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Add a line style layer from GeoJSON

A newer version of the Maps SDK is available

This page uses v6.4.1 of the Mapbox Maps SDK. A newer version of the SDK is available. Learn about the latest version, v11.3.0, in the Maps SDK documentation.

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 {
preconditionFailure("Failed to load local GeoJSON file")

guard let jsonData = try? Data(contentsOf: jsonUrl) else {
preconditionFailure("Failed to parse GeoJSON file")

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 }

guard let shapeFromGeoJSON = try? MGLShape(data: geoJson, encoding: String.Encoding.utf8.rawValue) else {
fatalError("Could not generate MGLShape")

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)