虽然我一直在堆栈溢出处寻找答案，但感觉到我的情况对于该错误是唯一的。
我一直在学习如何在SwiftUI中使用CoreData来拥有持久数据。我首先制作了一个基本的电影列表，当您单击“添加电影”按钮时，它会添加带有任意文本的电影，目的是使其正常工作。
我的ContentView中有一个列表，该列表对ForEach实体进行了Movie，但是在添加以下代码行时：
List {
ForEach(movies, id: \.self) { (movie: Movie) in
Text(movie.title ?? "Unknown Movie")
}
}
我收到一个错误：
PotentialCrashError: MyMovieList.app may have crashed
MyMovieList.app may have crashed. Check ~/Library/Logs/DiagnosticReports for any crash logs
from your application.
==================================
| Error Domain=com.apple.dt.ultraviolet.service Code=12 "Rendering service was interrupted"
UserInfo={NSLocalizedDescription=Rendering service was interrupted}
但是，当我构建并运行该应用程序时，它可以完美运行。似乎只有在添加该特定代码行时预览才会中断。将其注释掉将使预览再次起作用。
我的Movie实体仅由属性title组成，该属性是一个可选的字符串。
ContentView的完整代码：
import SwiftUI
struct ContentView: View {
// 1
@FetchRequest(
// 2
entity: Movie.entity(),
// 3
sortDescriptors: [
NSSortDescriptor(keyPath: \Movie.title, ascending: true)
]
// 4
) var movies: FetchedResults<Movie>
@Environment(\.managedObjectContext) var managedObjectContext
var body: some View {
NavigationView {
VStack {
Button(action: {
self.addMovie(title: "Generic Movie")
}) {
Text("Add Movie")
}
List {
ForEach(movies, id: \.self) { (movie: Movie) in
Text(movie.title ?? "Unknown Movie")
}
}
}.navigationBarTitle("My Movies")
}
}
func deleteItem(at offsets: IndexSet) {
// 1
offsets.forEach { index in
// 2
let movie = self.movies[index]
// 3
self.managedObjectContext.delete(movie)
}
// 4
saveContext()
}
func saveContext() {
do {
try managedObjectContext.save()
} catch {
print("Error saving managed object context: \(error)")
}
}
func addMovie(title: String) {
// 1
let newMovie = Movie(context: managedObjectContext)
// 2
newMovie.title = title
// 3
saveContext()
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
我的AppDelegate和SceneDelegate是创建项目时生成的默认值，但是无论如何我都会在这里共享它们，因为我知道人们会问。
AppDelegate：
import UIKit
import CoreData
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
// Override point for customization after application launch.
return true
}
// MARK: UISceneSession Lifecycle
func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
// Called when a new scene session is being created.
// Use this method to select a configuration to create the new scene with.
return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
}
func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
// Called when the user discards a scene session.
// If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
// Use this method to release any resources that were specific to the discarded scenes, as they will not return.
}
// MARK: - Core Data stack
lazy var persistentContainer: NSPersistentContainer = {
/*
The persistent container for the application. This implementation
creates and returns a container, having loaded the store for the
application to it. This property is optional since there are legitimate
error conditions that could cause the creation of the store to fail.
*/
let container = NSPersistentContainer(name: "MyMovieList")
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)")
}
})
return container
}()
// MARK: - Core Data Saving support
func saveContext () {
let context = persistentContainer.viewContext
if context.hasChanges {
do {
try context.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)")
}
}
}
}
SceneDelegate：
import UIKit
import SwiftUI
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
var window: UIWindow?
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()
}
}
func sceneDidDisconnect(_ scene: UIScene) {
// Called as the scene is being released by the system.
// This occurs shortly after the scene enters the background, or when its session is discarded.
// Release any resources associated with this scene that can be re-created the next time the scene connects.
// The scene may re-connect later, as its session was not neccessarily discarded (see `application:didDiscardSceneSessions` instead).
}
func sceneDidBecomeActive(_ scene: UIScene) {
// Called when the scene has moved from an inactive state to an active state.
// Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.
}
func sceneWillResignActive(_ scene: UIScene) {
// Called when the scene will move from an active state to an inactive state.
// This may occur due to temporary interruptions (ex. an incoming phone call).
}
func sceneWillEnterForeground(_ scene: UIScene) {
// Called as the scene transitions from the background to the foreground.
// Use this method to undo the changes made on entering the background.
}
func sceneDidEnterBackground(_ scene: UIScene) {
// Called as the scene transitions from the foreground to the background.
// Use this method to save data, release shared resources, and store enough scene-specific state information
// to restore the scene back to its current state.
// Save changes in the application's managed object context when the application transitions to the background.
(UIApplication.shared.delegate as? AppDelegate)?.saveContext()
}
}
答案 0 :(得分：2)
您需要以与应用程序相同的方式为预览设置上下文，因此这是一个解决方案
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
let context = (UIApplication.shared.delegate as! AppDelegate)
.persistentContainer.viewContext
return ContentView()
.environment(\.managedObjectContext, context)
}
}