Zooms out a MKMapView to enclose all its annotations (inc. current location)

iOSMapKitFitAnnotations.m

MKMapRect zoomRect = MKMapRectNull;
for (id <MKAnnotation> annotation in mapView.annotations) {
    MKMapPoint annotationPoint = MKMapPointForCoordinate(annotation.coordinate);
    MKMapRect pointRect = MKMapRectMake(annotationPoint.x, annotationPoint.y, 0, 0);
    if (MKMapRectIsNull(zoomRect)) {
        zoomRect = pointRect;
    } else {
        zoomRect = MKMapRectUnion(zoomRect, pointRect);
    }
}
[mapView setVisibleMapRect:zoomRect animated:YES];

Unfortunately, showAnnotations is too zoomed out and doesn't look good. Thanks for this code everyone, here's my condensed version:

mapView.setVisibleMapRect(
    mapView.annotations.reduce(MKMapRectNull) { result, next in
        let point = MKMapPointForCoordinate(next.coordinate)
        let rect = MKMapRectMake(point.x, point.y, 0, 0)
        guard !MKMapRectIsNull(result) else { return rect }
        return MKMapRectUnion(result, rect)
    },
    edgePadding: UIEdgeInsetsMake(20, 20, 40, 20),
    animated: true
)

 extension MKMapView {

    var MERCATOR_OFFSET : Double {
        return 268435456.0
    }

    var MERCATOR_RADIUS : Double  {
        return 85445659.44705395
    }

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

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

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

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

    private func coordinateSpan(withMapView mapView: MKMapView, centerCoordinate: CLLocationCoordinate2D, zoomLevel: UInt) ->MKCoordinateSpan {
        let centerPixelX = longitudeToPixelSpaceX(longitude: centerCoordinate.longitude)
        let centerPixelY = latitudeToPixelSpaceY(latitude: centerCoordinate.latitude)

        let zoomExponent = Double(20 - zoomLevel)
        let zoomScale = pow(2.0, zoomExponent)

        let mapSizeInPixels = mapView.bounds.size
        let scaledMapWidth =  Double(mapSizeInPixels.width) * zoomScale
        let scaledMapHeight = Double(mapSizeInPixels.height) * zoomScale

        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;

        let minLat = pixelSpaceYToLatitude(pixelY: topLeftPixelY)
        let maxLat = pixelSpaceYToLatitude(pixelY: topLeftPixelY + scaledMapHeight)
        let latitudeDelta = -1 * (maxLat - minLat);

        let span = MKCoordinateSpanMake(latitudeDelta, longitudeDelta)
        return span
    }

    func zoom(toCenterCoordinate centerCoordinate:CLLocationCoordinate2D ,zoomLevel: UInt) {
        let zoomLevel = min(zoomLevel, 20)
        let span = self.coordinateSpan(withMapView: self, centerCoordinate: centerCoordinate, zoomLevel: zoomLevel)
        let region = MKCoordinateRegionMake(centerCoordinate, span)
        self.setRegion(region, animated: true)

    }
}

Original Code Credit: http://troybrant.net/blog/2010/01/set-the-zoom-level-of-an-mkmapview/
Swift 2.3 Translation Credit: https://stackoverflow.com/users/2729171/apinho
Swift 4.x Swift Translation: By me. 👍

This is the easiest way:

map.layoutMargins = UIEdgeInsets(top: 10, right: 10, bottom: 10, left: 10)
map.showAnnotations(map.annotations, animated: true)

It works! I promise.

Thanks for this!
In my particular case, I had annotations and overlays for them, like the user's location blue circle overlay (userLocation.horizontalAccuracy). To fit all the annotations and their overlays, I modified this a bit and got these two extensions.

extension MKMapView {
    func setVisibleMapRectToFitAllAnnotations(animated: Bool = true,
                                              shouldIncludeUserAccuracyRange: Bool = true,
                                              edgePadding: UIEdgeInsets = UIEdgeInsets(top: 35, left: 35, bottom: 35, right: 35)) {
        var mapOverlays = overlays

        if shouldIncludeUserAccuracyRange, let userLocation = userLocation.location {
            let userAccuracyRangeCircle = MKCircle(center: userLocation.coordinate, radius: userLocation.horizontalAccuracy)
            mapOverlays.append(MKOverlayRenderer(overlay: userAccuracyRangeCircle).overlay)
        }

        let zoomRect = MKMapRect(bounding: mapOverlays)
        setVisibleMapRect(zoomRect, edgePadding: edgePadding, animated: animated)
    }
} 

extension MKMapRect {
 init(bounding overlays: [MKOverlay]) {
        self = .null
        overlays.forEach { overlay in
            let rect: MKMapRect = overlay.boundingMapRect
            self = self.union(rect)
        }
    }
}

I just modified SimonJanevski's answer to show all annotations and overlays present on MapView.

extension MKMapView {
  func setVisibleMapRectToFitAllAnnotations(animated: Bool = true,
                                            shouldIncludeUserAccuracyRange: Bool = true,
                                            shouldIncludeOverlays: Bool = true,
                                            edgePadding: UIEdgeInsets = UIEdgeInsets(top: 35, left: 35, bottom: 35, right: 35)) {
    var mapOverlays = overlays
    
    if shouldIncludeUserAccuracyRange, let userLocation = userLocation.location {
      let userAccuracyRangeCircle = MKCircle(center: userLocation.coordinate, radius: userLocation.horizontalAccuracy)
      mapOverlays.append(MKOverlayRenderer(overlay: userAccuracyRangeCircle).overlay)
    }
    
    if shouldIncludeOverlays {
      let annotations = self.annotations.filter { !($0 is MKUserLocation) }
      annotations.forEach { annotation in
        let cirlce = MKCircle(center: annotation.coordinate, radius: 1)
        mapOverlays.append(cirlce)
      }
    }
    
    let zoomRect = MKMapRect(bounding: mapOverlays)
    setVisibleMapRect(zoomRect, edgePadding: edgePadding, animated: animated)
  }
}

extension MKMapRect {
  init(bounding overlays: [MKOverlay]) {
    self = .null
    overlays.forEach { overlay in
      let rect: MKMapRect = overlay.boundingMapRect
      self = self.union(rect)
    }
  }
}