Get & set the zoom level of a MKMapView

MKMapView+Extensions.swift

import MapKit

// http://troybrant.net/blog/2010/01/set-the-zoom-level-of-an-mkmapview/

private let mercadorRadius: Double = 85445659.44705395
private let mercadorOffset: Double = 268435456

private func longitudeToPixelSpaceX(longitude: Double) -> Double {
    return round(mercadorOffset + mercadorRadius * longitude * .pi / 180.0);
}

private func latitudeToPixelSpaceY(latitude: Double) -> Double {
    return round(mercadorOffset - mercadorRadius * log((1 + sin(latitude * .pi / 180.0)) / (1 - sin(latitude * .pi / 180.0))) / 2.0);
}

private func pixelSpaceXToLongitude(pixelX: Double) -> Double {
    return ((round(pixelX) - mercadorOffset) / mercadorRadius) * 180.0 / .pi;
}

private func pixelSpaceYToLatitude(pixelY: Double) -> Double {
    return (.pi / 2.0 - 2.0 * atan(exp((round(pixelY) - mercadorOffset) / mercadorRadius))) * 180.0 / .pi;
}

private func coordinateSpan(with mapView: MKMapView, center: CLLocationCoordinate2D, zoomLevel: Double) -> MKCoordinateSpan {
    // convert center coordiate to pixel space
    let centerPixelX = longitudeToPixelSpaceX(longitude: center.longitude)
    let centerPixelY = latitudeToPixelSpaceY(latitude: center.latitude)
    
    // determine the scale value from the zoom level
    let zoomExponent = 20 - zoomLevel
    let zoomScale = pow(2, zoomExponent)
    
    // scale the map’s size in pixel space
    let mapSizeInPixels = mapView.bounds.size
    let scaledMapWidth = Double(mapSizeInPixels.width) * zoomScale
    let scaledMapHeight = Double(mapSizeInPixels.height) * zoomScale
    
    // figure out the position of the top-left pixel
    let topLeftPixelX = centerPixelX - (scaledMapWidth / 2)
    let topLeftPixelY = centerPixelY - (scaledMapHeight / 2)
    
    // find delta between left and right longitudes
    let minLng = pixelSpaceXToLongitude(pixelX: topLeftPixelX)
    let maxLng = pixelSpaceXToLongitude(pixelX: topLeftPixelX + scaledMapWidth)
    let longitudeDelta = maxLng - minLng
    
    // find delta between top and bottom latitudes
    let minLat = pixelSpaceYToLatitude(pixelY: topLeftPixelY)
    let maxLat = pixelSpaceYToLatitude(pixelY: topLeftPixelY + scaledMapHeight)
    let latitudeDelta = -1 * (maxLat - minLat)
    
    // create and return the lat/lng span
    return MKCoordinateSpan(latitudeDelta: latitudeDelta, longitudeDelta: longitudeDelta)
}

extension MKMapView {
    func setCenter(_ coordinate: CLLocationCoordinate2D, zoomLevel: Int, animated: Bool) {
        var zoomLevel = zoomLevel
        // clamp large numbers to 28
        zoomLevel = min(zoomLevel, 28);
        
        // use the zoom level to compute the region
        let span = coordinateSpan(with: self, center: coordinate, zoomLevel: Double(zoomLevel))
        let region = MKCoordinateRegion(center: coordinate, span: span)
        
        // set the region like normal
        setRegion(region, animated: animated)
    }
}