Core Location + MapKit

How it works — Core Location

• CLLocationManager: strong ref, created on main (callbacks use the creating thread’s run loop). locationManagerDidChangeAuthorization (iOS 14) fires at creation too — read authorizationStatus there.
• Info.plist NSLocationWhenInUseUsageDescription (+ …AlwaysAndWhenInUse… for Always); missing → no prompt.
• requestLocation() = one fix, then stops (needs didFailWithError). startUpdatingLocation() = stream, tuned by desiredAccuracy, distanceFilter, activityType.
• iOS 17: CLLocationUpdate.liveUpdates() = AsyncSequence, no delegate; a held CLBackgroundActivitySession keeps it alive in the background. CLMonitor (actor) replaces startMonitoring(for:): add(condition, identifier:), then iterate events (.satisfied/.unsatisfied).
• Background: mode location + allowsBackground LocationUpdates = true, started in the foreground.
• Quality: horizontalAccuracy < 0 = invalid; first fix often cached (check timestamp). CLGeocoder: network, rate-limited.

How it works — MapKit

• Annotation (MKAnnotation point) → viewFor: → reused MKAnnotationView (register + dequeueReusable…; MKMarkerAnnotationView). Overlay (MKPolyline/Polygon/ Circle/MKTileOverlay) → rendererFor: → MKOverlayRenderer, scales with zoom.
• Clustering (iOS 11): shared clusteringIdentifier → MKClusterAnnotation; displayPriority.
• Search: MKLocalSearchCompleter (as you type) → MKLocalSearch → [MKMapItem]. Directions: MKDirections.Request → calculate → MKRoute (polyline, expectedTravelTime); or hand off: openInMaps(launchOptions:).
• SwiftUI Map (iOS 17): content builder (Marker, Annotation, MapPolyline, MapCircle, UserAnnotation), MapCameraPosition (.automatic, .region, .userLocation(fallback:)), .onMapCameraChange, .mapControls. iOS 16 Map(coordinateRegion:) is deprecated.

Example — iOS 17 async + SwiftUI Map

let bg = CLBackgroundActivitySession() // hold: bg updates
for try await u in CLLocationUpdate.liveUpdates() {
  guard let loc = u.location else { continue }
  track(loc) }                         // + u.isStationary
let m = await CLMonitor("stores")      // actor, 20 max
await m.add(CLMonitor.CircularGeographicCondition(
  center: shop, radius: 150), identifier: "shop-7")
for try await e in await m.events {
  if case .satisfied = e.state { arrived(e.identifier) } }
Map(position: $camera, selection: $picked) {
  ForEach(shops) { s in
    Marker(s.name, coordinate: s.coord).tag(s.id) }
  MapPolyline(route.polyline).stroke(.blue, lineWidth: 4)
  UserAnnotation() }

Interview traps

• Reading authorizationStatus right after request… — still .notDetermined; the answer comes in the delegate.
• No first prompt offers “Always”. A cold requestAlways gives provisional Always; the upgrade prompt shows once — after that, and after Don’t Allow, only Settings.
• Blue pill: mandatory for While-Using background use; off by default for Always (opt in via showsBackgroundLocationIndicator).
• allowsBackgroundLocationUpdates without the location mode crashes; pausesLocationUpdatesAutomatically silently stops a background tracker.
• 20 regions per app: monitor the nearest 19 + one big region round the user whose exit re-plans. Geofences lag by minutes; radii under ≈100 m are unreliable.
• Returning a view for MKUserLocation hides the blue dot — return nil. CLGeocoder per keystroke is throttled.

Remember

Cheapest service that answers it · permission can shrink any time · points = reused views, shapes = renderers.

Likely questions

1. Track a run, screen off? — While Using + location mode, start in foreground, .fitness; blue pill.
2. Remind at 300 stores? — nearest-19 rotation + re-plan region; Always.
3. Log places all day cheaply? — visits or significant change.
4. 10 000 pins? — clustering + reuse, or aggregate server-side.