SwiftUI List selection always nil

Question

In my macOS app project, I have a SwiftUI List view of NavigationLinks build with a foreach loop from an array of items:

struct MenuView: View {
    @EnvironmentObject var settings: UserSettings
    
    var body: some View {
        List(selection: $settings.selectedWeek) {
            ForEach(settings.weeks) { week in
                NavigationLink(
                    destination: WeekView(week: week)
                        .environmentObject(settings)
                    tag: week,
                    selection: $settings.selectedWeek)
                {
                    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())
    }
}

This view creates the sidebar menu for a overall NavigationView.

In this List view, I would like to use the selection mechanic together with tag from NavigationLink. Week is a custom model class:

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

And UserSettings looks like this:

class UserSettings: ObservableObject {
    @Published var weeks: [Week] = [
        Week(name: "test week 1"),
        Week(name: "foobar"),
        Week(name: "hello world")
    ]
    
    @Published var selectedWeek: Week? = UserDefaults.standard.object(forKey: "week.selected") as? Week {
        didSet {
            var a = oldValue
            var b = selectedWeek
            UserDefaults.standard.set(selectedWeek, forKey: "week.selected")
        }
    }
}

My goal is to directly store the value from List selection in UserDefaults. The didSet property gets executed, but the variable is always nil. For some reason the selected List value can't be stored in the published / bindable variable. Why is $settings.selectedWeek always nil?

Answer 1

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.

Answer 2

Bumped into a situation like this where multiple item selection didn't work on macOS. Here's what I think is happening and how to workaround it and get it working

Background

So on macOS NavigationLinks embedded in a List render their Destination in a detail view (by default anyway). e.g.

struct ContentView: View {
    let beatles = ["John", "Paul", "Ringo", "George", "Pete"]
    @State var listSelection = Set<String>()

    var body: some View {
        NavigationView {
            List(beatles, id: \.self, selection: $listSelection) { name in
                NavigationLink(name) {
                    Text("Some details about \(name)")
                }
            }
        }
    }
}

Renders like so

Problem

When NavigationLinks are used it is impossible to select multiple items in the sidebar (at least as of Xcode 13 beta4).

... but it works fine if just Text elements are used without any NavigationLink embedding.

What's happening

The detail view can only show one NavigationLink View at a time and somewhere in the code (possibly NavigationView) there is piece of code that is enforcing that compliance by stomping on multiple selection and setting it to nil, e.g.

let selectionBinding = Binding {
    backingVal
} set: { newVal in
    guard newVal <= 1 else {
        backingVal = nil
        return
    }
    backingVal = newVal
}

What happens in these case is to the best of my knowledge not defined. With some Views such as TextField it goes out of sync with it's original Source of Truth (for more), while with others, as here it respects it.

Workaround/Fix

To enable multiple selection, use a Zstack to hide the NavigationLinks behind a Text based List and drive the NavigationLinks programatically. e.g.

struct ContentView: View {
    let beatles = ["John", "Paul", "Ringo", "George", "Pete"]
    @State var listSelection = Set<String>()

    var detailShow: String? {
        return listSelection.count == 1 ? listSelection.first
            : listSelection.count > 1 ? "tooMany"
            : nil
    }

    var body: some View {
        NavigationView {
            /// Workaround for inability to select multiple NavigationLinks in a List on macOS
            ZStack {
                /// Hidden List with programatically driven NavigationLinks
                List {
                    ForEach(beatles, id: \.self) { name in
                        NavigationLink(
                            "",
                            tag: name,
                            selection: .constant(detailShow)
                        ) {
                            Text("Some details on \(name)")
                        }
                    }
                    NavigationLink(
                        "",
                        tag: "tooMany",
                        selection: .constant(detailShow)
                    ) {
                        Text("Too many selected")
                    }
                }
                .opacity(0.0)

                /// List that is selected from
                List(beatles, id: \.self, selection: $listSelection) { name in
                    Text(name)
                }
            }
        }
    }
}

Have fun.