OPENER-next / offline_geocoder

A simple package to reverse geocode locations offline.

offline_geocoder

An agnostic package to reverse geocode locations from a presupplied geojson file. No internet connection or external services required as your geocoder is generated once ahead of time.

How does it work

This uses dart's build tools to pregenerated source code from a GeoJSON file. The GeoJSON is converted into constant code on compile time wherefore no parsing or loading has to be done at runtime. The package allows you to specify which GeoJSON properties you want to extract and turns them into typed class properties.

Usage

1. Get your source GeoJSON FeatureCollection

You can get public domain licensed geo data from https://github.com/nvkelso/natural-earth-vector like country borders.

Example .geojson file:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "NAME": "Fiji",
        "LEVEL": 2
      },
      "geometry": {
        "type": "MultiPolygon",
        "coordinates": [ ... ]
      }
    },
    ...
  ]
}

Loading

2. Create build.yaml config file

Add the build_runner package as a dev_dependency to your pubspec.yaml. Then create a build.yaml file to make use of the offline_geocoder builder.

Example build.yaml

targets:
  $default:
    builders:
      offline_geocoder:
        options:
          # define the location of the GeoJSON file that contains your FeatureCollection
          source_file: 'assets/ne_110m_admin_0_countries.geojson'
          # define the output location of the generated GeoCoder dart file
          output_file: 'lib/services/offline_geocoder.dart'
          # specify which properties you want to extract from the GeoJSON including their data type
          extract_properties: {"NAME": "String", "LEVEL": "int"}

3. Generate and use the GeoCoder

Run dart run build_runner build or dart watch build_runner build from your project directory to generate the GeoCoder class. Import the generated dart file and use the geocoder like this:

final result = GeoCoder.getFromLocation(point);

Example main.dart:

import 'package:latlong2/latlong.dart';
import '/services/offline_geocoder.dart';

void main() {
  final result = GeoCoder.getFromLocation(const LatLng(-18.1248, 178.4501));
  print(result?.name);
  print(result?.level);
}