PrepareAsyncViewData.swift

struct PrepareAsyncViewData: ViewModifier {
  @State var hasPrepared = false
  @State var task: Task<Void, Never>?
    
  let action: (() async -> Void)
    
  func body(content: Content) -> some View {
    content
      .onAppear {
        guard !hasPrepared else { return }
        guard task == nil else { return }
        task = Task {
          await action()
          hasPrepared = true
        }
      }
      .onDisappear {
        task?.cancel()
        task = nil
      }
  }
}

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

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