Limitations of the turf geotypes package

[josxha]

Description

This discussion is about the use of geotypes for geo features. There are some problems or limitations of the package I'd like to collect and discuss.

Possible solutions

0. Implement implementations and workaround (currently solution and not suitable on the long term).
1. Talk to the dart turf team about these limitations and find a solution together.
2. Try other package for geojson types (e.g. geobase).
3. Use an own base package for geojson types (most work but better integrity).

[josxha]

nullable feature.id field

geotypes uses null as a feature id if no id is set.
From the geojson specification:

If a Feature has a commonly used identifier, that identifier SHOULD be included as a member of the Feature object with the name "id", and the value of this member is either a JSON string or number.

This means if there is no identifier, the "id" field should not exist in the json.

MapLibre Native does not allow null as a feature id and excepts that the id key does not exist in the json. We currently have a workaround to mitigate this problem in our example app.

flutter-maplibre/example/lib/animation_page.dart

Lines 40 to 49 in aba6de7

	final response = await get(
	Uri.parse('https://maplibre.org/maplibre-gl-js/docs/assets/hike.geojson'),
	);
	final geojsonLine = response.body;
	final geojson = FeatureCollection.fromJson(
	jsonDecode(geojsonLine) as Map<String, Object?>,
	);
	final lineString = geojson.features.first.geometry! as LineString;
	// TODO: setting the id is currently required as geotypes would set it to null, Feature.id documented at https://datatracker.ietf.org/doc/html/rfc7946#section-3.2
	geojson.features.first.id = 1;

[josxha]

const constructors

Many of the data classes in maplibre have const constructors. The const keyword can never be used when initializing new instances because the types from geotypes don't have const constructors. This mostly affects showcases and does not affect implementations where the data has to get loaded first.

flutter-maplibre/example/lib/layers_circle_page.dart

Lines 36 to 42 in aba6de7

	CircleLayer(
	points: _points,
	color: Colors.orange.withOpacity(0.5),
	radius: 20,
	strokeColor: Colors.red,
	strokeWidth: 2,
	),

This is mostly a nit as it has limited usage in real world usages.

[josxha]

No toString() overrides

We use toString() in our example app, for example to show the Position that has been clicked by the user. Because geotypes does not provide a toString() representation itself, we need to do it.

flutter-maplibre/example/lib/events_page.dart

Lines 72 to 73 in aba6de7

	String _formatPosition(Position point) =>
	'${point.lng.toStringAsFixed(3)}, ${point.lat.toStringAsFixed(3)}';

flutter-maplibre/lib/src/map_events.dart

Lines 72 to 73 in aba6de7

	@override
	String toString() => 'MapEventClick(point: $point)';

[josxha]

missing == and hashCode overrides for LineString and Polygon

While some geotypes classes have a hash code and equals override, LineString and Polygon does not. It is necessary to compare if instances contain the same data. Currently both are only equal or have the same hashCode if they are the same object.
Comparing large lists of Positions can be expensive, there could be some additional optimizations like comparing the list lengths first before checking every item of the list.

[josxha]

Using num

From the geojson specs:

JavaScript Object Notation (JSON), and the terms object, member, name, value, array, number, true, false, and null, are to be interpreted as defined in [RFC7159].

and from the JSON specs:

The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. [...]

From a JavaScript perspective, num as a data type for the number type described in the geojson specs seems natural. Though, as dart is not only used in the JavaScript world, all other programming languages like swift, kotlin or c expect a double or int. When interacting with those languages, all nums need to explicitly converted to a double using toDouble().

While the definition of the number data type applies to num as it is a super type of int and double, it applies to double itself as well.

flutter-maplibre/lib/src/platform/android/widget_state.dart

Lines 206 to 209 in aba6de7

	: pigeon.LngLat(
	lng: _options.initCenter!.lng.toDouble(),
	lat: _options.initCenter!.lat.toDouble(),
	),