MGLGeoJSONSource should have a features initializer

[1ec5]

MGLGeoJSONSource currently has four initializers: two that accept a GeoJSON-formatted NSData object, and two that accept a URL to a local or remote GeoJSON file. MGLGeoJSONSource should have an additional initializer that accepts an array of MGLFeatures and would accompany the existing read-only features property.

Consider this basic use case: the developer wants to create a GeoJSON source containing a particular polyline feature. Currently, they have to either compose a GeoJSON string and stuff it into an NSData object or write out a GeoJSON file and refer to it by its path. Instead, it would be much more convenient to create the source using an MGLPolyline object directly.

To simplify MGLGeoJSONSource, we should remove -initWithSourceIdentifier:geoJSONData: and -initWithSourceIdentifier:URL: and make the options parameter on the remaining initializers nullable. MGLGeoJSONSource would be left with three initializers corresponding to the three properties features, geoJSONData, and URL.

We should also make MGLFeature.attributes read-write so that it’s possible to create a feature with specific attributes that a layer might filter for.

/cc @boundsj @incanus @frederoni

[1ec5]

This will require functionality for writing out MGLFeature objects as mapbox::geojson types or, alternatively, as GeoJSON strings. Each of the MGLFeature subclasses would implement a -geoJSONFeature method similar to the existing -annotationObjectWithDelegate: method, but using the attributes dictionary instead of MGLMultiPointDelegate.

In order to convert the attribute dictionary into a mapbox::geojson::property_map, we can implement category methods similar to -[NSNumber(MGLStyleAttributeAdditions) mbgl_boolPropertyValue]. Or, if that proves to be too cumbersome, a single recursive method would also work.

[1ec5]

This will require functionality for writing out MGLFeature objects as mapbox::geojson types

The manual MGLFeature-to-GeoJSON conversion in nitaliano/react-native-mapbox-gl#419 could serve as a template for the kind of transformation we’d need to do here.