SwiftUI FetchRequest错误：“提取请求必须具有实体”

问题描述

我正在使用SwiftUI和CoreData构建一个小型应用程序，并具有SwiftUI应用程序生命周期（无场景或应用程序委托）。运行并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文件名与传递给NSPersistentContainer NSPersistentCloudKitContainer(name: [app name])的文件名相同
• 实体的名称Car正是我传递给FetchRequest的名称

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

• 我正在为实体Codegen选择“手动/无”，并且生成的类是

public class Car: NSManagedObject {}

在Car上的扩展名为Identifiable。

这是我的整个视图结构（据我了解）应该将环境传递给所有“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)
            }
        }
    }
}

和我的@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)
        }
    }
}

当我逐步使用调试器时，一旦返回WindowGroup，崩溃似乎就会出现。我不确定这是否对您有帮助。

感谢您的帮助，谢谢。

解决方法

好的，因此从lazy的{​​{1}}行中删除lazy var persistentContainer: NSPersistentCloudKitContainer = {...}()有助于我的情况。因为似乎苹果公司在AppDelegate行中传递了NSManagedObjectModel的样板代码，并且FetchRequest在该对象被闭包正确实例化之前访问了该对象，因此无法找到{{1} }。从变量中删除let contentView = ContentView().environment(\.managedObjectContext,persistentContainer.viewContext)会立即执行关闭操作。

希望这对您有所帮助，并能解决您所遇到的问题。

,

在选中“核心数据”支持后，尝试从SwiftUI应用项目模板中获取代码：

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)")
            }
        })
    }
}

,

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

示例

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)!
    }
}