Core Data is persistent after I remove it and restart my app

Question

Good morning,

I'm working for the first time using CoreData with Swift 2.2. At the moment, I can add objects to my entity "Item" and I can remove all the items after I press a button "Remove everything". That's correct, but when I delete all the objects and then I restart my app, I still have the same Core Data before the "Remove everything" action.

That's how I delete the Core Data objects:

@IBAction func removeWishlist(sender: AnyObject) {
    deleteAllData("ItemsWishlist")
    getWishlist()
    self.tableView.reloadData()
}

func deleteAllData(entity: String) {

    let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
    let managedContext = appDelegate.managedObjectContext
    let fetchRequest = NSFetchRequest(entityName: entity)
    fetchRequest.returnsObjectsAsFaults = false

    do
    {
        let results = try managedContext.executeFetchRequest(fetchRequest)
        for managedObject in results
        {
            let managedObjectData:NSManagedObject = managedObject as! NSManagedObject
            managedContext.deleteObject(managedObjectData)
            print("Deleted")
        }

    } catch let error as NSError {
        print(error)
    }
}

And it's working because when I press the button, the list is empty, but when I restart the app, it shows again the same items as before I pressed the "clear" button.

And that's my AppDelegate with the Core Data part:

// MARK: - Core Data stack

lazy var applicationDocumentsDirectory: NSURL = {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "-.test" in the application's documents Application Support directory.
    let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
    return urls[urls.count-1]
}()

lazy var managedObjectModel: NSManagedObjectModel = {
    // The managed object model for the application. This property is not optional. It is a fatal error for the application not to be able to find and load its model.
    let modelURL = NSBundle.mainBundle().URLForResource("wishlist", withExtension: "momd")!
    return NSManagedObjectModel(contentsOfURL: modelURL)!
}()

lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator = {
    // The persistent store coordinator for the application. This implementation creates and returns a coordinator, having added 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.
    // Create the coordinator and store
    let coordinator = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
    let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("SingleViewCoreData.sqlite")
    var failureReason = "There was an error creating or loading the application's saved data."
    do {
        try coordinator.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil)
    } catch {
        // Report any error we got.
        var dict = [String: AnyObject]()
        dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
        dict[NSLocalizedFailureReasonErrorKey] = failureReason

        dict[NSUnderlyingErrorKey] = error as NSError
        let wrappedError = NSError(domain: "YOUR_ERROR_DOMAIN", code: 9999, userInfo: dict)
        // Replace this with code to handle the error appropriately.
        // abort() 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.
        NSLog("Unresolved error \(wrappedError), \(wrappedError.userInfo)")
        abort()
    }

    return coordinator
}()

lazy var managedObjectContext: NSManagedObjectContext = {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.) This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.
    let coordinator = self.persistentStoreCoordinator
    var managedObjectContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)
    managedObjectContext.persistentStoreCoordinator = coordinator
    return managedObjectContext
}()

// MARK: - Core Data Saving support

func saveContext () {
    if managedObjectContext.hasChanges {
        do {
            try managedObjectContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // abort() 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
            NSLog("Unresolved error \(nserror), \(nserror.userInfo)")
            abort()
        }
    }
}

What am I doing wrong with the Core Data? How can I resolve this issue?

Much appreciated,

Regards.

Answer 1

The reason why you got those objects back after restarting your app is because you did not save your NSManagedObjectContext after performing all the deletion so that those changes can be persisted to the persistence store you are using.

The context is like a virtual board. Think of your managed objects on a context as toys that you put on a table to play with. You can move them around, break them, move them out of the table, and bring new toys in. That table is your managed object context, and you can save its state when you are ready. When you save the state of the managed object context, this save operation will be communicated to the persistent store coordinator to which the context is connected. Then the persistent store coordinator will store the information to the persistent store and subsequently to disk.

func deleteAllData(entity: String) {

let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
let managedContext = appDelegate.managedObjectContext
let fetchRequest = NSFetchRequest(entityName: entity)
fetchRequest.returnsObjectsAsFaults = false

do
  {
    let results = try managedContext.executeFetchRequest(fetchRequest)
    for managedObject in results
    {
        let managedObjectData:NSManagedObject = managedObject as! NSManagedObject
        managedContext.deleteObject(managedObjectData)
        print("Deleted")
    }

  } catch let error as NSError {
    print(error)
  }
  appDelegate.saveContext()
}

For performance benefits, you should look into NSBatchDeleteRequest.