当应用程序重新启动时，数据在 Core Data 中不持久

Question

我第一次在 XCode 8，swift 3 制作的项目中使用 Core Data。我使用了后台上下文（调用 container.performBackgroundTask 块..）来保存数据和主上下文来获取数据。当我的应用程序重新启动时，我保存在私有后台上下文中的数据将被删除。

请告诉我哪里出错了！！！

这里我调用了 CoreDataManager 类在 applicationDidEnterBackground 中的 save context 方法和 AppDelegate 类的 applicationWillTerminate 方法：

class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
lazy var coreDataMgrInstance = CoreDataManager.sharedInstance

func applicationDidEnterBackground(_ application: UIApplication) {

    coreDataMgrInstance.saveContext()
}
func applicationWillTerminate(_ application: UIApplication) {
    coreDataMgrInstance.saveContext()
}}

这是我的单例类 CoreDataManager 来启动 NSpersistentContainer

class CoreDataManager: NSObject {    
class var sharedInstance: CoreDataManager {
    struct Singleton {
        static let instance = CoreDataManager()
    }

    return Singleton.instance
}
   private override init() {

    super.init()
}

 lazy var persistentContainer: NSPersistentContainer = {

   let container = NSPersistentContainer(name: "E_CareV2")

    let description = NSPersistentStoreDescription() // enable auto lightweight migratn
    description.shouldInferMappingModelAutomatically = true
    description.shouldMigrateStoreAutomatically = true

    container.persistentStoreDescriptions = [description]

    container.loadPersistentStores(completionHandler: { (storeDescription, error) in
        if let error = error as NSError? {
            fatalError("Unresolved error \(error), \(error.userInfo)")
        }
    })
    container.viewContext.automaticallyMergesChangesFromParent = true
    return container
}()
func saveContext(){

    print("saveContext")
    let context = persistentContainer.viewContext
    if context.hasChanges {
        do {
            try context.save()
        } catch {
            let nserror = error as NSError
            fatalError("Failure to save main context \(nserror), \(nserror.userInfo)")
        }
}}

现在这是我从 Core Data 保存和获取数据的类

class SenderViewController: UIViewController {

var persistentContainer: NSPersistentContainer!

override func viewDidLoad() {
    super.viewDidLoad()

 persistentContainer = CoreDataManager.sharedInstance.persistentContainer
 let results = self.fetchPersistentData(entityName: "School", withPredicate: nil)
    print("results \n \(results)")       
  }

 @IBAction func enterPressed(_ sender: Any) {

  self.persistentContainer.performBackgroundTask({ (backgroundContext) in

        backgroundContext.mergePolicy = NSMergePolicy.mergeByPropertyStoreTrump

        let schoolentity = NSEntityDescription.insertNewObject(forEntityName: "School", into: backgroundContext) as! School

                schoolentity.schoolName = "ABC"
                schoolentity.schoolWebSite = "http://anywebsite.com/"

            do {
                try backgroundContext.save()
            } catch {
                fatalError("Failure to save background context: \(error)")
                }
    })
}

func fetchPersistentData(entityName: String?, withPredicate: NSPredicate?) -> [NSManagedObject]{

        let context = self.persistentContainer.viewContext
        let request: NSFetchRequest<School> = School.fetchRequest()
        let newentity = NSEntityDescription.entity(forEntityName: entityName!, in: context)

        request.entity = newentity
        request.returnsObjectsAsFaults = false

        do {
            let searchResults = try context.fetch(request) as [NSManagedObject]
            return searchResults               
        } catch {
            print("Error with request: \(error)")
        }
 return []
}

Answer 1

实际上，正如您在屏幕截图中看到的那样，默认情况下启用了轻量级迁移

所以你可以安全地删除这些行：

let description = NSPersistentStoreDescription() // enable auto lightweight migratn
description.shouldInferMappingModelAutomatically = true
description.shouldMigrateStoreAutomatically = true

container.persistentStoreDescriptions = [description]

之后，一切都应该工作。