SwiftUI TabView index view not respecting page view index - swiftui

Question

My goal is to keep the source of truth for the current TabView index within app state. But I'm not seeing the behaviour I expect from the SwiftUI TabView index indicator.
What I can do is have one of either two outcomes: Have the binding update the source of truth properly, OR have the indicator work as expected. I can't seem to have both.
I've stripped down my code to a test case below. I expect the TabView index view (the indicator) to update and display the current view's index position in the array of the TabView's content (relatively of course, so + 1).
What happens is that the index doesn't update at all. In the original code it actually lags by one..
Can anyone with more experience with this setup offer any suggestions or point out where I'm going wrong?
import SwiftUI
class Item: Identifiable {
let id = Int.random(in: 0..<1000)
}
class SourceOfTruth: ObservableObject {
#Published var items: [Item] = [Item(), Item(), Item()]
/// Source of truth for app state
var _selectedItem: Int? {
willSet {
print("\(newValue ?? 0)")
}
}
/// Exposed SwiftUI binding
public lazy var selectedItem: Binding<Int> = Binding<Int>(get: {
self._selectedItem ?? -1
}, set: {
self._selectedItem = $0
})
init () {
_selectedItem = items.first?.id
}
}
struct BindingTestView: View {
#EnvironmentObject var appState: SourceOfTruth
var body: some View {
if appState.items.count > 0 {
TabView(selection: appState.selectedItem) {
ForEach(appState.items) { item in
Text("\(item.id)")
.tag(item.id)
}
}
.tabViewStyle(PageTabViewStyle(indexDisplayMode: .always))
.indexViewStyle(PageIndexViewStyle(backgroundDisplayMode: .always))
} else {
Text("No items")
}
}
}
#main
struct SwiftUICombineTestingApp: App {
let appState = SourceOfTruth()
var body: some Scene {
WindowGroup {
BindingTestView().environmentObject(appState)
}
}
}

Answer 1

You need to add #Published so the variable can be observed by the view:
#Published var _selectedItem: Int? {
willSet {
print("\(newValue ?? 0)")
}
}