SwiftUI List view return error Type '()' cannot conform to 'View with core data

Question

I have implemented core data with required function. I am trying to call function into view , when we do not have data into core data , that time I am making network call and save the data into core data. I am following MVVM Architecture pattern. Inside the view I also have search functionality as well. The problem is when I try to call the save function I am getting following error ..

Type '()' cannot conform to 'View'

I am expecting when we make network call that time , it should configure the data into list and also save it into core data storage..

Here is the core data function.

import Foundation
import CoreData

class CoreDataViewModel: ObservableObject {

    let persistentContainer: NSPersistentContainer

    init() {
        persistentContainer = NSPersistentContainer(name: "ProductDataManager")
        persistentContainer.loadPersistentStores { (description, error) in
            if let error = error {
                print("Core Data Store failed \(error.localizedDescription)")
            }
        }
    }
func saveProduct(products: Product) {

        let product = ProductEntity(context: persistentContainer.viewContext)
        product.id = Int16(products.id)
        product.title = products.title
        product.price = Int32(products.price)
        product.quantity = Int16(products.quantity)
        product.total = Int32(products.total)
        product.discountPercentage = products.discountPercentage
        product.discountedPrice = Int32(products.discountedPrice)

        do {
            try persistentContainer.viewContext.save()
        } catch {
            print("Failed to save movie \(error)")
        }
    }
}

Here is my view Model code .. It has dependency on ProductRepository if required I can add that code as well ..

import Foundation

protocol ProductListViewModelAction: ObservableObject {
    func getProductList(urlStr: String) async
}

@MainActor
final class ProductListViewModel {
    
    @Published private(set) var productLists: [Product] = []
    @Published private var filteredProductLists: [Product] = []
    @Published private(set) var customError: NetworkError?
    
    private let repository: ProductRepository
    init(repository: ProductRepository) {
        self.repository = repository
    }
}

extension ProductListViewModel: ProductListViewModelAction {
    func getProductList(urlStr: String) async {

        guard let url = URL(string: urlStr) else {
        self.customError = NetworkError.invalidURL
        return
        }
        do {
            let lists = try await repository.getList(for: url)
            productLists = lists.carts.first?.products ?? []
            
        } catch {
            customError = error as? NetworkError
        }
    }
}

extension ProductListViewModel {
    
    func performSearch(keyword: String) {
        filteredProductLists = self.productLists.filter {
            $0.title.localizedCaseInsensitiveContains(keyword)
        }
    }
    var productList: [Product] {
        filteredProductLists.isEmpty ? productLists: filteredProductLists
    }
}

Here is my view code ..

import SwiftUI

struct ProductListView: View {
    
    @StateObject var viewModel = ProductListViewModel(repository: ProductRepositoryImplementation(networkManager: NetworkManager()))

    @StateObject var coreData = CoreDataViewModel()
    @State var searchText = ""

     var body: some View  {

        NavigationStack {
            VStack {
                    if viewModel.productLists.count > 0 {

                        List(viewModel.productList, id: \.self) { product in
                            NavigationLink(destination: ProductDetailsView(product: product)) {
                                ProductListViewCell(productData: product)
                                coreData.saveProduct(products: product) // error is here ..
                            }

                        } .listStyle(.grouped)
                    }
                }
            }
            .searchable(text: $searchText)
            .onChange(of: searchText, perform: viewModel.performSearch)
            .navigationTitle(Text("Product List"))
        }.task {
            await getDataFromAPI()
        }
        .refreshable {
            await getDataFromAPI()
        }
    }
      func getDataFromAPI() async {
        await viewModel.getProductList(urlStr: NetworkURL.productUrl)
    }
}

Here is the screenshot of the error ..

Answer 1

It's a common mistake when someone starts using SwiftUI: attempting to call a function where only View types are allowed.

You can easily fix this by calling the function when the view is appearing:

List(viewModel.productList, id: \.self) { product in
    NavigationLink(destination: ProductDetailsView(product: product)) {
           ProductListViewCell(productData: product)
               .onAppear {
                  coreData.saveProduct(products: product)
               }
           }
}

However due to the behavior of Lists, it may appear multiple times while scrolling through the view. I would check whether it's already saved or not to prevent unnecessary save actions.

Just a note: the Product has an 'id' variable, so you can conform it to the Identifiable protocol. After that, you don't need to specify 'id' in List or ForEach.

List(viewModel.productList)