FetchedResults crash: Dispatch queue: com.apple.main-thread (0)

Question

I have followed this tutorial to learn about dynamic filters and CoreData: https://www.hackingwithswift.com/quick-start/ios-swiftui/dynamically-filtering-fetchrequest-with-swiftui

I have the following code. ContentView.swift:

import SwiftUI

struct ContentView: View {

    @Environment(\.managedObjectContext) var moc
    @State var lastNameFilter = "A"

    var body: some View {

        VStack {
            FilteredList(filter: lastNameFilter)

            Button("Add Examples") {
                let taylor = Singer(context: self.moc)
                taylor.firstName = "Taylor"
                taylor.lastName = "Swift"

                let ed = Singer(context: self.moc)
                ed.firstName = "Ed"
                ed.lastName = "Sheeran"

                let adele = Singer(context: self.moc)
                adele.firstName = "Adele"
                adele.lastName = "Adkins"

                try? self.moc.save()
            }

            Button("Show A") {
                self.lastNameFilter = "A"
            }

            Button("Show S") {
                self.lastNameFilter = "S"
            }
        }

    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

FilteredList.swift:

import CoreData
import SwiftUI

struct FilteredList: View {

    var fetchRequest: FetchRequest<Singer>
    var singers: FetchedResults<Singer> { fetchRequest.wrappedValue }

    var body: some View {
        List(singers, id: \.self) { singer in
            Text("\(singer.wrappedFirstName) \(singer.wrappedLastName)")
        }
    }

    init(filter: String) {
        fetchRequest = FetchRequest<Singer>(entity: Singer.entity(), sortDescriptors: [], predicate: NSPredicate(format: "lastName BEGINSWITH %@", filter))
    }

}

//struct FilteredList_Previews: PreviewProvider {
//    static var previews: some View {
//    }
//}

Singer+CoreDataClass.swift:

import Foundation
import CoreData

@objc(Singer)
public class Singer: NSManagedObject {

}

Singer+CoreDataProperties.swift:

import Foundation
import CoreData


extension Singer {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Singer> {
        return NSFetchRequest<Singer>(entityName: "Singer")
    }

    @NSManaged public var firstName: String?
    @NSManaged public var lastName: String?

    var wrappedFirstName : String {
        firstName ?? "Unknown"
    }

    var wrappedLastName : String {
        lastName ?? "Unknown"
    }

}

Core Data has 1 entity: Singer with 2 String attributes: firstName and lastName. Module: Current Product Module. Codegen: Manual/None.

When I try to preview ContentView, preview crashes. When I open crash logs file and go to Debug Navigator, I can see following error:

Dispatch queue: com.apple.main-thread (0)

On lines:

FilteredList.swift:

var singers: FetchedResults<Singer> { fetchRequest.wrappedValue }

List(singers, id: \.self) { singer in

AppDelegate.swift:

class AppDelegate: UIResponder, UIApplicationDelegate {

What can be the problem? wrappedValue? What is wrong with the code?

EDIT: Here is a link to crash log file on my Dropbox: crash-logs

Hi, Asperi. I have just edited my question and added link to crash logs file at the end. — mallow
– mallow, Commented Dec 26, 2019 at 0:24

mallow · Accepted Answer · 2020-01-29 05:56:06Z

2

What helped me was changing struct ContentView_Previews(...) to:

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return ContentView().environment(\.managedObjectContext, context)
    }
}

I have also changed struct FilteredList_Previews(...) to:

struct FilteredList_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return FilteredList(filter: "A").environment(\.managedObjectContext, context)
    }
}

Kudos to @Gergely as he helped me with similar problem here: https://stackoverflow.com/a/59629730/12315994

answered Jan 29, 2020 at 5:56

mallow

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Comments

nine stones · Accepted Answer · 2020-01-29 18:30:19Z

0

I had the same scenario. Providing the context from the AppDelegate to the preview and adding data to that fixed the crash for me:

struct FilteredList_Previews: PreviewProvider {
  static var previews: some View {
    let moc = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    let taylor = Singer(context: moc)
    taylor.firstName = "Taylor"
    taylor.lastName = "Swift"
    return FilteredList().environment(\.managedObjectContext, moc)
  }
}

Unfortunately, the data does not show the exact data I would expect (it shows duplicated entries). But for me it's good enough to preview the design of the view. Hope that still helps.

edited Jan 29, 2020 at 18:30

answered Jan 28, 2020 at 18:46

nine stones

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2 Comments

mallow Over a year ago

Thanks, but on line let taylor = Singer(context: self.moc) I was getting an error Type 'FilteredList_Previews' has no member 'moc' I have solved this another way thanks to help from @Gergely. Please check my answer to this question. Maybe you will find it useful as well.

nine stones Over a year ago

Except that I forgot to strip self. from the context parameter that's exactly my suggestion. I'll still update my response if someone else finds it helpful.

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