I have a user sign-up form that changes the color of the requested user name when its availability is checked. The color is based on an @State enum in the view:

struct SignUpView: View
{
    enum ValueState
    {
        case unchecked
        case good
        case noGood
    }

    ...
    @State private var          usernameCheck: ValueState = .unchecked

I build the bulk of the UI in a ViewBuilder function, so the body only contains this:

var body: some View
{
    Group
    {
        ScrollView
        {
            signUpForm()
            Text("This is some BS.")
                .foregroundColor(usernameCheck == .unchecked ? .yellow :
                                    usernameCheck == .good ? .green : .red)
        }
    }
}

As you can see, that text field’s color depends on that state variable. It works fine. If I put the exact same dependency on a text field added by a ViewBuilder, it doesn’t work.

@ViewBuilder func signUpForm() -> some View
{
    VStack
    {
        Text("Sign up!")
            .font(AppStyle.titleFont)
            .padding(.bottom)
        
        Group
        {
            Text("Desired username:")
                .font(AppStyle.labelFont)
                .frame(maxWidth: .infinity, alignment: .leading)
                .padding(1)
            HStack
            {
                TextField("", text: $tempUser.username)
                    .onChange(of: tempUser.username, perform: { value in
                        usernameChanged(toString: value) })
                    .foregroundColor(usernameCheck == .unchecked ? .yellow :
                                        usernameCheck == .good ? .green : .red) <- DOES NOT WORK

I even tried passing userNameCheck into the ViewBuilder. No dice. So how are ViewBuilders supposed to work?