I have a TableView. By tapping an entry in this table, another view opens in which that entry is editable.
What's the proper way to handle this? I am aware that I can fetch entries from Core Data via predicates like this:
func fetchEntriesStartingWith(startingDate:Date) -> [FuelEntry] {
let fetchRequest =
NSFetchRequest<NSManagedObject>(entityName: "FuelEntry")
fetchRequest.predicate = NSPredicate(format: "date >= %#", startingDate as CVarArg)
let sort = NSSortDescriptor(key: #keyPath(FuelEntry.date), ascending: false)
fetchRequest.sortDescriptors = [sort]
do {
return try (CoreDataHandler.managedContext.fetch(fetchRequest) as? [FuelEntry] ?? [FuelEntry]())
} catch let error as NSError {
print("Could not fetch. \(error), \(error.userInfo)")
return []
}
}
However, this seems error prone: What if I have two entries with the exact same values (might happen in my application) and thus, the NSFetchRequest returns two entries?
Obviously, those two entries will have different IDs, but I'm unsure how to work with those and infer which entry will correspond to what indexPath.
Related
I have an odd crash in my app that's difficult to reproduce. I did run into some threading issues before which seem to be mostly fixed now but every now and then I receive a crash log, even on my own devices.
My code fetches Librarys descending by the last time a book was added. For each of these libraries I want to see the most recently added books. The code works (except for the crash) but I can't think of a reason for the crash itself.
I apologize for the weird unwrapping at if let libraries1 = libraries0 { etc. but to me it seemed to have improved the situation which also may however only be imagination.
The last possible cause of issue that I may spot by myself is to declare libraries outside of the thread of the mainContext however I'm not sure if that can be the root of the problem since I would expect the crash to be way more present if it was a hard "don't".
Below you will see my code, the // CRASH: comment indicates the line that Xcode blames the crash on.
func readEntries( count: Int = 3, completion: #escaping ([Data], [String]) -> () ) {
var libraries = [Library]()
let fetchRequest = makeFetchRequest(LIBRARY)
let sort = NSSortDescriptor(key: "lastAddedTime", ascending: false)
fetchRequest.sortDescriptors = [sort]
fetchRequest.fetchLimit = count
mainContext.perform {
do {
let libraries0 = try self.mainContext.fetch(fetchRequest) as? [Library]
if let libraries1 = libraries0 {
libraries = libraries1
for lib in libraries {
if(lib.lastAddedTime > 0) {
let fetchRequest = self.makeFetchRequest(self.BOOK)
fetchRequest.fetchLimit = 5
let predicate = NSPredicate(format: "hostLibrary == %#", lib)
fetchRequest.predicate = predicate
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "time", ascending: false)]
if var books = try? self.mainContext.fetch(fetchRequest) as? [Book] { // CRASH: "Dispatch queue: SQLQueue 0x1020206e0 for DBBookModel.sqlite (0)"
print(books)
}
}
}
//completion ...
}
} catch let error as NSError {
print("Could not load. \(error), \(error.userInfo)")
}
}
}
I know how to fetch from core data with predicates using a
let predicate = NSPredicate(format: "MyEntityAttribute == %#", "Matching Value"). I want to know if it's possible to fetch all of the values for a particular attribute without using a Matching Value. I want to get a count of the total number of values for a particular attribute.
This is what I got so far, but I am only getting back what is matching the name attribute.
let filter = "wayne"
let fetchRequest = NSFetchRequest<Likes>(entityName: "Likes")
let predicate = NSPredicate(format: "name == %#", filter)
fetchRequest.predicate = predicate
do {
let nameCount = try context.fetch(fetchRequest)
if nameCount.count >= 0 {
print("name exist")
}
} catch{
print(error.localizedDescription)
}
I took a different approach and use NSFetchRequestResult to get back the result of the single attribute.
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Likes")
fetchRequest.resultType = .dictionaryResultType
fetchRequest.propertiesToFetch = ["name"] // Single attribute I wanted to fetch
fetchRequest.returnsDistinctResults = true
do {
let result = try context.fetch(fetchRequest)
let resultDic = result as! [[String:String]]
print(resultDic.count)
print(resultDic)
} catch{
print(error.localizedDescription)
}
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
}
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.
Ok, I was searching and trying in that case for the last 1-2 weeks and I didn't get it work. I would be able to achieve what I want without NSFRC but for performance reasons and convienience I would like to do it with the NSFRC.
So, I have a DataModel with 2 Entities - see the picture
There is one Account and one account can have many accountchanges - which is quite obvious.
So I want to be able to choose an Account and then show all AccountChanges for that specific Account.
So far I was able to get the Account and also accessing the NSSet in cellForRow Function but I am not getting the correct sections and numberOfRowsInSection - this is the main issue.
Here is some code:
func numberOfSections(in tableView: UITableView) -> Int {
print("Sections : \(self.fetchedResultsController.sections?.count)")
if (self.fetchedResultsController.sections?.count)! <= 0 {
print("There are no objects in the core data - do something else !!!")
}
return self.fetchedResultsController.sections?.count ?? 0
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
print("Section Name")
print(self.fetchedResultsController.sections![section].name)
let sectionInfo = self.fetchedResultsController.sections![section]
print("Section: \(sectionInfo) - Sections Objects: \(sectionInfo.numberOfObjects)")
return sectionInfo.numberOfObjects
}
There are some print statements which are only for information!
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let myCell = myTable.dequeueReusableCell(withIdentifier: "myCell")! as UITableViewCell
let accountBalanceChanges = self.fetchedResultsController.object(at: indexPath)
print("AccountBalanceChanges from cell....")
print(accountBalanceChanges)
let details = accountBalanceChanges.accountchanges! as NSSet
print("Print out the details:")
print(details)
let detailSet = details.allObjects
let detailSetItem = detailSet.count // Just for information!
let myPrint = detailSet[indexPath.row] as! AccountChanges
let myVal = myPrint.category
myCell.textLabel?.text = myVal
return myCell
}
So, I am able to get the data but always only one item and not the whole set - I guess due to the fact that the sections/ numberOfRows are wrong.
Here is my NSFRC
var fetchedResultsController: NSFetchedResultsController<Accounts> {
if _fetchedResultsController != nil {
return _fetchedResultsController!
}
let fetchRequest: NSFetchRequest<Accounts> = Accounts.fetchRequest()
// Set the batch size to a suitable number.
fetchRequest.fetchBatchSize = 20
// Edit the sort key as appropriate.
let sortDescriptor = NSSortDescriptor(key: "aName", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
let predicate = NSPredicate(format: "(ANY accountchanges.accounts = %#)", newAccount!)
fetchRequest.predicate = predicate
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.coreDataStack.context, sectionNameKeyPath: nil, cacheName: nil)
aFetchedResultsController.delegate = self
_fetchedResultsController = aFetchedResultsController
do {
try _fetchedResultsController!.performFetch()
} 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)")
}
return _fetchedResultsController!
}
I am assuming it is the SortDescriptor or the predicate - or maybe both?
Any help or at least directions are well appreciated.
I already tried many different approaches but none was giving me the correct results.
I would do the opposite, I mean using the FRC to fetch all the changes for an account with a certain Id, and use the following predicate:
let predicate = NSPredicate(format: "accounts.aId = %#", ACCOUNTID)
or
let predicate = NSPredicate(format: "accounts = %#", account.objectID)
I would rename Accounts entity to Account and same for the relationship since it's a to-one relationship.
That's assuming you have a table view with all the accounts and when you click on one it gives you back its changes.
var fetchedResultsController: NSFetchedResultsController<AccountChanges> {
if _fetchedResultsController != nil {
return _fetchedResultsController!
}
let fetchRequest: NSFetchRequest<AccountChanges> = AccountChanges.fetchRequest()
// Set the batch size to a suitable number.
fetchRequest.fetchBatchSize = 20
// Edit the sort key as appropriate.
let sortDescriptor = NSSortDescriptor(key: "aName", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
let predicate = NSPredicate(format: "accounts.aId = %#", ACCOUNTID)
fetchRequest.predicate = predicate
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.coreDataStack.context, sectionNameKeyPath: nil, cacheName: nil)
aFetchedResultsController.delegate = self
_fetchedResultsController = aFetchedResultsController
do {
try _fetchedResultsController!.performFetch()
} 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)")
}
return _fetchedResultsController!
}
Cheers
My managedObjectContext hierarchy is as follows: (PSC)<-(writerMOC -- private)<-(mainMOC -- main)<-(backgroundMOC -- private)
I have an NSManagedObject who "name" property is "Banana".
In the backgroundMOC, I get a reference to the object with backgroundMOC.objectWithID, change the NSManagedObject's "name" property to "Apple", and subsequently set it's "syncStatus" property to 1 (flagged for synchronization), then recursively save the moc's with the following routine:
func saveManagedContext(moc: NSManagedObjectContext, shouldSync: Bool = true, completion: (() -> Void)? = nil)
{
print("\nSaving managed object context...")
do {
try moc.save()
if let parentContext = moc.parentContext {
parentContext.performBlock {
self.saveManagedContext(parentContext, shouldSync: shouldSync, completion: completion)
}
}
else {
if shouldSync { SyncEngine.sharedInstance.synchronize(shouldPushUpdates: true) }
completion?()
}
print("Finished saving managed object context...")
} catch {
logger.error("\(error)")
}
}
Once the last moc is saved, a sync routine is called which does its work on the backgroundMOC, which queries the local store for all objects whose syncStatus is 1, again this fetch is called on the backgroundMOC.
let fetchRequest = NSFetchRequest(entityName: entity.name)
let syncPredicate = NSPredicate(format: "%K == %d", JSONKey.SyncStatus.rawValue, 1)
fetchRequest.predicate = syncPredicate
return try backgroundMOC.executeFetchRequest(fetchRequest) as? [SyncableManagedObject] ?? []
This correctly returns the updated object in the array, however, that object's syncStatus property equals 0, and its "name" property is still set to "Banana".
This is really causing me headaches, I felt like i had totally understood how managedObjectContext blocks should work, but this has proven to be quite a puzzle.
UPDATE
Here's the code that prompts the update. This is called from the main thread when the cell is tapped.
func updateNameForCell(cell: UITableViewCell)
{
///gets the object id from the fetchedResultsController
guard let fruitMetaID = tableController.objectIDForCell(cell) else { return }
let backgroundMOC = CoreDataController.sharedInstance.newBackgroundManagedObjectContext()
backgroundMOC.performBlock {
do {
guard let fruit = (backgroundMOC.objectWithID(fruitMetaID) as? FruitMetaData)?.fruit else {
throw //Error
}
print(fruit.name) // "Banana"
fruit.name = "Apple"
fruit.needsSynchronization() //Sets syncStatus to 1
CoreDataController.sharedInstance.saveManagedContext(backgroundMOC)
}
catch {
//handle error
}
}
}
UPDATE AGAIN
Maybe I'm not creating the contexts right. Enlighten me please!
/// The parent to all other NSManagedObjectContexts. Responsible for writting to the store.
lazy var writerManagedObjectContext: NSManagedObjectContext =
{
let managedObjectContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
managedObjectContext.performBlockAndWait {
managedObjectContext.persistentStoreCoordinator = self.persistentStoreCoordinator
}
return managedObjectContext
}()
lazy var mainManagedObjectContext: NSManagedObjectContext =
{
let managedObjectContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)
managedObjectContext.performBlockAndWait {
managedObjectContext.parentContext = self.writerManagedObjectContext
}
return managedObjectContext
}()
/// The context associated with background syncing..
func newBackgroundManagedObjectContext() -> NSManagedObjectContext
{
let backgroundManagedObjectContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
backgroundManagedObjectContext.performBlockAndWait {
backgroundManagedObjectContext.parentContext = self.mainManagedObjectContext
}
return backgroundManagedObjectContext
}
Holding onto child MOCs (children of the main context) is fraught with issues. I would recommend creating a new child (aka backgroundMOC) for each operation that you do.
Without seeing all of your code this looks like an issue with the child context getting out of sync.
Update
Assuming that your creation of the backgroundMOC sets the mainMOC as its parent then I wonder about the -objectWithID: and what it is returning.
I also wonder about your -performBlock: calls. In my head the threading looks fine but better to test. Try changing to -performBlockAndWait: just to test and see if there is a threading race condition. Not a permanent change but eliminates that part of the code as a source of the issue.
Before fetchRequest is called, you should reset context.
backgroundMOC.reset() // add this line
let fetchRequest = NSFetchRequest(entityName: entity.name)
let syncPredicate = NSPredicate(format: "%K == %d", JSONKey.SyncStatus.rawValue, 1)
fetchRequest.predicate = syncPredicate
return try backgroundMOC.executeFetchRequest(fetchRequest) as? [SyncableManagedObject] ?? []
The reason is FruitMetaData is an object(or class) so changing one of its properties/Core Data attributes does not register as a change to the results array ... the object references in the array remain the same.
And NSFetchRequest still returns the same result(by using cache). When use context.reset().This tells the context in the extension to fetch new data every time and ignore the cache.