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Generalizing FilteredList with protocol - help needed

Forums > SwiftUI

Hey ho. I'm trying to make the famous filtered list more reusable and did so in a first step. Now I want to try out something more advanced - but am stuck.

Have a look here:

/// A container that presents rows of data from a Core Data fetch request, which is generated with the provided sort descriptors and predicate.
///
/// In its simplest form, it is used inside a view providing typical Core Data assets.
/// ```
/// var body: some View {
///     FilteredList(sortDescriptors: []) { (item: Item) in
///         Text(item.title)
///     }
/// }
/// ```
/// More commonly you specify how the list is sorted and filtered, in which case Swift can infer the type of your items.
/// As an option, swipe to delete can be enabled, using the environments managed object context to delete the selected item.
/// You can also provide a special view in case the result set is empty.
/// ```
/// var body: some View {
///     FilteredList(sortDescriptors: sortDescriptors, predicate: predicate, canDelete: true) { item in
///         Text(item.title)
///     } emptyView: {
///         Text("There are currently no items available.")
///     }
/// }
/// ```
struct FilteredList<T: NSManagedObject, Content: View, Empty: View>: View {
    @Environment(\.managedObjectContext) var viewContext
    @FetchRequest var fetchRequest: FetchedResults<T>
    let canDelete: Bool
    let content: (T) -> Content
    let emptyView: Empty

    var body: some View {
        if fetchRequest.isEmpty {
            emptyView
        } else {
            List {
                ForEach(fetchRequest, id: \.self) { item in
                    self.content(item)
                }
                .onDelete(perform: canDelete ? delete : nil)
            }
        }
    }

    init(sortDescriptors: [SortDescriptor<T>] = [], predicate: NSPredicate? = nil, canDelete: Bool = false,
         @ViewBuilder content: @escaping (T) -> Content,
         @ViewBuilder emptyView: () -> Empty
    ) {
        _fetchRequest = FetchRequest<T>(sortDescriptors: sortDescriptors, predicate: predicate, animation: .default)
        self.canDelete = canDelete
        self.content = content
        self.emptyView = emptyView()
    }

    func delete(_ offsets: IndexSet) {
        for offset in offsets {
            let item = fetchRequest[offset]
            viewContext.delete(item)
        }
        try? viewContext.save()
    }
}

extension FilteredList where Content: View, Empty == EmptyView {
    init(sortDescriptors: [SortDescriptor<T>] = [], predicate: NSPredicate? = nil, canDelete: Bool = false,
         @ViewBuilder content: @escaping (T) -> Content) {
        self.init(sortDescriptors: sortDescriptors, predicate: predicate, canDelete: canDelete,
             content: content, emptyView: { EmptyView() })
    }
}

When I want to use it together with a view model, I ended up with this integration code:

FilteredList(sortDescriptors: viewModel.sortDescriptors, predicate: viewModel.predicate, canDelete: true) { player in
    HStack {
        NavigationLink(
            destination: PlayerDetailView(
                viewModel: PlayerViewModel(dataController: dataController, player: player)
            )
        ) {
            Label(player.playerName, systemImage: "person")
        }
        Spacer()
    }
} emptyView: {
    NoDataView(text: "There are currently no players available.")
}

and thought: everything could be encapsulated in the view model.

Now my next approach would be to

  • provide a FilteredListViewModelProtocol which requires certain properties/functions
  • pass in the viewmodel as only parameter to the FilteredList.
  • When there is an optional delete function defined in the view model, enable swipe to delete, otherwise not.

and I started here:

protocol FilteredListViewModel {
    associatedtype Entity: NSManagedObject
    var sortDescriptors: [SortDescriptor<Entity>] { get }
    var predicate: NSPredicate? { get }
    var allowsDeletion: Bool { get }
    func delete(_ offsets: IndexSet)
}

with a new initializer

init<VM: FilteredListViewModel>(viewModel: VM,
     @ViewBuilder content: @escaping (T) -> Content,
     @ViewBuilder emptyView: () -> Empty
) {
    _fetchRequest = FetchRequest<T>(sortDescriptors: viewModel.sortDescriptors,  // <--- Cannot convert value of type '[SortDescriptor<VM.Entity>]' to expected argument type '[NSSortDescriptor]'
                                    predicate: viewModel.predicate,
                                    animation: .default)
    self.canDelete = viewModel.allowsDeletion
    self.content = content
    self.emptyView = emptyView()
}
  1. I receive the inline mentioned compile error and wonder: there is an initializer for SortDescriptor rather than NSSortDescriptor. Why am I getting this and how to get around?
  2. What would be a feasible way - if at all possible - to avoid having to include allowDeletion but rather check whether the (then optional) delete function is defined in the view model? If I provide a default implementation, it would still be defined and I can't check whether it does something or not (or is there a way?). If I mark it as @objc optional I have to mark the whole protocol as @objc and get compile errors
Associated type 'Entity' cannot be declared inside '@objc' protocol 'FilteredListViewModel
Property cannot be a member of an @objc protocol because its type cannot be represented in Objective-C

on the associatedtype and sortDescriptors declaration respectively.

2      

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