my code is here:

https://stackoverflow.com/questions/76391885/swiftui-state-binding-selectedindex-state-lost-on-rotation-layout

basicslly what i think is happening is that as i rotate the device the views are thrown away and recreated and as such lose all state. how do i fix this without a 'global' somewhere?

Hi,

You need to go "one level up" with the selectedItem @Binding, try with this:

QuickCalculationsView

struct QuickCalculationsView: View
{
    @Environment(\.horizontalSizeClass) var hSizeClass
    @Environment(\.verticalSizeClass)   var vSizeClass
    @State private var selectedItem = 0

    @ObservedObject var settings = MySettingsSingleton.shared

    var body: some View
    {
        ZStack
        {
            Color.brown
                .edgesIgnoringSafeArea(.all)

            GeometryReader
            { geometry in
                if geometry.size.height > geometry.size.width // PORTRAIT
                {
                    if      UIScreen.main.traitCollection.userInterfaceIdiom == .phone
                    {
                        portraitArrangement(Width: geometry.size.width, Height: geometry.size.height)
                    }
                    else if UIScreen.main.traitCollection.userInterfaceIdiom == .pad
                    {
                        if settings.isLeftHanded == YES
                        {
                            if hSizeClass == .compact
                            {
                                portraitArrangement(Width: geometry.size.width, Height: geometry.size.height)
                            }
                            else
                            {
                                landscapeArrangementLeftHanded(Width: geometry.size.width, Height: geometry.size.height)
                            }
                        }
                        else
                        {
                            if hSizeClass == .compact
                            {
                                portraitArrangement(Width: geometry.size.width, Height: geometry.size.height)
                            }
                            else
                            {
                                landscapeArrangementRightHanded(Width: geometry.size.width, Height: geometry.size.height)
                            }
                        }
                    }
                }
                else                                          // LANDSCAPE
                {
                    if      UIScreen.main.traitCollection.userInterfaceIdiom == .phone
                    {
                        if settings.isLeftHanded == YES
                        {
                            landscapeArrangementLeftHanded(Width: geometry.size.width, Height: geometry.size.height)
                        }
                        else
                        {
                            landscapeArrangementRightHanded(Width: geometry.size.width, Height: geometry.size.height)
                        }
                    }
                    else if UIScreen.main.traitCollection.userInterfaceIdiom == .pad
                    {
                        if settings.isLeftHanded == YES
                        {
                            if hSizeClass == .compact
                            {
                                portraitArrangement(Width: geometry.size.width, Height: geometry.size.height)
                            }
                            else
                            {
                                landscapeArrangementLeftHanded(Width: geometry.size.width, Height: geometry.size.height)
                            }
                        }
                        else
                        {
                            if hSizeClass == .compact
                            {
                                portraitArrangement(Width: geometry.size.width, Height: geometry.size.height)
                            }
                            else
                            {
                                landscapeArrangementRightHanded(Width: geometry.size.width, Height: geometry.size.height)
                            }
                        }
                    }
                }
            }
        }
    }

    @ViewBuilder func portraitArrangement(Width width: CGFloat, Height height: CGFloat) -> some View
    {
        VStack
        {
            CalculationTabView(selectedItem: $selectedItem)
                .frame(width: width, height: 0.6 * height)

            NumberPadView()
                .frame(width: width, height: 0.4 * height)
        }
    }

    @ViewBuilder func landscapeArrangementLeftHanded(Width width: CGFloat, Height height: CGFloat) -> some View
    {
        HStack
        {
            NumberPadView()
                .frame(width: 0.4 * width, height: height)

            CalculationTabView(selectedItem: $selectedItem)
                .frame(width: 0.6 * width, height: height)
        }
    }

    @ViewBuilder func landscapeArrangementRightHanded(Width width: CGFloat, Height height: CGFloat) -> some View
    {
        HStack
        {
            CalculationTabView(selectedItem: $selectedItem)
                .frame(width: 0.6 * width, height: height)

            NumberPadView()
                .frame(width: 0.4 * width, height: height)
        }
    }
}

CalculationTabView

struct CalculationTabView: View
{
  private        let id                   : UUID = UUID()
  @State private var numberOfCalculations : Int    = 9 // Indexed from 0
  @Binding public  var selectedItem         : Int
  private        let width                : CGFloat = 64.0
  private        let arrayOfColors        : [Color] = [Color.pink, Color.mint, Color.purple, Color.cyan, Color.yellow, Color.teal,
                                                       Color.red,      Color.blue,      Color.green,  Color.gray, Color.indigo, Color.white]
  private        let arrayOfText          : [Text]  = [Text("One"),   Text("Two"),   Text("Three"), Text("Four"), Text("Five"),
                                                       Text("Six"),   Text("Seven"), Text("Eight"), Text("Nine"), Text("Ten")]

  var body: some View
  {
    HStack
    {
      VStack
      {
        VerticalSelectableTabBar(numberOfItems: $numberOfCalculations, selectedIndex: $selectedItem, content: arrayOfText)
          .frame(width: width)
      }

      arrayOfColors[selectedItem]
        .padding(.top, 20)
        .padding(.trailing, 10)
    }
  }
}

Hi. thank you so much for your reply.

Doesn't this solution break the concept of it being a reusable view? If the core mechanic is stored 'outside' of the view itself, this would break core software engineering concepts.

is there another way that doesnt violate them and keeps the component reusable?

Well the other way i could think of is using @Appstorage and storing the selected item that way, but maybe somebody else has a better idea.

Doesn't this seem like a fundamental flaw in SwiftUI?