先來看看 MKGeodesicPolyline 在 Apple Developer Documentation 上的介紹:
A line-based shape that follows the contours of the Earth to create the shortest path between the specified points.

繪製 Polyline

首先我們在建置 MKGeodesicPolyline 的時候,
給予它一個 [CLLocationCoordinate2D],並宣告要繪製幾個點;
接著讓 MKMapView 新增進去。

let geodesicPolyline = MKGeodesicPolyline.init(coordinates: [start, end], count: 2)

再來我們需要透過 MKMapViewDelegate 的 function 來定義 MKGeodesicPolyline 的 UI:

func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
    guard let polyline = overlay as? MKPolyline else {
        return MKOverlayRenderer(overlay: overlay)
    let renderer = MKPolylineRenderer.init(polyline: polyline)
    renderer.lineWidth = 1
    renderer.strokeColor = .red
    return renderer

取得 Polyline 中間的經緯度

MKGeodesicPolyline 的繪製是由很多個點所連起來的,
可以利用 points() -> UnsafeMutablePointer 來取得。

@available(iOS 4.0, *)
open class MKMultiPoint : MKShape {
    open func points() -> UnsafeMutablePointer<MKMapPoint>
    open var pointCount: Int { get }
    // Unproject and copy points into the provided array of coordinates that
    // must be large enough to hold range.length coordinates.
    open func getCoordinates(_ coords: UnsafeMutablePointer<CLLocationCoordinate2D>, range: NSRange)

我們可以透過 pointCount 來得知有幾個點,


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