SwiftUI: ForEach of FetchedResults 给出错误 - "Value of type ' NSManagedObject' has no member ..."

Question

我是 SwiftUI 的新手，部分是 Swift，所以希望有人能对我明显缺少的东西有所了解。原谅乱码!我还没有机会重构。

我正在使用 @FetchRequest属性包装器拉出所有列表 Memories从 CoreData 回来模型，带 NSFetchRequest<Memory>在其上定义的方法，从 NSManagedObject 扩展.

当我遍历 FetchRequest<Memory> 的结果时在 SwiftUI 中通过 ForEach ，它返回一个 NSManagedObject 的实例进入循环变量而不是 Memory ，从而显示错误:Value of type 'NSManagedObject' has no member 'createdAt'对于以下 SwiftUI 类:

import SwiftUI

struct MemoryView: View {

    @Environment(
        \.managedObjectContext
    ) var managedObjectContext

    @FetchRequest(
        fetchRequest: Memory.allMemories()
    ) var allMemories: FetchedResults<Memory>

    @State private var newMemory = ""

    var body: some View {
        NavigationView {
            List {
                Section(header: Text("What's on your mind?")) {
                    HStack {
                        TextField("New memory", text: self.$newMemory)

                        Button(action: {
                            let memory = Memory(context: self.managedObjectContext)

                            memory.title = self.newMemory
                            memory.createdAt = Date()

                            do {
                                try self.managedObjectContext.save()
                            } catch {
                                print(error)
                            }

                            self.newMemory = ""
                        }) {
                            Image(systemName: "plus.circle.fill")
                                .foregroundColor(.green)
                                .imageScale(.large)
                        }
                    }
                }
                .font(.headline)

                Section(header: Text("My Memories")) {
                    ForEach(self.allMemories) { memoryItem in
                        MemoryRow(title: memoryItem.title, createdAt: memoryItem.createdAt)
                        //               /\ /\ /\ /\ /\ /\            /\ /\ /\ /\ /\ /\ /\
                        // >>> Error:
                        // >>> Value of type 'NSManagedObject' has no member 'createdAt'
                    }
                }
            }
            .navigationBarTitle(Text("Memories"))
            .navigationBarItems(trailing: EditButton())
        }
    }
}

struct MemoryView_Previews: PreviewProvider {
    static var previews: some View {
        MemoryView()
    }
}

供引用， Memory有问题的模型如下:

import Foundation
import CoreData
import CoreLocation

// Define the CoreData Object Model.
// Ensuring that this class name matches
// the entity model name.
public class Memory: NSManagedObject, Identifiable {
    @NSManaged public var title: String?
    @NSManaged public var body: String?

    @NSManaged public var latitude: Double
    @NSManaged public var longitude: Double

    @NSManaged public var createdAt: Date?
    @NSManaged public var updatedAt: Date?

//    @NSManaged public var type: Type?
//    @NSManaged public var tags: [Tags]?
}

// Create an extension in which to place things out of the context
// of the initial class declaration within.
extension Memory {
    // Define a helper method for accessing a generated
    // CLLocationCoordinate2D instance from the objects
    // latitude and longitude values.
    var locationCoordinate: CLLocationCoordinate2D {
        CLLocationCoordinate2D(
            latitude: latitude,
            longitude: longitude
        )
    }

    // Define a helper method that grabs all instances of a Memory
    // sorted by their created date in ascending order.
    static func allMemories() -> NSFetchRequest<Memory> {
        let request: NSFetchRequest<Memory> = Memory.fetchRequest() as! NSFetchRequest<Memory>

        let sortDescriptor = NSSortDescriptor(key: "createdAt", ascending: true)

        request.sortDescriptors = [sortDescriptor]

        return request
    }
}

对此的任何帮助将不胜感激，因为我完全没有想法。我认为明天的问题有一个新的头脑!

编辑:这是 MemoryRow类(class):

import SwiftUI

struct MemoryRow: View {
    var title: String = ""
    var createdAt: String = ""

    var body: some View {
        HStack {
            VStack(alignment: .leading) {
                Text(title)
                    .font(.headline)

                Text(createdAt)
                    .font(.caption)
            }
        }
    }
}

struct MemoryRow_Previews: PreviewProvider {
    static var previews: some View {
        MemoryRow(title: "Test Title", createdAt: "Test Date")
    }
}

Answer 1

就我而言，这是因为 optional当我们在数据模型中添加属性时默认选择的数据类型，因此在 NSManagedObjectClass它看起来像这样@NSManaged public var name: String?当然，要解决这个问题，可以删除 ?从上面一行或使用 ??使用该属性时的 nil 合并运算符，例如 ClassName.name ?? "Unknown"根据给出的答案和评论，我在类里面检查了我的属性名称是否有任何错字，然后我意识到了这个问题。现在必须处理 SwiftUI 的这种行为它不会显示实际错误或将显示在任何其他行上。🤯
希望这可以帮助像我这样的人。