MKMapView with multiple overlays memory-issue

Question

There seems to be an \"issue\" with overlays and the MapKit. Unlike annotations, overlays aren\'t reused and therefore when adding multiple overlays would cause

Answer 1

A Swift 4 version of the Objective-C code posted by @wkberg:

MultiPolygon.swift:

import MapKit

/// A concatenation of multiple polygons to allow a single overlay to be drawn in the map,
/// which will consume less resources
class MultiPolygon: NSObject, MKOverlay {
    var polygons: [MKPolygon]?

    var boundingMapRect: MKMapRect

    init(polygons: [MKPolygon]?) {
        self.polygons = polygons
        self.boundingMapRect = MKMapRect.null

        super.init()

        guard let pols = polygons else { return }
        for (index, polygon) in pols.enumerated() {
            if index == 0 { self.boundingMapRect = polygon.boundingMapRect; continue }
            boundingMapRect = boundingMapRect.union(polygon.boundingMapRect)
        }
    }

    var coordinate: CLLocationCoordinate2D {
        return MKMapPoint(x: boundingMapRect.midX, y: boundingMapRect.maxY).coordinate
    }
}

MultiPolygonPathRenderer.swift:

import MapKit

/// A MKOverlayPathRenderer that can draw a concatenation of multiple polygons as a single polygon
/// This will consume less resources
class MultiPolygonPathRenderer: MKOverlayPathRenderer {
    /**
     Returns a `CGPath` equivalent to this polygon in given renderer.

     - parameter polygon: MKPolygon defining coordinates that will be drawn.

     - returns: Path equivalent to this polygon in given renderer.
     */
    func polyPath(for polygon: MKPolygon?) -> CGPath? {
        guard let polygon = polygon else { return nil }
        let points = polygon.points()

        if polygon.pointCount < 3 { return nil }
        let pointCount = polygon.pointCount

        let path = CGMutablePath()

        if let interiorPolygons = polygon.interiorPolygons {
            for interiorPolygon in interiorPolygons {
                guard let interiorPath = polyPath(for: interiorPolygon) else { continue }
                path.addPath(interiorPath, transform: .identity)
            }
        }

        let startPoint = point(for: points[0])
        path.move(to: CGPoint(x: startPoint.x, y: startPoint.y), transform: .identity)

        for i in 1..

Usage - Adding the overlay to your MKMapView:

// Add the overlay to mapView
let polygonsArray: [MKPolygon] = self.buildMKPolygons()
let multiPolygons = MultiPolygon.init(polygons: polygonsArray)
self.mapView.addOverlay(multiPolygons)

Usage - Implementing the viewForOverlay in your MKMapViewDelegate:

// Method viewForOverlay:
func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
    if overlay is MultiPolygon {
        let polygonRenderer = MultiPolygonPathRenderer(overlay: overlay)
        polygonRenderer.lineWidth = 0.5
        polygonRenderer.strokeColor = .mainGreen
        polygonRenderer.miterLimit = 2.0
        polygonRenderer.fillColor = UIColor.mainGreen.withAlphaComponent(0.2)
        return polygonRenderer
    }

    return MKOverlayRenderer()
}