CoreData 和 SwiftUI：环境中的上下文未连接到持久存储协调器

Question

我正在尝试通过构建作业管理应用程序来自学 Core Data。我的代码构建良好，应用程序运行正常，直到我尝试向列表添加新分配。我得到这个错误Thread 1: EXC_BREAKPOINT (code=1, subcode=0x1c25719e8)以下行：ForEach(courses, id: \.self) { course in。控制台也有这个错误：Context in environment is not connected to a persistent store coordinator: <NSManagedObjectContext: 0x2823cb3a0>。

我对 Core Data 知之甚少，不知道可能是什么问题。我在数据模型中设置了“作业”和“课程”实体，其中课程与作业具有一对多关系。每个作业将被归类到特定的课程下。

这是向列表添加新分配的视图的代码：

    struct NewAssignmentView: View {

    @Environment(\.presentationMode) var presentationMode
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(entity: Course.entity(), sortDescriptors: []) var courses: FetchedResults<Course>

    @State var name = ""
    @State var hasDueDate = false
    @State var dueDate = Date()
    @State var course = Course()

    var body: some View {
        NavigationView {
            Form {
                TextField("Assignment Name", text: $name)
                Section {
                    Picker("Course", selection: $course) {
                        ForEach(courses, id: \.self) { course in
                            Text("\(course.name ?? "")").foregroundColor(course.color)
                        }
                    }
                }
                Section {
                    Toggle(isOn: $hasDueDate.animation()) {
                        Text("Due Date")
                    }
                    if hasDueDate {
                        DatePicker(selection: $dueDate, displayedComponents: .date, label: { Text("Set Date:") })
                    }
                }
            }
            .navigationBarTitle("New Assignment", displayMode: .inline)
            .navigationBarItems(leading: Button(action: {
                self.presentationMode.wrappedValue.dismiss()
            }, label: { Text("Cancel") }),
                                trailing: Button(action: {
                                    let newAssignment = Assignment(context: self.moc)
                                    newAssignment.name = self.name
                                    newAssignment.hasDueDate = self.hasDueDate
                                    newAssignment.dueDate = self.dueDate
                                    newAssignment.statusString = Status.incomplete.rawValue
                                    newAssignment.course = self.course
                                    self.presentationMode.wrappedValue.dismiss()
                                }, label: { Text("Add").bold() }))
        }
    }
}

编辑：这是 AppDelegate 中设置持久容器的代码：

lazy var persistentContainer: NSPersistentCloudKitContainer = {
    let container = NSPersistentCloudKitContainer(name: "test")
    container.loadPersistentStores(completionHandler: { (storeDescription, error) in
        if let error = error as NSError? {
            fatalError("Unresolved error \(error), \(error.userInfo)")
        }
    })
    return container
}()

以及设置环境的 SceneDelegate 中的代码：

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
    // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
    // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).

    // Get the managed object context from the shared persistent container.
    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

    // Create the SwiftUI view and set the context as the value for the managedObjectContext environment keyPath.
    // Add `@Environment(\.managedObjectContext)` in the views that will need the context.
    let contentView = ContentView().environment(\.managedObjectContext, context)

    // Use a UIHostingController as window root view controller.
    if let windowScene = scene as? UIWindowScene {
        let window = UIWindow(windowScene: windowScene)
        window.rootViewController = UIHostingController(rootView: contentView)
        self.window = window
        window.makeKeyAndVisible()
    }
}

Answer 1

像您的 moc 这样的环境值只会自动传递给层次结构中的其他视图。因此，如果您显示工作表或任何不属于您的视图层次结构的内容，您将失去环境，并且您需要将 moc 传递给新的层次结构，就像您为 ContentView 所做的那样。检查此代码片段：

.sheet(isPresented: self.$showSheet) {
            SheetView()
                .environment(\.managedObjectContext, self.moc)
        }

Answer 2

您实际上并没有保存上下文。您应该执行以下操作：

let newAssignment = Assignment(context: self.moc)
newAssignment.name = self.name
newAssignment.hasDueDate = self.hasDueDate
newAssignment.dueDate = self.dueDate
newAssignment.statusString = Status.incomplete.rawValue
newAssignment.course = self.course

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

你@FetchRequest(...)也可以看起来像这样：

@FetchRequest(fetchRequest: CourseItem.getCourseItems()) var courses: FetchedResults<CourseItem>

你可以修改你的CourseItem类来处理sortDescriptors如下：

public class CourseItem: NSManagedObject, Identifiable {
    @NSManaged public var name: String?
    @NSManaged public var dueDate: Date?
    // ...etc
}

extension CourseItem {
    static func getCourseItems() -> NSFetchRequest<CourseItem> {
        let request: NSFetchRequest<CourseItem> = CourseItem.fetchRequest() as! NSFetchRequest<CourseItem>

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

        request.sortDescriptors = [sortDescriptor]

        return request
    }
}

然后，您可以修改ForEach(...)以下内容，也可以很容易地处理项目的删除：

ForEach(self.courses) { course in
    // ...
}.onDelete { indexSet in
    let deleteItem = self.courses[indexSet.first!]
    self.moc.delete(deleteItem)

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

您要确保的一件事是“班级名称”设置为“CourseItem”，它与CourseItem我们之前创建的班级相匹配。

只需点击实体在您的.xcdatamodeId文件，并设置一切以下（含模块到“当前产品模块”和代码生成“手动/无”）：