SwiftUI - 应用程序进入前台时获取位置和调用函数

Question

我是 SwiftUI 的新手，我对一些我认为相当简单的事情感到困难。 我创建了一个显示用户当前位置的应用程序。如果用户将应用程序置于后台，并且用户的物理位置发生了变化，则应用程序在进入前台时不会显示新的位置。如果我终止应用程序，然后重新启动，它会显示新位置。

当检测到位置更改时，我还想调用一个函数以从 Firebase 获取更新的数据集。这是我正在使用的代码的简化版本。请让我知道我错过了什么。我想我应该使用更多的组合功能来实现这个结果，但我不确定如何？

我的关键问题是：“我如何在我的视图中检测位置变化并根据位置变化调用函数？”

MyView.swift

import SwiftUI

struct MyView: View {

    @ObservedObject var locationManager = LocationManager()

    var userLatitude: String {
        return "\(locationManager.lastLocation?.coordinate.latitude ?? 0)"
    }

    var userLongitude: String {
        return "\(locationManager.lastLocation?.coordinate.longitude ?? 0)"
    }

    var body: some View {
        VStack {
            Text("location status: \(locationManager.statusString)")
            HStack {
                Text("latitude: \(userLatitude)")
                Text("longitude: \(userLongitude)")
            }
            Text("place: \(locationManager.placemark?.thoroughfare ?? "")") 
        }
     
    }
    
    func refreshData() {
        // call firebase listener with new location data
    }

}

LocationManager.swift

import Foundation
import CoreLocation
import Combine

class LocationManager: NSObject, ObservableObject {
    
    let objectWillChange = PassthroughSubject<Void, Never>()
    
    private let locationManager = CLLocationManager()
    
    override init() {
        super.init()
        self.locationManager.delegate = self
        self.locationManager.desiredAccuracy = kCLLocationAccuracyThreeKilometers
        self.locationManager.requestWhenInUseAuthorization()
        self.locationManager.startUpdatingLocation()
    }
    
    @Published var locationStatus: CLAuthorizationStatus? {
        willSet {
            objectWillChange.send()
        }
    }
    
    @Published var lastLocation: CLLocation? {
        willSet {
            objectWillChange.send()
        }
    }
    
    @Published var placemark: CLPlacemark? {
        willSet {
            objectWillChange.send()
        }
    }
    
    var statusString: String {
        guard let status = locationStatus else {
            return "unknown"
        }
        
        switch status {
        case .notDetermined: return "notDetermined"
        case .authorizedWhenInUse: return "authorizedWhenInUse"
        case .authorizedAlways: return "authorizedAlways"
        case .restricted: return "restricted"
        case .denied: return "denied"
        default: return "unknown"
        }
        
    }
    
}

extension LocationManager: CLLocationManagerDelegate {
    
    func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
        self.locationStatus = status
        print(#function, statusString)
    }
    
    func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        guard let location = locations.last else { return }
        self.lastLocation = location
        print(#function, location)
        
        let geocoder = CLGeocoder()
        geocoder.reverseGeocodeLocation(location) { (placemarks, error) in
            if (error != nil){
                print("Error in reverseGeocode: \(error)")
                return
            }
            
            let placemark = placemarks! as [CLPlacemark]
            
            if placemark.count > 0 {
                let placemark = placemarks![0]
                self.placemark = placemark
            }
        }
        locationManager.stopUpdatingLocation()
    }
    
    func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {
        if let error = error as? CLError, error.code == .denied {
            // Location updates are not authorized.
            locationManager.stopUpdatingLocation()
            return
        }
        // Notify the user of any errors.
    }
    
}

Answer 1

您可以订阅 LocationManager 类中的 lastLocation 属性，以便在发生更改时收到通知。使用 Combine，您可以像这样实现：

// inside the location manager class
private var cancellables: [AnyCancellable] = [] // new property to keep track of your subscriptions

// inside the initialiser of the class
$lastLocation
    .receive(on: RunLoop.main) // main thread so view gets an update
    .sink { lastLocation in // subscribe to the published property. Anytime it changes you will get notified
       refreshData()
       // do more with the new value of last location or not :)
    }
    .store(in: cancellabels) // store subscription in property so it doesn’t go out of scope immediately 

另外，在您的视图中，请确保绕过计算属性并直接订阅已发布的位置属性。根据我的经验，计算属性和订阅不能很好地协同工作。 在您的文本视图中直接引用位置坐标：

Text(locationManager.lastLocation?.coordinate.latitude ?? 0)