MapLibreParticleLab/platform/ios/MapLibre.docc/GeoJSON.md

Working with GeoJSON Data

Adding a GeoJSON file to the map

MapLibre iOS offers several ways to work with GeoJSON files. GeoJSON is a standard file format for representing geographic data.

You can host a GeoJSON file or bundle it with your application, you can use a GeoJSON file as the basis of an MLNShapeSource object. Pass the file’s URL into the MLNShapeSource/initWithIdentifier:URL:options: initializer and add the shape source to the map using the MLNStyle/addSource: method. The URL may be a local file URL, an HTTP URL, or an HTTPS URL.

Once you’ve added the GeoJSON file to the map via an MLNShapeSource object, you can configure the appearance of its data and control what data is visible using MLNStyleLayer objects. You can also access the data programmatically.

Converting GeoJSON data into shape objects

If you have GeoJSON data in the form of source code (also known as “GeoJSON text”), you can convert it into an MLNShape, MLNFeature, or MLNShapeCollectionFeature object that the MLNShapeSource class understands natively. First, create an NSData object out of the source code string or file contents, then pass that data object into the MLNShape/shapeWithData:encoding:error: method. Finally, you can pass the resulting shape or feature object into the MLNShapeSource/initWithIdentifier:shape:options: initializer and add it to the map, or you can use the object and its properties to power non-map-related functionality in your application.

To include multiple shapes in the source, create and pass an MLNShapeCollection or MLNShapeCollectionFeature object to MLNShapeSource/initWithIdentifier:shape:options:. Alternatively, use the MLNShapeSource/initWithIdentifier:features:options: or MLNShapeSource/initWithIdentifier:shapes:options: method to create a shape source with an array. MLNShapeSource/initWithIdentifier:features:options: accepts only MLNFeature instances, such as MLNPointFeature objects, whose attributes you can use when applying a predicate to MLNVectorStyleLayer or configuring a style layer’s appearance.

Extracting GeoJSON data from the map

Any MLNShape, MLNFeature, or MLNShapeCollectionFeature object has an MLNShape/geoJSONDataUsingEncoding: method that you can use to create a GeoJSON source code representation of the object. You can extract a feature object from the map using a method such as MLNMapView/visibleFeaturesAtPoint:.

About GeoJSON deserialization

The process of converting GeoJSON text into MLNShape, MLNFeature, or MLNShapeCollectionFeature objects is known as “GeoJSON deserialization”. GeoJSON geometries, features, and feature collections are known in this SDK as shapes, features, and shape collection features, respectively.

Each GeoJSON object type corresponds to a type provided by either this SDK or the Core Location framework:

GeoJSON object type	SDK type
Position (longitude, latitude)	CLLocationCoordinate2D (latitude, longitude)
Point	MLNPointAnnotation
MultiPoint	MLNPointCollection
LineString	MLNPolyline
MultiLineString	MLNMultiPolyline
Polygon	MLNPolygon
MultiPolygon	MLNMultiPolygon
GeometryCollection	MLNShapeCollection
Feature	MLNFeature
FeatureCollection	MLNShapeCollectionFeature

A Feature object in GeoJSON corresponds to an instance of an MLNShape subclass conforming to the MLNFeature protocol. There is a distinct MLNFeature-conforming class for each type of geometry that a GeoJSON feature can contain. This allows features to be used as raw shapes where convenient. For example, some features can be added to a map view as annotations. Note that identifiers and attributes will not be available for feature querying when a feature is used as an annotation.

In contrast to the GeoJSON standard, it is possible for MLNShape subclasses other than MLNPointAnnotation to straddle the antimeridian.

The following GeoJSON data types correspond straightforwardly to Foundation data types when they occur as feature identifiers or property values:

GeoJSON data type	Objective-C representation	Swift representation
null	NSNull	NSNull
true, false	NSNumber.boolValue	Bool
Integer	NSNumber.unsignedLongLongValue, NSNumber.longLongValue	UInt64, Int64
Floating-point number	NSNumber.doubleValue	Double
String	NSString	String