SortDescriptor from a generic method crashes Xcode Previews but not Simulator.

Hi,

Sorry to hear you are having problems getting previews working. This most commonly is seen when previews don't have a SwiftData model container configured correctly.

Could you show how you are setting up your preview? I am looking for your #Preview definition + any custom PreviewModifier s you are using.

Thank you for your time replying. Yeah I had many problems a year or two ago in getting the container right. But I think it ok at the moment especially as it works fine creating a SortDescriptor one way, but creating it the other way (without changing anything else) causes the crash. I copied only the necessary code to a basic app today so the whole thing fits in here. There are actually 2 files with previews and they add the model container in different ways: .modelContainer(PreviewDataController.previewContainer) .modelContainer(for: Item.self, inMemory: true)

So the problem is probably else where.

import SwiftData
 
@main
struct MyAppApp: App {
    var sharedModelContainer: ModelContainer = {
        let schema = Schema([
            Item.self,
        ])
        let modelConfiguration = ModelConfiguration(schema: schema, isStoredInMemoryOnly: false)
 
        do {
            return try ModelContainer(for: schema, configurations: [modelConfiguration])
        } catch {
            fatalError("Could not create ModelContainer: \(error)")
        }
    }()
 
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(sharedModelContainer)
    }
}

import SwiftData
 
struct ContentView: View {
    @Query private var items: [Item]
    
    var body: some View {
        NavigationView{
            ItemsList(sortStyle: .alphabetical)
        }
    }
}
 
#Preview {
    ContentView()
        .modelContainer(for: Item.self, inMemory: true)
}

import SwiftUI
import SwiftData
 
struct ItemsList: View {
    @Environment(\.modelContext) private var modelContext
    @Query private var items:[Item] = []
    init(sortStyle:ListSortingStyle<Item> = .alphabetical) {
           let sortDescriptor1 = SortDescriptor<Item>(\.name, comparator: .localized, order: .forward) //this works in both
           let sortDescriptor2 = sortStyle.sortDescriptor() //this works in simulator but not in preview
           print(sortDescriptor1,"\n",sortDescriptor2)
           let fetchDescriptor = FetchDescriptor<Item>(sortBy:[sortDescriptor1]) // <-- change between 1&2 to reproduce the crash
           
           self._items = Query(fetchDescriptor)
       }
 
    var body: some View {
        List {
            ForEach(items) { item in
                NavigationLink {
                    Text(item.name)
                } label: {
                    Text("\(item.name)")
                }
            }
        }
        .toolbar {
            ToolbarItem {
                Button(action: addItem) {
                    Label("Add Item", systemImage: "plus")
                }
            }
        }
    }
    private func addItem() {
        withAnimation {
            let newItem = Item(name: "New Item")
            modelContext.insert(newItem)
        }
    }
}
 
#Preview {
    NavigationView{
        ItemsList(sortStyle: .alphabetical)
        .modelContainer(PreviewDataController.previewContainer)
    }
}

import Foundation
import SwiftData
 
@MainActor
class PreviewDataController {
    static let previewContainer: ModelContainer = {
        do {
            let schema = Schema([
                Item.self,
            ])
            
            let modelConfiguration = ModelConfiguration(schema: schema, isStoredInMemoryOnly: true)
            
            let container = try ModelContainer(for: schema, configurations: [modelConfiguration])
                        
            return container
        } catch {
            fatalError("Failed to create container for preview: \(error.localizedDescription)")
        }
    }()

 
/// A Protocol for models to help get predicates etc to sort
protocol ListSorting {
    var name:String {get}
}
 
extension Item:ListSorting {}
 
enum ListSortingStyle<T>:String,CaseIterable where T:ListSorting {
    case alphabetical = "A…Z"
    case alphabeticalReverse = "Z…A"
    
    /// Returns a SortDescriptor matching the case
    func sortDescriptor() -> SortDescriptor<T> {
        switch self {
        case .alphabetical:
            SortDescriptor<T>(\.name, comparator: .localized, order: .forward)
        case .alphabeticalReverse:
            SortDescriptor<T>(\.name, comparator: .localized, order: .reverse)
        }
    }
}

import Foundation
import SwiftData
 
@Model
final class Item {
    var name:String = ""
    
    init(name: String) {
        self.name = name
    }
}

It looks like a similar issue I am facing: https://developer.apple.com/forums/thread/775624