I was trying to use @StateObject to recreate the functionality of Landmarks app, which Apple released as tutorial last year. I have the following code. It's a basic app that you can see JSON objects' isFavorite value true or false. In Apple's example it's possible to change the isFavorite value by tapping on a button, and then persist the change.

Still, I do not fully understand Apple's example here. How it is possible to persist the value without rewriting the JSON itself? Isn't it a bad example?

import SwiftUI
import Combine
import Foundation

struct Item: Codable, Identifiable, Equatable {
    var id: Int
    var name: String
    var isFavorite: Bool
}

final class UserData: ObservableObject {
    @Published var items = Bundle.main.decode([Item].self, from: "data.json")
    @Published var showFavorites = false
}

struct ContentView: View {
    @State var itemID = Item.ID()
    @StateObject var userData = UserData()

    var body: some View {
        NavigationView {
            VStack {
                Toggle(isOn: $userData.showFavorites) {
                    Text("Show Favorites Only")
                }
                List {
                    ForEach(userData.items) { item in
                        if !userData.showFavorites || item.isFavorite {
                            NavigationLink(destination: ContentDetail(itemID: item.id - 1)) {
                                ContentRow(item: item)
                            }
                        }
                    }
                }
            }
        }
    }
}

struct ContentRow: View {
    var item: Item

    var body: some View {
        HStack {
            Text(item.name)
            Spacer()
            if item.isFavorite {
                Image(systemName: "star.fill")
                    .imageScale(.medium)
                    .foregroundColor(.yellow)
            }
        }
    }
}

struct ContentDetail: View {
    @State var itemID = Item.ID()
    @StateObject var userData = UserData()

    var body: some View {
        VStack {
            Button {
                userData.items[itemID].isFavorite.toggle()
            } label: {
                if userData.items[itemID].isFavorite {
                    Image(systemName: "star.fill")
                        .foregroundColor(Color.yellow)
                } else {
                    Image(systemName: "star")
                        .foregroundColor(Color.gray)
                }
            }
            Text(userData.items[itemID].name)
        }
    }
}

extension Bundle {
    func decode<T: Decodable>(_ type: T.Type, from file: String, dateDecodingStrategy: JSONDecoder.DateDecodingStrategy = .deferredToDate, keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy = .useDefaultKeys) -> T {
        guard let url = self.url(forResource: file, withExtension: nil) else {
            fatalError("Failed to locate \(file) in bundle.")
        }

        guard let data = try? Data(contentsOf: url) else {
            fatalError("Failed to load \(file) from bundle.")
        }

        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = dateDecodingStrategy
        decoder.keyDecodingStrategy = keyDecodingStrategy

        do {
            return try decoder.decode(T.self, from: data)
        } catch DecodingError.keyNotFound(let key, let context) {
            fatalError("Failed to decode \(file) from bundle due to missing key '\(key.stringValue)' not found – \(context.debugDescription)")
        } catch DecodingError.typeMismatch(_, let context) {
            fatalError("Failed to decode \(file) from bundle due to type mismatch – \(context.debugDescription)")
        } catch DecodingError.valueNotFound(let type, let context) {
            fatalError("Failed to decode \(file) from bundle due to missing \(type) value – \(context.debugDescription)")
        } catch DecodingError.dataCorrupted(_) {
            fatalError("Failed to decode \(file) from bundle because it appears to be invalid JSON")
        } catch {
            fatalError("Failed to decode \(file) from bundle: \(error.localizedDescription)")
        }
    }
}