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

Question

我按照本教程学习了动态过滤器和 CoreData: https://www.hackingwithswift.com/quick-start/ios-swiftui/dynamically-filtering-fetchrequest-with-swiftui

我有以下代码。 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"
    }

}

核心数据有 1 个实体:Singer，具有 2 个字符串属性:firstName 和 lastName。模块:当前产品模块。 Codegen:手动/无。

当我尝试预览 ContentView 时，预览崩溃。当我打开崩溃日志文件并转到调试导航器时，我可以看到以下错误:

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

上线:

FilteredList.swift:

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

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

AppDelegate.swift:

class AppDelegate: UIResponder, UIApplicationDelegate {

可能是什么问题？包裹值(value)？代码有什么问题？

编辑:以下是我的 Dropbox 上崩溃日志文件的链接: crash-logs

Answer 1

对我有帮助的是将 struct ContentView_Previews(...) 更改为:

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

我还将 struct FilteredList_Previews(...) 更改为:

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)
    }
}

感谢@Gergely，因为他在这里帮助我解决了类似的问题:https://stackoverflow.com/a/59629730/12315994