SwiftUI 中的@FetchRequest

Question

在使用 Core Data 运行我的 SwiftUI 应用程序时，我不断收到以下错误。

[error] error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'Movie' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?

在我的 MovieAppApp 文件中，我有以下代码。

 var body: some Scene {
        WindowGroup {
            MoviesScreen().environment(\.managedObjectContext, CoreDataManager.shared.viewContext)
        }
    }

最后，在 MoviesScreen 中，我使用了@FetchRequest，如下所示：

struct MoviesScreen: View {
    
    @Environment(\.managedObjectContext) var managedObjectContext
    
    @FetchRequest(entity: Movie.entity(), sortDescriptors: [NSSortDescriptor(key: "title", ascending: true)])
    var movies: FetchedResults<Movie>
    
    var body: some View {
        List(movies, id: \.self) { movie in
            Text(movie.title ?? "")
        }
    }
}

CoreDataManager 如下图所示：

class CoreDataManager {
    
    let persistentContainer: NSPersistentContainer
    
    static let shared = CoreDataManager()
    
    private init() {
        
        persistentContainer = NSPersistentContainer(name: "MovieAppModel")
        persistentContainer.loadPersistentStores { (description, error) in
            if let error = error {
                fatalError("Failed to initialize Core Data \(error)")
            }
        }
        
        let directories = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)
        print(directories[0])
    }
    
    var viewContext: NSManagedObjectContext {
        return persistentContainer.viewContext 
    }
    
}

为什么我总是收到错误消息？

Answer 1

您应该从 main 包中获取您的 模型，并将其传递给 NSPersistentContainer 初始化程序。

试试下面的代码：

class DataControllers:ObservableObject{
   
    let container: NSPersistentContainer
    
    init(inMemory:Bool = false) {
       
        container = NSPersistentContainer(name: "Main", managedObjectModel: DataController.model)
        
        if inMemory{
            container.persistentStoreDescriptions.first?.url = URL(fileURLWithPath: "/dev/null")
        }
        
       
        container.loadPersistentStores { (storeDescription, error) in
            if let error = error{
                fatalError("fatal error loading data:\(error.localizedDescription)")
            }
           
        }
    }
    
    static let model: NSManagedObjectModel = {
        guard let url = Bundle.main.url(forResource: "Main", withExtension: "momd") else{
            fatalError("Failed to locate model file.")
        }
        
        guard let managedObjectModel = NSManagedObjectModel(contentsOf: url) else {
            fatalError("Failed to load model file.")
        }
        
        return managedObjectModel
    }()
}