polygon = polygonBuilder.toGeometry();
    } else if (feature.geometry.type == CountryGeometryType.multiPolygon) {
      // if country is a multipart polygon
      final polygonBuilderFromParts =
          PolygonBuilder.fromSpatialReference(SpatialReference.wgs84);
      for (final part in coffeeFeatureCoordinatesList) {
        final mutablePart =
            MutablePart.withSpatialReference(SpatialReference.wgs84);
        for (final list in part) {
          for (final coordinates in list) {
            final lat = coordinates[0];
            final long = coordinates[1];
            mutablePart.addPoint(
              ArcGISPoint(
                x: lat,
                y: long,
                spatialReference: SpatialReference.wgs84,
              ),
            );
          }
        }
        polygonBuilderFromParts.parts.addPart(mutablePart: mutablePart);
      }
      polygon = polygonBuilderFromParts.toGeometry();
    }
    return polygon;
  }

  // create, display and add attributes to graphic from geometry
  void _createAndDisplayGraphic(Geometry geometry, CoffeeFeature coffeeFeature) {
    final simpleFillSymbol = SimpleFillSymbol(
      style: SimpleFillSymbolStyle.solid,
      color: _colorManager.getUniqueColor(),
      outline: _borderSymbol,
    );

    final graphic = Graphic(geometry: geometry, symbol: simpleFillSymbol);

    graphic.attributes.addAll({
      'coffeeBagsProduced': coffeeFeature.properties.coffeeProduction,
      'countryName': coffeeFeature.properties.admin,
    });

    _graphicsOverlay.graphics.add(graphic);
  }