【发布时间】：2017-06-24 09:07:17
【问题描述】：
我在 XCode 8, swift 3 中制作的项目中第一次使用 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appDelegate类的applicationDidEnterBackground和applicationWillTerminate方法中保存上下文方法：
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
lazy var coreDataMgrInstance = CoreDataManager.sharedInstance
func applicationDidEnterBackground(_ application: UIApplication) {
coreDataMgrInstance.saveContext()
}
func applicationWillTerminate(_ application: UIApplication) {
coreDataMgrInstance.saveContext()
}}
这是我用来启动 NSpersistentContainer 的 Singleton 类 CoreDataManager
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 []
}
【问题讨论】：
-
return searchResults在viewDidLoad中没有效果，我怀疑代码甚至可以编译。不相关，但为什么要将提取返回的特定类型[School]转换为更不特定的类型[NSManagedObject]？ -
我刚刚编辑了代码。我得到的结果很好。但是当我重新运行应用程序时，我没有得到相同的结果。
-
很可能是这样。因为您依赖应用生命周期事件（例如进入后台）来保存更改，而不是在进行更改时保存更改。
-
我在 SenderViewController 类中进行更改时保存了背景上下文