doubled savings with core data swift 4 - core-data

Question

developing a little app for my comic collection encountered this issue:
in my second "add comic" VC I have a button and the func below, but I save TWICE entities in manged context (ate least, I think this is the issue)
for example if I have 2 comics yet shown in main VC tableview, go to "add comic VC" and save a third one, going back to main VC I'll print 3 objects with title, number etc but also print 2 new objects with no data as I had saved twice a manger context a "right one" and another one with same number of object but empty. If I keep adding a 4th comic, I'll get 6 complete comic + the 4th and more 6 "blank itmes" with default values "no title"
let kComicEntityName = "Comic"
func addingSingleComic(gotTitle: String, gotIssue: Int16, gotInCollection: Bool ) {
guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {return}
let managedContext = appDelegate.persistentContainer.viewContext
let entity = NSEntityDescription.entity(forEntityName: kComicEntityName, in: managedContext)!
let comicToAdd = Comic(entity: entity, insertInto: managedContext)
comicToAdd.comicTitle = gotTitle
comicToAdd.issueNumber = gotIssue
comicToAdd.inCollection = gotInCollection
do {
try managedContext.save()
} catch let error as NSError {
print("could not save. \(error), \(error.userInfo)")
}
print("new single comic crated: title: \(comicToAdd.comicTitle ?? "!! not title !!"), n. \(comicToAdd.issueNumber), owned?: \(comicToAdd.inCollection)")
}
in the main VC I use this to check items in core data
func asyncPrintEntities() {
self.asyncComicEntityArray.removeAll(keepingCapacity: true)
guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {return}
let managedContext = appDelegate.persistentContainer.viewContext
let comicFetch : NSFetchRequest<Comic> = Comic.fetchRequest()
asyncFetchRequest = NSAsynchronousFetchRequest<Comic>(fetchRequest: comicFetch) {
[unowned self] (result: NSAsynchronousFetchResult) in
guard let AllComicEntityResult = result.finalResult else {
return
}
self.asyncComicEntityArray = AllComicEntityResult
//************************************
do {
self.asyncComicEntityArray = try managedContext.fetch(comicFetch)
if self.asyncComicEntityArray.count > 0 {
print("Ok! model is not empty!")
} else {
print("No entites availabe")
}
} catch let error as NSError {
print("Fetch error: \(error) description: \(error.userInfo)")
}
guard self.asyncComicEntityArray != nil else {return}
for comicFoundInArray in self.asyncComicEntityArray {
let entity = NSEntityDescription.entity(forEntityName: self.kComicEntityName, in: managedContext)!
var comicTouse = Comic(entity: entity, insertInto: managedContext)
// var comicTouse = Comic() //to be deleted since this kind of init is not allowed, better above insertInto
comicTouse = comicFoundInArray as! Comic
print("comic title: \(comicTouse.comicTitle ?? "error title"), is it in collection? : \(comicTouse.inCollection)")
}
self.MyTableView.reloadData()
//************************************
}
// MARK: - async fetch request 3
do {
try managedContext.execute(asyncFetchRequest)
} catch let error as NSError {
print("Could not fetch \(error), \(error.userInfo)")
}
//end of function
}

Answer 1

In your addingSingleComic you create a new Comic here:
let comicToAdd = Comic(entity: entity, insertInto: managedContext)
Then you assign values to the object's properties.
Separately, in asyncPrintEntities, you also create new Comic objects here:
var comicTouse = Comic(entity: entity, insertInto: managedContext)
This time you do not assign values to the object's properties. They will have no title, etc, because you created them but never assigned a title. This line executes once for every object in asyncComicEntityArray, so if the array has two objects, you create two new objects that contain no data. You don't use comicToUse anywhere except in the one print, but it still exists in the managed object context and will still get saved the next time you save changes.
This is why you're getting extra entries-- because you're creating them in this line of code. It's not clear why you're creating them here. You just executed a fetch request, and then you immediately create a bunch of no-data entries which you don't use. It looks like that entire for loop could just be deleted, because the only thing it does is create these extra entries.