SwiftUI FetchRequest 错误：“获取请求必须有一个实体”

Question

我正在使用 SwiftUI 和 CoreData 以及 SwiftUI 应用程序生命周期（无场景或应用程序委托）构建一个小型应用程序。当我运行+构建我的应用程序时出现以下错误：

'executeFetchRequest:error: A fetch request must have an entity.'

我已经检查/验证/重新检查了以下内容：

• 我的[app name].xcdatamodeld 文件名与我传入 NSPersistentContainer NSPersistentCloudKitContainer(name: [app name]) 的文件名相同
• 实体名称 Car 正是我传递给 FetchRequest 的内容

@FetchRequest(entity: Car.entity(), sortDescriptors: []) var car: FetchedResults<Car>

• 我正在为我的实体 Codegen 选择 Manual/None，生成的类是

public class Car: NSManagedObject {}

在 Car 上的扩展名为 Identifiable。

这是我的整个视图结构，应该（据我理解）将环境传递给它的所有“子”视图。

struct AppView: View {
    
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(entity: Car.entity(), sortDescriptors: []) var car: FetchedResults<Car>
    
    var body: some View {
        List {
            ForEach(car, id:\.self) { item in
                RootView(carData: item)
                    .environment(\.managedObjectContext, self.moc)
            }
        }
    }
}

还有我的@main 结构

@main
struct AutoMateApp: App {
    
    @StateObject var coreData = PersistentCloudKitContainer()
    
    let persistence = PersistentCloudKitContainer()
    
    var body: some Scene {
        WindowGroup {
            AppView()
                .environment(\.managedObjectContext, coreData.persistentContainer.viewContext)
        }
    }
}

当我使用调试器单步执行时，一旦返回 WindowGroup，就会出现崩溃。我不确定这是否是有用的信息。

感谢您的所有帮助，谢谢。

Answer 1

好的，所以从AppDelegate 中的lazy var persistentContainer: NSPersistentCloudKitContainer = {...}() 行中删除lazy 对我的情况有帮助。因为似乎 Apple 的样板代码在 let contentView = ContentView().environment(\.managedObjectContext, persistentContainer.viewContext) 行中传递了 NSManagedObjectModel 并且 FetchRequest 在它被闭包正确实例化之前访问了该对象，因此无法找到 NSManagedObject subclass。从变量中删除lazy 会立即执行闭包。

希望这仍然可以帮助您并解决您遇到的问题。

Answer 2

将 Car.entity() 替换为静态 NSEntityDescription。

例子

struct AppView: View {
    
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(entity: AppView_Previews.entity, sortDescriptors: []) var car: FetchedResults<Car>
    
    var body: some View {
        List {
            ForEach(car, id:\.self) { item in
                RootView(carData: item)
                    .environment(\.managedObjectContext, self.moc)
            }
        }
    }
}

struct AppView_Previews: PreviewProvider {
    static var entity: NSEntityDescription {
        return NSEntityDescription.entity(forEntityName: "Car", in: yourViewContext)!
    }
}

Answer 3

在选中 Core Data 支持时尝试 SwiftUI 应用项目模板中的代码：

MyApp.swift

import SwiftUI

@main
struct MyApp: App {
    let persistenceController = PersistenceController.shared

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext, persistenceController.container.viewContext)
        }
    }
}

Persistance.swift

import CoreData

struct PersistenceController {
    static let shared = PersistenceController()

    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        let viewContext = result.container.viewContext
        for _ in 0..<10 {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()
        }
        do {
            try viewContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nsError = error as NSError
            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
        }
        return result
    }()

    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "MyApp")
        if inMemory {
            container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        }
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                /*
                Typical reasons for an error here include:
                * The parent directory does not exist, cannot be created, or disallows writing.
                * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                * The device is out of space.
                * The store could not be migrated to the current model version.
                Check the error message to determine what the actual problem was.
                */
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
    }
}