Add a line style layer from GeoJSON

Related examples: drawing a GeoJSON line and animated line.

import Mapbox
class ViewController: UIViewController, MGLMapViewDelegate {
var mapView: MGLMapView!
override func viewDidLoad() {
mapView = MGLMapView(frame: view.bounds)
mapView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
CLLocationCoordinate2D(latitude: 45.5076, longitude: -122.6736),
zoomLevel: 11,
animated: false)
mapView.delegate = self
// Wait until the map is loaded before adding to the map.
func mapView(_ mapView: MGLMapView, didFinishLoading style: MGLStyle) {
func loadGeoJson() { {
// Get the path for example.geojson in the app’s bundle.
guard let jsonUrl = Bundle.main.url(forResource: "example", withExtension: "geojson") else { return }
guard let jsonData = try? Data(contentsOf: jsonUrl) else { return }
DispatchQueue.main.async {
self.drawPolyline(geoJson: jsonData)
func drawPolyline(geoJson: Data) {
// Add our GeoJSON data to the map as an MGLGeoJSONSource.
// We can then reference this data from an MGLStyleLayer.
// is optional, so you must guard against it not being set.
guard let style = else { return }
let shapeFromGeoJSON = try! MGLShape(data: geoJson, encoding: String.Encoding.utf8.rawValue)
let source = MGLShapeSource(identifier: "polyline", shape: shapeFromGeoJSON, options: nil)
// Create new layer for the line.
let layer = MGLLineStyleLayer(identifier: "polyline", source: source)
// Set the line join and cap to a rounded end.
layer.lineJoin = NSExpression(forConstantValue: "round")
layer.lineCap = NSExpression(forConstantValue: "round")
// Set the line color to a constant blue color.
layer.lineColor = NSExpression(forConstantValue: UIColor(red: 59/255, green:178/255, blue:208/255, alpha:1))
// Use `NSExpression` to smoothly adjust the line width from 2pt to 20pt between zoom levels 14 and 18. The `interpolationBase` parameter allows the values to interpolate along an exponential curve.
layer.lineWidth = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'linear', nil, %@)",
[14: 2, 18: 20])
// We can also add a second layer that will draw a stroke around the original line.
let casingLayer = MGLLineStyleLayer(identifier: "polyline-case", source: source)
// Copy these attributes from the main line layer.
casingLayer.lineJoin = layer.lineJoin
casingLayer.lineCap = layer.lineCap
// Line gap width represents the space before the outline begins, so should match the main line’s line width exactly.
casingLayer.lineGapWidth = layer.lineWidth
// Stroke color slightly darker than the line color.
casingLayer.lineColor = NSExpression(forConstantValue: UIColor(red: 41/255, green:145/255, blue:171/255, alpha:1))
// Use `NSExpression` to gradually increase the stroke width between zoom levels 14 and 18.
casingLayer.lineWidth = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'linear', nil, %@)",
[14: 1, 18: 4])
// Just for fun, let’s add another copy of the line with a dash pattern.
let dashedLayer = MGLLineStyleLayer(identifier: "polyline-dash", source: source)
dashedLayer.lineJoin = layer.lineJoin
dashedLayer.lineCap = layer.lineCap
dashedLayer.lineColor = NSExpression(forConstantValue: UIColor.white)
dashedLayer.lineOpacity = NSExpression(forConstantValue: 0.5)
dashedLayer.lineWidth = layer.lineWidth
// Dash pattern in the format [dash, gap, dash, gap, ...]. You’ll want to adjust these values based on the line cap style.
dashedLayer.lineDashPattern = NSExpression(forConstantValue: [0, 1.5])
style.insertLayer(casingLayer, below: layer)