iOS - SwiftUI 在位置更改时更新文本

Question

I am working with SwiftUI and a CLLocationManager. Here is my LocationModel:

class LocationViewModel: NSObject, ObservableObject{
  
  @Published var userLatitude: Double = 0
  @Published var userLongitude: Double = 0
  @Published var userTown: String = ""
    var objectWillChange = ObservableObjectPublisher()
  
  private let locationManager = CLLocationManager()
  
  override init() {
    super.init()
    self.locationManager.delegate = self
    self.locationManager.desiredAccuracy = kCLLocationAccuracyBest
    self.locationManager.requestWhenInUseAuthorization()
    self.locationManager.startUpdatingLocation()
  }
}

extension LocationViewModel: CLLocationManagerDelegate {
    
    struct ReversedGeoLocation {
        let name: String            // eg. Apple Inc.
        let streetName: String      // eg. Infinite Loop
        let streetNumber: String    // eg. 1
        let city: String            // eg. Cupertino
        let state: String           // eg. CA
        let zipCode: String         // eg. 95014
        let country: String         // eg. United States
        let isoCountryCode: String  // eg. US

        var formattedAddress: String {
            return """
            \(name),
            \(streetNumber) \(streetName),
            \(city), \(state) \(zipCode)
            \(country)
            """
        }

        // Handle optionals as needed
        init(with placemark: CLPlacemark) {
            self.name           = placemark.name ?? ""
            self.streetName     = placemark.thoroughfare ?? ""
            self.streetNumber   = placemark.subThoroughfare ?? ""
            self.city           = placemark.locality ?? ""
            self.state          = placemark.administrativeArea ?? ""
            self.zipCode        = placemark.postalCode ?? ""
            self.country        = placemark.country ?? ""
            self.isoCountryCode = placemark.isoCountryCode ?? ""
        }
    }
  
  func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    guard let location = locations.last else { return }
    userLatitude = location.coordinate.latitude
    userLongitude = location.coordinate.longitude
    userTown = getTown(lat: CLLocationDegrees.init(userLatitude), long: CLLocationDegrees.init(userLongitude))

    print(location)
  }
    
    func getTown(lat: CLLocationDegrees, long: CLLocationDegrees) -> String
    {
        var town = ""
        let location = CLLocation.init(latitude: lat, longitude: long)
        CLGeocoder().reverseGeocodeLocation(location) { placemarks, error in

            guard let placemark = placemarks?.first else {
                let errorString = error?.localizedDescription ?? "Unexpected Error"
                print("Unable to reverse geocode the given location. Error: \(errorString)")
                return
            }

            let reversedGeoLocation = ReversedGeoLocation(with: placemark)
            print(reversedGeoLocation.city)
            
            town = reversedGeoLocation.city
            self.objectWillChange.send()
        }
        
        return town
    }
}

Now I would like to display the current coordinates and the city, but the city is just not being displayed, seems that the variable is not being updated properly. How to do that? Here is my view:

@ObservedObject var locationViewModel = LocationViewModel()

    var latitude: Double  { return(locationViewModel.userLatitude ) }
    var longitude: Double { return(locationViewModel.userLongitude ) }
    var town: String { return(locationViewModel.userTown) }

    var body: some View {
        VStack {
            Text("Town: \(town)")
            Text("Latitude: \(latitude)")
            Text("Longitude: \(longitude)")
        }
    }

I do not completely understand how to pass the updated variable into the view when the location changes or when the completion of the getTown functions closes.

Answer 1

You have made things more complicated than they need to be. You don't need to explicitly publish changes in your model; The properties are marked as @Published , so changing them will automatically fire the property change.

The reason you aren't seeing updates in your view is because you have tried to use computed properties to access your model; This won't work. There is nothing to consume the published changes to your model properties and nothing to tell the view that it should refresh.

If you simply access the view model properties directly in your Text views it will work the way you want.

Your final problems are related to reverse geocoding. First, a reverse geocoding request completes asynchronously. This means you can't return town . Again, you can simply update the userTown property directly, dispatching onto the main queue since you can't guarantee that the reverse geocoding handler will be called on the main queue and all UI updates must be performed on the main queue.

Putting all of this together gets

class LocationViewModel: NSObject, ObservableObject{
  
  @Published var userLatitude: Double = 0
  @Published var userLongitude: Double = 0
  @Published var userTown: String = ""
  
  private let locationManager = CLLocationManager()
  
  override init() {
    super.init()
    self.locationManager.delegate = self
    self.locationManager.desiredAccuracy = kCLLocationAccuracyBest
    self.locationManager.requestWhenInUseAuthorization()
    self.locationManager.startUpdatingLocation()
  }
}

extension LocationViewModel: CLLocationManagerDelegate {
    
    struct ReversedGeoLocation {
        let name: String            // eg. Apple Inc.
        let streetName: String      // eg. Infinite Loop
        let streetNumber: String    // eg. 1
        let city: String            // eg. Cupertino
        let state: String           // eg. CA
        let zipCode: String         // eg. 95014
        let country: String         // eg. United States
        let isoCountryCode: String  // eg. US

        var formattedAddress: String {
            return """
            \(name),
            \(streetNumber) \(streetName),
            \(city), \(state) \(zipCode)
            \(country)
            """
        }

        // Handle optionals as needed
        init(with placemark: CLPlacemark) {
            self.name           = placemark.name ?? ""
            self.streetName     = placemark.thoroughfare ?? ""
            self.streetNumber   = placemark.subThoroughfare ?? ""
            self.city           = placemark.locality ?? ""
            self.state          = placemark.administrativeArea ?? ""
            self.zipCode        = placemark.postalCode ?? ""
            self.country        = placemark.country ?? ""
            self.isoCountryCode = placemark.isoCountryCode ?? ""
        }
    }
  
  func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    guard let location = locations.last else { return }
    userLatitude = location.coordinate.latitude
    userLongitude = location.coordinate.longitude
    getTown(lat: CLLocationDegrees.init(userLatitude), long: CLLocationDegrees.init(userLongitude))

    print(location)
  }
    
    func getTown(lat: CLLocationDegrees, long: CLLocationDegrees) -> Void
    {
        let location = CLLocation.init(latitude: lat, longitude: long)
        CLGeocoder().reverseGeocodeLocation(location) { placemarks, error in

            guard let placemark = placemarks?.first else {
                let errorString = error?.localizedDescription ?? "Unexpected Error"
                print("Unable to reverse geocode the given location. Error: \(errorString)")
                return
            }

            let reversedGeoLocation = ReversedGeoLocation(with: placemark)
            print(reversedGeoLocation.city)
            DispatchQueue.main.async {
                self.userTown = reversedGeoLocation.city
            }
        }

    }
}

struct ContentView: View {
    @ObservedObject var locationViewModel = LocationViewModel()
    
    var body: some View {
        VStack {
            Text("Town: \(locationViewModel.userTown)")
            Text("Latitude: \(locationViewModel.userLatitude)")
            Text("Longitude: \(locationViewModel.userLongitude)")
        }
    }
}

The final problem with reverse geocoding is that it is rate limited; you can only call it so many times in a period before you will start getting errors. Location updates arrive about once per second, even when you aren't moving. Most of the time you will be needlessly looking up the same or almost the same location.

One approach is to check the distance travelled since the last reverse location look up and only perform a new look up if you exceed some threshold, say 500m

(We can also be a bit smarter with getTown - There is no point in splitting a location into latitude/longitude only to create a CLLocation in getTown )

private var lastTownLocation: CLLocation? = nil

func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        guard let location = locations.last else { return }
        userLatitude = location.coordinate.latitude
        userLongitude = location.coordinate.longitude
        
        if self.lastTownLocation == nil || self.lastTownLocation!.distance(from: location) > 500 {
            getTown(location)
        }
    }
    
    func getTown(_ location: CLLocation) -> Void
    {
        CLGeocoder().reverseGeocodeLocation(location) { placemarks, error in
            
            guard let placemark = placemarks?.first else {
                let errorString = error?.localizedDescription ?? "Unexpected Error"
                print("Unable to reverse geocode the given location. Error: \(errorString)")
                return
            }
            self.lastTownLocation = location
            let reversedGeoLocation = ReversedGeoLocation(with: placemark)
            print(reversedGeoLocation.city)
            DispatchQueue.main.async {
                self.userTown = reversedGeoLocation.city
            }
        }
        
    }