如何让 SwiftUI 与 Core Data 一起工作（在启动项目后）？

Question

我在尝试让 Xcode 与 Core Data 一起工作时收到此错误。任何帮助将不胜感激。

Context in environment is not connected to a persistent store coordinator: <NSManagedObjectContext: 0x6000008a8820>

在注释掉所有其他环境后，持久化环境只在顶层使用，我仍然有同样的错误。

这是应用程序顶级 Swift 文件：

导入 SwiftUI 导入核心数据

@main
struct Draw_DailyApp: App {
    
let persistenceController = PersistenceController.shared
@Environment(\.scenePhase) var scenePhase

@FetchRequest(entity: Drawing.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \Drawing.img, ascending: true)])



var orders: FetchedResults<Drawing>

@State var showOrderSheet = false


// Creating a global environment and placing it into the environment
@ObservedObject var searchObjectController: SearchObjectController = SearchObjectController()


var body: some Scene {
    WindowGroup {
        ContentView()
            .environment(\.managedObjectContext, persistenceController.container.viewContext)
            .environmentObject(self.searchObjectController)
    }
    .onChange(of: scenePhase) { _ in
        persistenceController.save()
    }
}

}

这里是使用持久化控制器的文件（Core Data xcdatamodeld 有一个名为 img 的二进制数据属性）

Persistence.Swift

    import CoreData
import UIKit

struct PersistenceController {
    static let shared = PersistenceController()
    
    func save() {
        let context = container.viewContext

        if context.hasChanges {
            do {
                try context.save()
            } catch {
                // Show some error here
            }
        }
    }

    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        let viewContext = result.container.viewContext
        for _ in 0..<10 {
            let newItem = Drawing(context: viewContext)
            newItem.img = Data()
            newItem.date = "02081999"
        }
        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: "DrawDaily")
        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)")
            }
        })
    }
}

PS：这个项目我不是用Core Data开始的，是后来添加的

Answer 1

您的@FetchRequest 在初始化时没有NSManagedObjectContext。您正在使用此行创建它

let persistenceController = PersistenceController.shared

您的@FetchRequest 必须向下移动到ContentView 才能正常工作。

必须调用此行

 .environment(\.managedObjectContext, persistenceController.container.viewContext)

初始化包含@FetchRequest的View时