SwiftUI - Editing a value of an item in a Codable Array Cannot use mutating member on immutable value

Question

I have a built up List with using an array of Codable type called Package. At some point, I need to update the isFavorite value of an Package item when user taps favorite button in the list. However I am getting the error of Cannot use mutating member on immutable value: 'package' is a 'let' constant. First I tried to directly change the value in the action. It did not work, so the point I got so far is below.

Obviously the code is longer than that but I just wanted make it simple and deleted irrelevant parts.

SwiftUIView

struct AView: View {

    @ObservedObject var store: JSONController

    var body: some View {
        NavigationView {
            List(self.store.packages) { (package) in
                return self.row(package)
            }
        }
    }

    private func row(_ package: Package) -> some View {
        HStack{
            Text(package.name)
            Spacer()
            Button(action: {
                package.changeFavoriteState()
            }) {
                Image(systemName: package.isFavorite ? "star.fill" : "star")
            }.buttonStyle(PlainButtonStyle())
        }
    }
}

Package.struct

struct Package: Codable, Identifiable {

    var id: UUID
    var isFavorite: Bool

    mutating func changeFavoriteState() {
        self.isFavorite.toggle()
    }
}

PackageList.struct

struct PackageList: Codable {

    var packages: [Package]

    private enum CodingKeys: CodingKey {
        case packages
    }

    init(){
        self.packages = [Package]()
    }

    init(from decoder: Decoder) throws {
        do {
            let container = try decoder.container(keyedBy: CodingKeys.self)
            self.packages = try container.decode([Package].self, forKey: .packages)
        }catch let error {
            print(error.localizedDescription+" Initializing packages with an empty array.")
            self.packages = [Package]()
        }
    }
}

JSONController.class

class JSONController: ObservableObject {

    @Published var packages: [Package] = [Package]()

    init(){
        self.readJSONFile(from: "packageList")
    }

    func readJSONFile(from url: String) {
        //stuff
    }

}

And finally the error I got,

Answer 1

it is simple, you need access the value, not its copy, provided by List

import SwiftUI

struct Package {
    var flag: Bool
}

struct ContentView: View {
    @State var arr = [Package(flag: true), Package(flag: false), Package(flag: true)]
    var body: some View {
        List(arr.indices, id:\.self) { (index) in
            Text(self.arr[index].flag.description).onTapGesture {
                self.arr[index].flag.toggle()
            }
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

or using the ObservableObject

import SwiftUI

struct Package: Identifiable {
    let id = UUID()
    var flag: Bool
}
class Model: ObservableObject {
    @Published var arr = [Package(flag: true), Package(flag: false), Package(flag: true)]
}

struct ContentView: View {
    @ObservedObject var model = Model()
    var body: some View {
        List(model.arr.indices) { (idx) in
            Text(self.model.arr[idx].flag.description)
                .onTapGesture {
                    self.model.arr[idx].flag.toggle()
            }
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

which is generally the same ... but in more complex scenarios preferable

UPDATE based on request You have to redefine row function

import SwiftUI

struct Package: Identifiable {
    let id = UUID()
    var flag: Bool
}
class Model: ObservableObject {
    @Published var arr = [Package(flag: true), Package(flag: false), Package(flag: true)]
}

struct ContentView: View {
    @ObservedObject var model = Model()
    var body: some View {
        List(model.arr.indices) { (idx) in
            self.row(flag: self.$model.arr[idx].flag)
        }
    }

    func row(flag: Binding<Bool>)-> some View {
        Text(flag.wrappedValue.description)
            .onTapGesture {
                flag.wrappedValue.toggle()
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Or the second option

import SwiftUI

struct Package: Identifiable {
    let id = UUID()
    var flag: Bool
}
class Model: ObservableObject {
    @Published var arr = [Package(flag: true), Package(flag: false), Package(flag: true)]
}

struct ContentView: View {
    @ObservedObject var model = Model()
    var body: some View {
        List(model.arr.indices) { (idx) in
            self.row(idx: idx)
        }
    }

    func row(idx: Int)-> some View {
        Text(model.arr[idx].flag.description)
            .onTapGesture {
                self.model.arr[idx].flag.toggle()
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

UPDATE with conditional sorting

import SwiftUI

struct Package: Identifiable {
    let id: Int
    var flag: Bool
}
class Model: ObservableObject {
    @Published var arr = [Package(id: 1, flag: true), Package(id: 3, flag: false), Package(id: 2, flag: true)]
    @Published var ascending = false

    var sorted: [Package] {
        arr.sorted { (p0, p1) -> Bool in
            let sort = (p0.id < p1.id)
            return ascending ? !sort : sort
        }
    }
}

struct ContentView: View {
    @ObservedObject var model = Model()

    var body: some View {

        VStack {
            Toggle(isOn: $model.ascending) {
                Text("ascending")
            }.padding(.horizontal)

            List(model.sorted.indices, id: \.self) { (idx) in
                self.row(idx: idx)
            }
        }
    }

    func row(idx: Int)-> some View {
        Text(verbatim: model.sorted[idx].flag.description + ": " + model.sorted[idx].id.description)
            .onTapGesture {
                if let i = self.model.arr.firstIndex (where: { (p) -> Bool in
                    self.model.sorted[idx].id == p.id
                }) {
                    self.model.arr[i].flag.toggle()
                }
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Answer 2

Change the signature of your function to

private func row(_ package: inout Package) -> some View

Why is that necessary? Because when you are passing a parameter to a function, that parameter is a let constant. Package is a struct so modifying any of its properties means actually modifying the instance itself. Which you cannot do, because it's a let constant.

Another thing is that since Package is a struct it means that when you are passing it to your function, you are actually passing a copy, not the original value.

An inout parameter has a value that is passed in to the function, is modified by the function, and is passed back out of the function to replace the original value.