PrepareViewData.swift

struct PrepareViewData: ViewModifier {
  @State var hasPrepared = false
    
  let action: (() -> Void)
    
  func body(content: Content) -> some View {
    content
      .onAppear {
        if !hasPrepared {
          action()
          hasPrepared = true
        }
      }
  }
}

extension View {
  func prepare(perform action: @escaping () -> Void) -> some View {
    modifier(PrepareViewData(action: action))
  }
}

struct ContentView: View {
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        VStack {
          // redacted
        }
        .prepare {
            viewModel.prepare()
        }
    }
}