Make anchorPoint a UnitPoint instead of CGPoint

`UnitPoint` - https://developer.apple.com/documentation/swiftui/unitpoint - is more SwiftUI-y than `CGPoint`, and will allow the anchorPoint to use the constants like `.center` and `.bottom`.

Extract extensions to make it trivial to convert the `UnitPoint` to a `centerOffset` within a rect, and to convert back to a `CGPoint` for the `UIView` `anchorPoint` property.

Sources/Annotations/MKMapAnnotationView.swift

@@ -38,25 +38,13 @@ class MKMapAnnotationView<Content: View>: MKAnnotationView {
         controller.view.backgroundColor = .clear
         #endif
 
-        if #available(iOS 16.0, *) {
-            anchorPoint = mapAnnotation.anchorPoint
+        if #available(iOS 16, *) {
+            anchorPoint = mapAnnotation.anchorPoint.toCGPoint()
         } else {
-            centerOffset = anchorPointToCenterOffset(mapAnnotation.anchorPoint, in: bounds)
+            centerOffset = mapAnnotation.anchorPoint.toCenterOffset(in: bounds)
         }
     }
 
-    // Only necessary for iOS < 16, where `anchorPoint` is not available on `UIView`
-    func anchorPointToCenterOffset(_ anchorPoint: CGPoint, in rect: CGRect) -> CGPoint {
-        assert((0.0...1.0).contains(anchorPoint.x), "Valid anchor point range is 0.0 to 1.0, received x value: \(anchorPoint.x)")
-        assert((0.0...1.0).contains(anchorPoint.y), "Valid anchor point range is 0.0 to 1.0, received y value: \(anchorPoint.y)")
-
-        return .init(
-            x: (0.5 - anchorPoint.x) * rect.width,
-            y: (0.5 - anchorPoint.y) * rect.height
-        )
-    }
-
-
     // MARK: Overrides
 
     #if canImport(UIKit)
@@ -77,7 +65,7 @@ class MKMapAnnotationView<Content: View>: MKAnnotationView {
         controller.view.frame = bounds
 
         if #unavailable(iOS 16.0), let mapAnnotation {
-            centerOffset = anchorPointToCenterOffset(mapAnnotation.anchorPoint, in: bounds)
+            centerOffset = mapAnnotation.anchorPoint.toCenterOffset(in: bounds)
         }
     }
 

Sources/Annotations/ViewMapAnnotation.swift

@@ -41,7 +41,7 @@ public struct ViewMapAnnotation<Content: View>: MapAnnotation {
     // MARK: Stored Properties
 
     public let annotation: MKAnnotation
-    let anchorPoint: CGPoint
+    let anchorPoint: UnitPoint
     let clusteringIdentifier: String?
     let displayPriority: MKFeatureDisplayPriority
     let collisionMode: MKAnnotationView.CollisionMode
@@ -53,7 +53,7 @@ public struct ViewMapAnnotation<Content: View>: MapAnnotation {
         coordinate: CLLocationCoordinate2D,
         title: String? = nil,
         subtitle: String? = nil,
-        anchorPoint: CGPoint = .init(x: 0.5, y: 0.5),
+        anchorPoint: UnitPoint = .center,
         clusteringIdentifier: String? = nil,
         displayPriority: MKFeatureDisplayPriority = .required,
         collisionMode: MKAnnotationView.CollisionMode = .rectangle,
@@ -69,7 +69,7 @@ public struct ViewMapAnnotation<Content: View>: MapAnnotation {
 
     public init(
         annotation: MKAnnotation,
-        anchorPoint: CGPoint = .init(x: 0.5, y: 0.5),
+        anchorPoint: UnitPoint = .center,
         clusteringIdentifier: String? = nil,
         displayPriority: MKFeatureDisplayPriority = .required,
         collisionMode: MKAnnotationView.CollisionMode = .rectangle,

Sources/Extensions/UnitPoint.swift

@@ -0,0 +1,25 @@
+//
+//  File.swift
+//  
+//
+//  Created by Darron Schall on 10/2/23.
+//
+
+import SwiftUI
+
+extension UnitPoint {
+
+    func toCGPoint() -> CGPoint {
+        return CGPoint(x: x, y: y)
+    }
+
+    func toCenterOffset(in rect: CGRect) -> CGPoint {
+        return CGPoint(
+            x: (0.5 - x) * rect.width,
+            y: (0.5 - y) * rect.height
+        )
+    }
+
+}
+
+