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MGLFeature

@protocol MGLFeature <MGLAnnotation>

The MGLFeature protocol is used to provide details about geographic features contained in an MGLShapeSource or MGLVectorTileSource object. Each concrete subclass of MGLShape in turn has a subclass that conforms to this protocol. A feature object associates a shape with an optional identifier and attributes.

You can add custom data to display on the map by creating feature objects and adding them to an MGLShapeSource using the -[MGLShapeSource initWithIdentifier:shape:options:] method or MGLShapeSource.shape property.

In addition to adding data to the map, you can also extract data from the map: -[MGLMapView visibleFeaturesAtPoint:] and related methods return feature objects that correspond to features in the source. This enables you to inspect the properties of features in vector tiles loaded by MGLVectorTileSource objects. You also reuse these feature objects as overlay annotations.

While it is possible to add MGLFeature-conforming objects to the map as annotations using -[MGLMapView addAnnotations:] and related methods, doing so has trade-offs:

  • Features added as annotations will not have identifier or attributes properties when used with feature querying.

  • Features added as annotations become interactive. Taps and selection can be handled in -[MGLMapViewDelegate mapView:didSelectAnnotation:].

identifier

An object that uniquely identifies the feature in its containing content source.

You can configure an MGLVectorStyleLayer object to include or exclude a specific feature in an MGLShapeSource or MGLVectorTileSource. In the MGLVectorStyleLayer.predicate property, compare the special $id attribute to the feature’s identifier.

In vector tiles loaded by MGLVectorTileSource objects, the identifier corresponds to the feature identifier (id). If the source does not specify the feature’s identifier, the value of this property is nil. If specified, the identifier may be an integer, floating-point number, or string. These data types are mapped to instances of the following Foundation classes:

In the tile source

This property

Integer

NSNumber (use the unsignedLongLongValue or longLongValue property)

Floating-point number

NSNumber (use the doubleValue property)

String

NSString

For details about the identifiers used in most Mapbox-provided styles, consult the Mapbox Streets layer reference.

The identifier should be set before adding the feature to an MGLShapeSource object; setting it afterwards has no effect on the map’s contents. While it is possible to change this value on feature instances obtained from -[MGLMapView visibleFeaturesAtPoint:] and related methods, doing so likewise has no effect on the map’s contents.

Declaration

Objective-C

@property (nonatomic, copy, readwrite, nullable) id identifier;

Swift

var identifier: Any? { get set }

attributes

A dictionary of attributes for this feature.

You can configure an MGLVectorStyleLayer object to include or exclude a specific feature in an MGLShapeSource or MGLVectorTileSource. In the MGLVectorStyleLayer.predicate property, compare a key of the attribute dictionary to the value you want to include. For example, if you want an MGLLineStyleLayer object to display only important features, you might assign a value above 50 to the important features’ importance attribute, then set MGLVectorStyleLayer.predicate to an NSPredicate with the format importance > 50.

You can also configure many layout and paint attributes of an MGLStyleLayer object to match the value of an attribute in this dictionary whenever it renders this feature. For example, if you display features in an MGLShapeSource using an MGLCircleStyleLayer, you can assign a halfway attribute to each of the source’s features, then set MGLCircleStyleLayer.circleRadius to an expression for the key path halfway.

The MGLSymbolStyleLayer.text and MGLSymbolStyleLayer.iconImageName properties allow you to use attributes yet another way. For example, to label features in an MGLShapeSource object by their names, you can assign a name attribute to each of the source’s features, then set MGLSymbolStyleLayer.text to an expression for the constant string value {name}. See the Predicates and Expressions guide for more information about expressions.

In vector tiles loaded by MGLVectorTileSource objects, the keys and values of each feature’s attribute dictionary are determined by the source. Each attribute name is a string, while each attribute value may be a null value, Boolean value, integer, floating-point number, or string. These data types are mapped to instances of the following Foundation classes:

In the tile source

In this dictionary

Null

NSNull

Boolean

NSNumber (use the boolValue property)

Integer

NSNumber (use the unsignedLongLongValue or longLongValue property)

Floating-point number

NSNumber (use the doubleValue property)

String

NSString

For details about the attribute names and values found in Mapbox-provided vector tile sources, consult the Mapbox Streets and Mapbox Terrain layer references.

When adding a feature to an MGLShapeSource, use the same Foundation types listed above for each attribute value. In addition to the Foundation types, you may also set an attribute to an NSColor (macOS) or UIColor (iOS), which will be converted into its CSS string representation when the feature is added to an MGLShapeSource. This can be convenient when using the attribute to supply a value for a color-typed layout or paint attribute via a key path expression.

Note that while it is possible to change this value on feature instances obtained from -[MGLMapView visibleFeaturesAtPoint:] and related methods, there will be no effect on the map. Setting this value can be useful when the feature instance is used to initialize an MGLShapeSource and that source is added to the map and styled.

Declaration

Objective-C

@property (nonatomic, copy, readwrite)
NSDictionary<NSString *, id> *_Nonnull attributes;

Swift

var attributes: [String : Any] { get set }

-attributeForKey:

Returns the feature attribute for the given attribute name.

See the attributes property’s documentation for details on keys and values associated with this method.

Declaration

Objective-C

- (nullable id)attributeForKey:(nonnull NSString *)key;

Swift

func attribute(forKey key: String) -> Any?

-geoJSONDictionary

Returns a dictionary that can be serialized as a GeoJSON Feature representation of an instance of an MGLFeature subclass.

The dictionary includes a geometry key corresponding to the receiver’s underlying geometry data, a properties key corresponding to the receiver’s attributes property, and an id key corresponding to the receiver’s identifier property.

Declaration

Objective-C

- (nonnull NSDictionary<NSString *, id> *)geoJSONDictionary;

Swift

func geoJSONDictionary() -> [String : Any]