SwiftUI：将值或键：值对写入 JSON 解码字典

Question

I am struggling with modifying a value in a Dictionary that is made up of data from JSON download data from a php query to SQL. I need to either create a new pair key:value pair (best approach) or reuse a field from the original SQL data that I am not using and rewrite the value in the pair. In the code below I am trying the second approach (rewrite the value in a key:value pair I am not using). The issue is in the getData function (near the bottom) and is noted by the error comment. There is a LocationManager to get the users current location and some extensions that help the search box capability that are not shown, but let me know if needed.

import SwiftUI
import MapKit

struct Response: Codable, Hashable {
    
    var District: String
    var BusinessName: String
    var LocationAddress: String
    var LocationCity: String
    
    func hash(into hasher: inout Hasher) {
        hasher.combine(BusinessName)
    }
}

struct ContentView: View {
    @ObservedObject var lm = LocationManager()
    
    @State var response = [Response]()
    @State var search: String = ""
    @State private var showCancelButton: Bool = false
    @State private var searchPerfromed: Bool = false
    @State var location2: CLLocationCoordinate2D?
    @State var distance: Double = 0
    @State var dist: [String: Double] = Dictionary()
    
    var body: some View {
        VStack {
            HStack {
                HStack {
                    Image(systemName: "magnifyingglass")
                    TextField("Search", text: $search, onEditingChanged: { isEditing in
                        self.showCancelButton = true
                    }, onCommit: {
                        self.searchPerfromed = true
                        getData()  // Function to get JSON data executed after search execution
                    }).foregroundColor(.primary)
                    Button(action: {
                        self.search = ""
                        self.searchPerfromed = false
                    }) {
                        Image(systemName: "xmark.circle.fill").opacity(search == "" ? 0 : 1)
                    }
                }
                .padding(EdgeInsets(top: 8, leading: 6, bottom: 8, trailing: 6))
                .foregroundColor(.secondary)
                .background(Color(.secondarySystemBackground))
                .cornerRadius(10.0)
                if showCancelButton  {
                    Button("Cancel") {
                        UIApplication.shared.endEditing(true)
                        self.search = ""
                        self.showCancelButton = false
                        self.searchPerfromed = false
                    }
                }
            }
            .padding(EdgeInsets(top: 10, leading: 0, bottom: 1, trailing: 0))
            .padding(.horizontal)
            Spacer()
            if searchPerfromed == true {
                List {
                    ForEach(response.sorted {$0.LocationAddress > $1.LocationAddress}, id: \.self) { item in
                        VStack(alignment: .leading) {
                            HStack(alignment: .center) {
                                Text("\(item.BusinessName)").font(.subheadline)
                                Spacer()
                                if dist[item.LocationAddress] == 0.0 {
                                    Text("Unknown").font(.caption).foregroundColor(.gray)
                                } else {
                                    Text("\(dist[item.LocationAddress] ?? 0, specifier: "%.0f") mi")
                                    .font(.caption).foregroundColor(.gray)
                                }
                            }
                            Text("\(item.LocationAddress), \(item.LocationCity)").font(.subheadline)
                        }
                    }
                }
            }
        }
    }
    
    func getLocation(from address: String, completion: @escaping (_ location: CLLocationCoordinate2D?)-> Void) {
        let geocoder = CLGeocoder()
        geocoder.geocodeAddressString(address) { (placemarks, error) in
            guard let placemarks = placemarks,
            let location = placemarks.first?.location?.coordinate else {
                completion(nil)
                return
            }
            completion(location)
        }
    }
    
    func getDistance() {
        let loc1 = CLLocation(latitude: lm.location1!.latitude, longitude: lm.location1!.longitude)
        let loc2 = CLLocation(latitude: location2?.latitude ?? lm.location1!.latitude, longitude: location2?.longitude ?? lm.location1!.longitude)
        let dist = loc1.distance(from: loc2)
        distance = dist * 0.00062 // convert from meters to miles
    }
    
    func getData() {
        let baseURL = "https://???"
        let combinedURL = baseURL + search
        let encodedURL = combinedURL.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)!
        let url = URL(string: encodedURL)!
        URLSession.shared.dataTask(with: url) { (data, response, error) in
            do {
                if let responseData = data {
                    let decodedData = try JSONDecoder().decode([Response].self, from: responseData)
                    DispatchQueue.main.async {
                        self.response = decodedData
                        print(decodedData)
                        for item in self.response {
                            let address = item.LocationAddress
                            getLocation(from: address) { coordinates in
                                self.location2 = coordinates
                                getDistance()
                                dist[address] = distance
                                let d = String(format: "%.1f", distance)
                                print(item.District)
                                item[District] = d  //Error: Cannot find 'District' in scope
                            }
                        }
                    }
                } else {
                    print("No data retrieved")
                }
            } catch {
                print("Error: \(error)")
            }
        }.resume()
    }
    
}

Just above the issue I assigned the distance value to a new Dictionary and match the values based on address, but this has limitations with other changes I need to make to get to the end state with the app. I have tried lots of different ways to assign the distance to an existing key:value pair in Response or create a new key value pair in Response. Some examples below:

item.District = d  //Error: Cannot assign to property: 'item' is a 'let' constant
item[Distance] = d  //Error: Cannot find 'Distance' in scope
item["Distance"] = d  //Error: Value of type 'Response' has no subscripts

How do I create a new key:value pair in Response, or assign the value of d to the District key? Thanks so much in advance for any help you can provide.

Answer 1

First thing, your instance variables on your Response struct are a little confusing because they don't follow the convention of having variables in all-lowercase, and types capitalized.

Second, for your first error in your list of 3, item is a let constant because it is inside a for loop. You can get around this by declaring the loop this way:

for var item in self.response {
    // I can modify item in this loop because it is declared a var
}

The other two errors are pretty self-explanatory, I think.

Third, it sounds like you want to alter your Response object programmatically after receiving it, which is also a bit of an anti-pattern. If you want to modify an object you have downloaded from a server, that's understandable, but it is confusing for someone reading your code to alter an object called "Response." (once you modify it, it no longer represents the server response for which it is named) At a minimum, you could change District to be a computed property of Response .

All that said, if you instantiate your loop using the var keyword, you should be able to do:

item.District = d