在使用MapKit往地图上画自定义的图钉时，有时候我们需要在图钉下面加些文字说明，可以用下面的方法。

        // 自定义的MKAnnotation作成
        func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
            guard annotation is MKPointAnnotation else { return nil }

            let identifier = "Annotation" + String(arc4random())
            var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: identifier)
            if annotationView == nil {
                annotationView = MKAnnotationView(annotation: annotation, reuseIdentifier: identifier)
            } else {
                annotationView!.annotation = annotation
            }

            let image = UIImage(named: "\(annotation.annotationType)")!
            annotationView!.image = image.scaleImage(scaleSize: annotation.iconScaleSize)
            if annotation.annotationType == "Point" {
                let offset = CGPoint(x: 0, y: -(image.size.height / 2))
                annotationView!.centerOffset = offset
            }

            // ※ 通过 annotationView!.addSubview 添加自定义文本
            let nameLbl: UILabel! = UILabel(frame: CGRect(x: -24, y: 40, width: 100, height: 30))
            nameLbl.text = "自由文本"
            nameLbl.textColor = UIColor.black
            nameLbl.font = UIFont(name: "Atari Classic Extrasmooth", size: 10)
            nameLbl.textAlignment = NSTextAlignment.center
            annotationView!.addSubview(nameLbl)
            annotationView!.displayPriority = .required
            return annotationView
        }