Display multiple icon images in a symbol layer

import UIKit
import MapboxMaps
 
public class DataDrivenSymbolsExample: UIViewController, ExampleProtocol {
private var cancelables = Set<AnyCancelable>()
 
internal var mapView: MapView!
 
override public func viewDidLoad() {
super.viewDidLoad()
 
let centerCoordinate = CLLocationCoordinate2D(latitude: 37.761, longitude: -119.624)
let options = MapInitOptions(cameraOptions: CameraOptions(center: centerCoordinate, zoom: 10.0),
styleURI: .outdoors)
 
mapView = MapView(frame: view.bounds, mapInitOptions: options)
mapView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
view.addSubview(mapView)
 
// Allows the delegate to receive information about map events.
mapView.mapboxMap.onMapLoaded.observeNext { [weak self] _ in
self?.setupExample()
}.store(in: &cancelables)
}
 
public func setupExample() {
// Constant used to identify the source layer
let sourceLayerIdentifier = "yosemite-pois"
 
// Add icons from the U.S. National Parks Service to the map's style.
// Icons are located in the asset catalog
try! mapView.mapboxMap.addImage(UIImage(named: "nps-restrooms")!, id: "restrooms")
try! mapView.mapboxMap.addImage(UIImage(named: "nps-trailhead")!, id: "trailhead")
try! mapView.mapboxMap.addImage(UIImage(named: "nps-picnic-area")!, id: "picnic-area")
 
// Access a vector tileset that contains places of interest at Yosemite National Park.
// This tileset was created by uploading NPS shapefiles to Mapbox Studio.
var source = VectorSource(id: sourceLayerIdentifier)
source.url = "mapbox://examples.ciuz0vpc"
try! mapView.mapboxMap.addSource(source)
 
// Create a symbol layer and access the layer contained.
// The source property refers to the identifier provided when the source was added.
var layer = SymbolLayer(id: sourceLayerIdentifier, source: sourceLayerIdentifier)
 
// Access the layer that contains the Point of Interest (POI) data.
// The source layer property is a unique identifier for a layer within a vector tile source.
layer.sourceLayer = "Yosemite_POI-38jhes"
 
// Expression that adds conditions to the source to determine styling.
/// `POITYPE` refers to a key in the data source. The values tell us which icon to use from the sprite sheet
let expression = Exp(.switchCase) { // Switching on a value
Exp(.eq) { // Evaluates if conditions are equal
Exp(.get) { "POITYPE" } // Get the current value for `POITYPE`
"Restroom" // returns true for the equal expression if the type is equal to "Restrooms"
}
"restrooms" // Use the icon named "restrooms" on the sprite sheet if the above condition is true
Exp(.eq) {
Exp(.get) { "POITYPE" }
"Picnic Area"
}
"picnic-area"
Exp(.eq) {
Exp(.get) { "POITYPE" }
"Trailhead"
}
"trailhead"
"" // default case is to return an empty string so no icon will be loaded
}
 
// MARK: Explanation of expression
// See https://docs.mapbox.com/mapbox-gl-js/style-spec/expressions/#case for expression docs
/*
The expression yields the following JSON
[case,
[==, [get, POITYPE], Restroom], restrooms,
[==, [get, POITYPE], Picnic Area], picnic-area,
[==, [get, POITYPE], Trailhead], trailhead
]
 
This is a switch statement that makes decisions on the Key `POITYPE`
It will map the value of `POITYPE` to the image name on the sprite sheet.
*/
 
// MARK: Alternative expression that yields the same visual Output
// See https://docs.mapbox.com/mapbox-gl-js/style-spec/expressions/#match for expression docs
//        let expression = Exp(.match) {
//            Exp(.get) { "POITYPE" }
//            "Restroom"
//            "restrooms"
//            "Picnic Area"
//            "picnic-area"
//            "Trailhead"
//            "trailhead"
//            ""
//          }
 
/*
The expression yields the following JSON
[match,
[get, POITYPE],
Restroom, restrooms,
Picnic Area, picnic-area,
Trailhead, trailhead
]
 
This gets the POITYPE and matches the result of it to image name on the sprite sheet.
*/
 
layer.iconImage = .expression(expression)
 
try! mapView.mapboxMap.addLayer(layer, layerPosition: nil)
 
// The below line is used for internal testing purposes only.
finish()
}
}