External Properties

ExternalProperties.swift

struct ToDo: Codable, Identifiable {
    let id: Int
    let userId: Int
    var title: String
    var completed: Bool
}

class SelectionManager<Item: Identifiable>: ObservableObject {
    @Published var status: [Item.ID:Bool] = [:]
    func isSelected(_ item: Item) -> Bool {
        status[item.id] ?? false
    }
    func toggle(_ item: Item) {
        status[item.id] = !isSelected(item)
    }
    func allSelected(_ items: [Item]) -> [Item] {
        items.filter { self.isSelected($0) }
    }
    func clear() {
        status = [:]
    }
}

struct SomeView: View {
    let todo: ToDo
    @EnvironmentObject var selections: SelectionManager<ToDo>
    var body: some View {
        VStack {
            Text("\(todo.title) is \(selections.isSelected(todo) ? "" : "not ") selected")
            Button("Toggle") {
                selections.toggle(todo)
            }
        }
    }
}

So one can also do something like this if you ask yourself the question:

In this case, is "selected" actually a property of the item, or is it a distinct and separate state maintained "about" the item (or items)?

Maintaining state separately could make it easier to save/restore the entire list of selections, clear them, and so on, without having to mutate an item from its persistent state to its "internal" state.