DataManager.swift

import Foundation
import SwiftUI

@main
struct MacApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}
 
struct ContentView: View {
    @StateObject var dataManager = DataManager()
    @State private var userName: String = ""
    
    var body: some View {
        VStack (alignment: .leading) {
            
            ForEach(dataManager.jsonArray) { jsonValue in
                Text("Username is \(jsonValue.username)")
            }
            
            TextField("Username", text: $userName)
            
            Button(action: {
                // Update array, just for testing update all userName
                for i in dataManager.jsonArray.indices {
                    dataManager.jsonArray[i].username = userName
                }
                // save to file
                dataManager.saveToFile()
                print("Done")
            })
            {
                Text("Save")
            }
        }.frame(width: 444, height: 444)
    }
    
}

class DataManager: ObservableObject {
    @Published var jsonArray: [JsonModel] = []
    
    var fileName = "settingsData"
    
    init() {
        readFromFile()
    }
    
    func readFromFile() {
        jsonArray = readJSONBundle(fileName) // do this only the first time you open the app
      //  readJSON() // read from the saved file
    }
    
    private func readJSONBundle<T: Decodable>(_ filename: String) -> T {
        let data: Data
        
        guard let file = Bundle.main.url(forResource: filename, withExtension: "json")
        else {
            fatalError("Couldn't find \(filename) in main bundle.")
        }
        do {
            print(file)
            data = try Data(contentsOf: file)
        } catch {
            fatalError("Couldn't load \(filename) from main bundle:\n\(error)")
        }
        do {
            let decoder = JSONDecoder()
            let jsonData = try decoder.decode(T.self, from: data)
            return jsonData
        } catch {
            fatalError("Couldn't parse \(filename) as \(T.self):\n\(error)")
        }
    }

    func saveToFile() {
        do {
            let furl = try FileManager.default
                .url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
                .appendingPathComponent(fileName)
                .appendingPathExtension("json")
            print("---> writing to: \(furl)")
            let data = try JSONEncoder().encode(jsonArray)
            try data.write(to: furl)
        } catch {
            print("---> error saveToFile: \(error)")
        }
    }
    
    func readJSON() {
        do {
            let furl = try FileManager.default
                .url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
                .appendingPathComponent(fileName)
                .appendingPathExtension("json")
            print("---> reading from: \(furl)")
            let data = try Data(contentsOf: furl)
            let decoder = JSONDecoder()
            let jsonData = try decoder.decode([JsonModel].self, from: data)
            jsonArray = jsonData
        } catch {
            print("---> error reading from file: \(error)")
        }
    }
    
}

struct JsonModel: Hashable, Codable, Identifiable {
    var id: Int
    var username: String
    var password: String
}