SwiftUI: setting userdefaults specifically to struct properties

Question

I want to set the variable "isfavorite" mapped to "id" as UserDefaults. I am able to save the whole of the struct encoded as JSON to UserDefaults. And then decode the UserDefaults JSON on init. There are two problems with this approach

1) I am using UserDefault to store a JSON file that could get bigger over time which is not the ideal usecase for UserDefaults

2) I am not able to update the content as during initialization, the last saved JSON on UserDefaults is loading

struct Labvalueslist: Codable, Identifiable, Equatable {
    var topic: String
    var sirange: String
    var usrange: String
    var id: Int
    var isfavorite: Bool }

var labvaluesdata: [Labvalueslist] = load("labvalues.json")

final class UserData: ObservableObject {
    @Published var labvaluesUserdata = labvaluesdata
        {
        didSet {
            let encoder = JSONEncoder()
            if let encoded = try?
                encoder.encode(labvaluesUserdata) {
                UserDefaults.standard.set(encoded, forKey: "isfavorite")
            }
        }
    }
    init () {
        if let labvaluesUserdata = UserDefaults.standard.data(forKey: "labvaluesUserdata")
        {
            let decoder = JSONDecoder()
            if let decoded = try?
                decoder.decode([Labvalueslist].self, from: labvaluesUserdata) {
                self.labvaluesUserdata = decoded
                return
            }
        }
        self.labvaluesUserdata = labvaluesdata
    }
}

struct ImageView: View {
    var list: Labvalueslist

    @ObservedObject var userData = UserData()

    var anatomyfavIndex: Int {
        userData.labvaluesUserdata.firstIndex(where: { $0.id == list.id})!
    }
// This is where I am confused; how do I get the "isfavorite" mapped to "id" and store it as UserDefaults and use only those values on init 

var body: some View {

            VStack {
        ScrollView {
            VStack(alignment: .leading)
            {
            Text("Normal Range:").font(.headline)
            HStack
                { Text("SI: " + list.sirange)
                Spacer()
                Text("US: " + list.usrange)
                }
            .padding(EdgeInsets(top: 10, leading: 0, bottom: 10, trailing: 0))
            }
        }.foregroundColor(.black)
            }.navigationBarTitle(Text(list.topic), displayMode: .inline)
           .padding(EdgeInsets(top: 10, leading: 10, bottom: 0, trailing: 5))
                .navigationBarItems(trailing: Button(action: {
                    self.userData.labvaluesUserdata[self.anatomyfavIndex].isfavorite.toggle()
                }) {
                    if self.userData.labvaluesUserdata[self.anatomyfavIndex].isfavorite {
                        Image(systemName: "star.fill").foregroundColor(.yellow).scaleEffect(1.5).padding (.trailing, 20)
                    }
                    else {
                        Image(systemName: "star")
                            .foregroundColor(.yellow)
                            .scaleEffect(1.5)
                            .padding (.trailing, 20)
                }
            })

}
}

Answer 1

1.) what about saving it to a file? 2.) why don't you just save the favorite id? (if it is just one, i am not sure) like this: key: "myFavoriteID" - id

Answer 2

You can use map to get an [Bool] array from you array of structs.

@State var isfavoriteArray : [Bool] = labvaluesdata.map{$0.isfavorite}

If you want to get isfavorite tied to id - you can get array of dictionaries [id: isfavorite] like this

@State var idIsFaveDict: [Int: Bool] = Dictionary(uniqueKeysWithValues: labvaluesdata.map { ($0.id, $0.isfavorite) })

did this help?

Answer 3

Here's a sample app which should do what something like what you're looking for. Instead of saving it as one entry in user defaults, I'm saving it as multiple. I think they key is writing the ObservableObject correctly.

struct ContentView: View {
    var body: some View {
        VStack {
            ButtonView(key: "favorite:1")
            ButtonView(key: "favorite:ab12")
            ButtonView(key: "favorite:2jjf")
        }
    }
}

struct ButtonView: View {
    @ObservedObject private var userDefaultsKey: UserDefaultsKey

    init(key: String) {
        userDefaultsKey = UserDefaultsKey(key: key)
    }

    var body: some View {
        Button(action: buttonTapped) {
            Text(userDefaultsKey.key + ": " + (userDefaultsKey.value ? "liked" : "not liked"))
        }
    }

    func buttonTapped() {
        UserDefaults.standard.set(!userDefaultsKey.value, forKey: userDefaultsKey.key)
    }
}

class UserDefaultsKey: ObservableObject {
    let key: String

    @Published var value = false

    init(key: String) {
        self.key = key

        value = UserDefaults.standard.bool(forKey: key)

        NotificationCenter.default.addObserver(forName: UserDefaults.didChangeNotification, object: nil, queue: nil) { [weak self] _ in
            guard let self = self else { return }
            self.value = UserDefaults.standard.bool(forKey: self.key)
        }
    }
}

Note that in the simulator, user defaults aren't necessarily saved unless you actually quit the app in the simulator (if you run again from Xcode, user defaults sometimes aren't saved: Xcode 9 simulator doesn't save userdefaults)