MKAnnotation 未在 MKMapView 上显示标注

在使用 MKMapView 进行地图标注时，有时候会遇到 MKAnnotation 未能在地图上显示的问题。本文将介绍一些常见的原因和解决方法，并提供一个案例代码来帮助读者更好地理解。

swift
let annotations = mapView.annotations
if !annotations.isEmpty {
    var zoomRect = MKMapRect.null
    for annotation in annotations {
        let annotationPoint = MKMapPoint(annotation.coordinate)
        let pointRect = MKMapRect(x: annotationPoint.x, y: annotationPoint.y, width: 0, height: 0)
        if zoomRect.isNull {
            zoomRect = pointRect
        } else {
            zoomRect = zoomRect.union(pointRect)
        }
    }
    mapView.setVisibleMapRect(zoomRect, edgePadding: UIEdgeInsets(top: 20, left: 20, bottom: 20, right: 20), animated: true)
}

swift
func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    let identifier = "AnnotationView"
    var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: identifier)
    if annotationView == nil {
        annotationView = MKPinAnnotationView(annotation: annotation, reuseIdentifier: identifier)
    } else {
        annotationView?.annotation = annotation
    }
    return annotationView
}

swift
class MyAnnotation: NSObject, MKAnnotation {
    var coordinate: CLLocationCoordinate2D
    var title: String?
    var subtitle: String?
    
    init(coordinate: CLLocationCoordinate2D) {
        self.coordinate = coordinate
    }
}

swift
import UIKit
import MapKit
class ViewController: UIViewController, MKMapViewDelegate {
    
    @IBOutlet weak var mapView: MKMapView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        mapView.delegate = self
        
        let coordinate = CLLocationCoordinate2D(latitude: 37.3318, longitude: -122.0312)
        let annotation = MyAnnotation(coordinate: coordinate)
        annotation.title = "Apple Inc."
        annotation.subtitle = "Cupertino, CA"
        mapView.addAnnotation(annotation)
        
        mapView.showAnnotations(mapView.annotations, animated: true)
    }
    
    func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
        let identifier = "AnnotationView"
        var annotationView = mapView.dequeueReusableAnnotationView(withIdentifier: identifier)
        if annotationView == nil {
            annotationView = MKPinAnnotationView(annotation: annotation, reuseIdentifier: identifier)
        } else {
            annotationView?.annotation = annotation
        }
        return annotationView
    }
    
}
class MyAnnotation: NSObject, MKAnnotation {
    var coordinate: CLLocationCoordinate2D
    var title: String?
    var subtitle: String?
    
    init(coordinate: CLLocationCoordinate2D) {
        self.coordinate = coordinate
    }
}