State variable that keeps track of list of published items - swiftui

Question

I am learning SwiftUI and having trouble with data flow.
I can't figure out how to create a "state"-variable to keep track of a list of published child items.
I need a variable that is true if at least one child item is selected. These items are mapped to SwiftUI toggle-switches, which works. (Making a computed property is not allowed)
Property wrapper cannot be applied to a computed property
class Main : ObservableObject {
#Published var items : [Item]
//This is what i want, but not allowed
#Published var selectedItemsExist : Bool {
get {
var exists = false
for item in items {
if item.selected {
exists = true
break
}
}
return exists
}
}
init(items: [Item]) {
self.items = items
}
}
class Item : Identifiable, ObservableObject {
var id = UUID()
var name : String!
#Published var selected : Bool = false
}
And then in my view be able to do something like
if main.selectedItemsExist {
Text("This text is shown if at least on toggle is on")
}
Any guidance appreciated!

Answer 1

Please find below the demo of approach how this could be done. Tested as worked with Xcode 11.2 / iOS 13.2
class Main : ObservableObject {
#Published var items : [Item] {
didSet { // << as item is value array will be updated on item changed
var exists = false
for item in items {
if item.selected {
exists = true
break
}
}
self.selectedItemsExist = exists
}
}
#Published var selectedItemsExist : Bool = false
init(items: [Item]) {
self.items = items
}
}
struct Item : Identifiable { // << value type, so changed on any property change
var id = UUID()
var name : String!
var selected : Bool = false
init(_ name: String) {
self.name = name
}
}
struct TestCalculablePublishing: View {
#ObservedObject var main = Main(items: [Item("1"), Item("2"), Item("3")])
var body: some View {
VStack {
Button("Test") {
self.main.items[1].selected.toggle() // << just for test purpose
}
if main.selectedItemsExist {
Text("This text is shown if at least on toggle is on")
}
}
}
}
struct TestCalculablePublishing_Previews: PreviewProvider {
static var previews: some View {
TestCalculablePublishing()
}
}