Xcode – SwiftUI List selection always nil

A couple of suggestions:

1. SwiftUI (specifically on macOS) is unreliable/unpredictable with certain List behaviors. One of them is selection — there are a number of things that either completely don’t work or at best are slightly broken that work fine with the equivalent iOS code. The good news is that NavigationLink and isActive works like a selection in a list — I’ll use that in my example.
2. @Published didSet may work in certain situations, but that’s another thing that you shouldn’t rely on. The property wrapper aspect makes it behave differently than one might except (search SO for "@Published didSet" to see a reasonable number of issues dealing with it). The good news is that you can use Combine to recreate the behavior and do it in a safer/more-reliable way.

A logic error in the code:

1. You are storing a Week in your user defaults with a certain UUID. However, you regenerate the array of weeks dynamically on every launch, guaranteeing that their UUIDs will be different. You need to store your week’s along with your selection if you want to maintain them from launch to launch.

Here’s a working example which I’ll point out a few things about below:

import SwiftUI
import Combine

struct ContentView : View {
    var body: some View {
        NavigationView {
            MenuView().environmentObject(UserSettings())
        }
    }
}

class UserSettings: ObservableObject {
    @Published var weeks: [Week] = []
    
    @Published var selectedWeek: UUID? = nil
    
    private var cancellable : AnyCancellable?
    private var initialItems = [
        Week(name: "test week 1"),
        Week(name: "foobar"),
        Week(name: "hello world")
    ]
    
    init() {
        let decoder = PropertyListDecoder()
                
        if let data = UserDefaults.standard.data(forKey: "weeks") {
            weeks = (try? decoder.decode([Week].self, from: data)) ?? initialItems
        } else {
            weeks = initialItems
        }
        
        if let prevValue = UserDefaults.standard.string(forKey: "week.selected.id") {
            selectedWeek = UUID(uuidString: prevValue)
            print("Set selection to: (prevValue)")
        }
        cancellable = $selectedWeek.sink {
            if let id = $0?.uuidString {
                UserDefaults.standard.set(id, forKey: "week.selected.id")
                let encoder = PropertyListEncoder()
                if let encoded = try? encoder.encode(self.weeks) {
                    UserDefaults.standard.set(encoded, forKey: "weeks")
                }
            }
        }
    }
    
    func selectionBindingForId(id: UUID) -> Binding<Bool> {
        Binding<Bool> { () -> Bool in
            self.selectedWeek == id
        } set: { (newValue) in
            if newValue {
                self.selectedWeek = id
            }
        }

    }
}

//Unknown what you have in here
struct Day : Equatable, Hashable, Codable {
    
}

struct Week: Identifiable, Hashable, Equatable, Codable {
    var id = UUID()
    var days: [Day] = []
    var name: String
}


struct WeekView : View {
    var week : Week
    
    var body: some View {
        Text("Week: (week.name)")
    }
}

struct MenuView: View {
    @EnvironmentObject var settings: UserSettings
    
    var body: some View {
        List {
            ForEach(settings.weeks) { week in
                NavigationLink(
                    destination: WeekView(week: week)
                        .environmentObject(settings),
                    isActive: settings.selectionBindingForId(id: week.id)
                )
                {
                    Image(systemName: "circle")
                    Text("(week.name)")
                }
            }
            .onDelete { set in
                settings.weeks.remove(atOffsets: set)
            }
            .onMove { set, i in
                settings.weeks.move(fromOffsets: set, toOffset: i)
            }
        }
        .navigationTitle("Weekplans")
        .listStyle(SidebarListStyle())
    }
}

1. In UserSettings.init the weeks are loaded if they’ve been saved before (guaranteeing the same IDs)
2. Use Combine on $selectedWeek instead of didSet. I only store the ID, since it seems a little pointless to store the whole Week struct, but you could alter that
3. I create a dynamic binding for the NavigationLinks isActive property — the link is active if the stored selectedWeek is the same as the NavigationLink‘s week ID.
4. Beyond those things, it’s mostly the same as your code. I don’t use selection on List, just isActive on the NavigationLink
5. I didn’t implement storing the Week again if you did the onMove or onDelete, so you would have to implement that.