Zooming in on MKMapView to the MIN possible zoom level keeping map 'centered' at a specified coordinate.

MKMapView+util.swift

import Foundation
import MapKit

extension MKMapView {
    
    /// Region with MIN possible zoom level, such that self.centerCoordinate == center
    func region(center: CLLocationCoordinate2D) -> MKCoordinateRegion {
        var oldRegion: MKCoordinateRegion?
        var newRegion: MKCoordinateRegion?
        
        for latitudeDelta in stride(from: 0.0, through: 180.0, by: 5.0) {
            let latitudeDelta = latitudeDelta == 0 ? 1 : latitudeDelta
            let span = MKCoordinateSpan(latitudeDelta: latitudeDelta, longitudeDelta: latitudeDelta * 2)
            newRegion = regionThatFits(MKCoordinateRegion(center: center, span: span))
            if oldRegion?.span.latitudeDelta.rounded(to: 3) == newRegion?.span.latitudeDelta.rounded(to: 3),
                oldRegion?.span.longitudeDelta.rounded(to: 3) == newRegion?.span.longitudeDelta.rounded(to: 3) {
                break
            }
            oldRegion = newRegion
        }
        return newRegion!
    }
}