Overlay around annotation

Hi I'm trying to create an overlay around my annotations, like apples reminders app, I have already created an MKCircle object which I think I should use to show the overlay but how do I convert my MKCircle object into an MKOVerlay object? maybe there's a better way to add annotations? I'm new to swift and programming. Any suggestions?

MKCircle is a MKOverlay object. You just need to add it as an overlay:

let circle = MKCircle(center: coordinate, radius: 1000)
mapView.add(circle)

Of course, you have to tell the map how to render it by implementing mapView(_:rendererFor:) in your delegate and instantiate a MKCircleRenderer for the MKCircle that’s passed as an overlay.

extension ViewController: MKMapViewDelegate {
    func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
        let renderer = MKCircleRenderer(overlay: overlay)
        renderer.fillColor = UIColor.cyan.withAlphaComponent(0.5)
        renderer.strokeColor = UIColor.cyan.withAlphaComponent(0.8)
        return renderer
    }
}

Clearly, make sure you specified the delegate for your MKMapView, too. And if you have other types of renderers, you might implement specific logic for those, too, e.g.

extension ViewController: MKMapViewDelegate {
    func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
        if let circle = overlay as? MKCircle {
            let renderer = MKCircleRenderer(circle: circle)
            renderer.fillColor = UIColor.cyan.withAlphaComponent(0.5)
            renderer.strokeColor = UIColor.cyan.withAlphaComponent(0.8)
            return renderer
        }

        if let polygon = overlay as? MKPolygon {
            let renderer = MKPolygonRenderer(polygon: polygon)
            renderer.fillColor = UIColor.blue.withAlphaComponent(0.5)
            renderer.strokeColor = UIColor.blue.withAlphaComponent(0.8)
            return renderer
        }

        if let polyline = overlay as? MKPolyline {
            let renderer = MKPolylineRenderer(polyline: polyline)
            renderer.fillColor = UIColor.red.withAlphaComponent(0.5)
            renderer.strokeColor = UIColor.red.withAlphaComponent(0.8)
            return renderer
        }

        fatalError("Unexpected overlay type")
    }
}