Showing alert with an error does not work when using a ViewModel

I am trying to find a way to use the new alert(isPresented:error:actions:message:). My motivation is to find a good way to throw a custom error and have a user facing alert using that error.

My issue is that this works when using local bindings but not when using a ViewModel. I don’t see where I am doing anything wrong, so I am wondering whether this is an error I should file a radar for?

This is my custom error, that implements the LocalizedError protocol:

struct CustomError: LocalizedError {
    var errorDescription: String?
    var failureReason: String?
    var helpAnchor: String?
    var recoverySuggestion: String?
}

This view is working just fine:

struct AlertViewWithoutViewModel: View {
    @State private var presentAlert = false
    @State private var error: CustomError?
    
    var body: some View {
        Button("Show alert without ViewModel") {
            error = CustomError(errorDescription: "Description of the error", failureReason: "It went wrong.")
            presentAlert = true
        }
        .padding()
        .alert(isPresented: $presentAlert, error: error) { error in
            Button("OK") {}
        } message: { error in
            if let failureReason = error.failureReason {
                Text(failureReason)
            }
        }

    }
}

What is not working is using this ViewModel––and I don’t understand why:

class ViewModel: ObservableObject {
    @Published var presentAlert = false
    @Published var error: CustomError?
}

struct AlertViewWithViewModel: View {
    @State private var vm = ViewModel()
    
    var body: some View {
        Button("Show alert with ViewModel") {
            vm.error = CustomError(errorDescription: "Fancy VM error title", failureReason: "Some fancy reason.")
            vm.presentAlert = true
        }
        .alert(isPresented: $vm.presentAlert, error: vm.error) { error in
            Button("OK") {}
        } message: { error in
            if let failureReason = error.failureReason {
                Text(failureReason)
            }
        }
    }
}

>Solution :

You need StateObject wrapper (which is observer for ObservableObject) instead of State (which for value types),

struct AlertViewWithViewModel: View {
    @StateObject private var vm = ViewModel()    // << here !!