I am developing a journaling app using Core Data and iCloud. A simplified view of the Core Data model looks as follows:
[Journal] <-->> [Post] <-->> [Content]
I use NSFetchedResultControllers to populate a UITableView with 'Content'-, and a UICollectionView with 'Journals' from the model.
The table view works perfectly and the fetched results controller's delegate is called whenever content changes, is added or deleted. In the collection view controller however, the fetched results controller is not calling the delegate when new journals are added or the posts or content within them changes (i realise the last part might be expected since the journal object itself doesn't change). However the delegate is called when journals are deleted.
I want the delegate to be called whenever journals are inserted, and have wrestled with this issue for hours without luck or further insight.
The fetched results controller is initialized and configured as follows:
// property declaration
lazy var fetchedResultsController: NSFetchedResultsController = self.initialFetchedResultsController()
// initialization
func initialFetchedResultsController() -> NSFetchedResultsController {
let request = NSFetchRequest(entityName: "Journal")
let nameSort = NSSortDescriptor(key: "name", ascending: true)
request.sortDescriptors = [nameSort]
return return NSFetchedResultsController(fetchRequest: request, managedObjectContext: DataModel.sharedModel().managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)
}
// configuration, called in viewDidLoad()
func configureFetchedResultsController() {
fetchedResultsController.delegate = collectionViewFetchedResultsControllerDelegate
do {
try fetchedResultsController.performFetch()
print("fetching")
} catch {
fatalError("Failed to initialize FetchedResultsController: \(error)")
}
}
The collectionViewFetchedResultsControllerDelegate passed to the fetchedResultsController during configuration is a standard NSFetchedResultsControllerDelegate managing the collection view of this view controller.
When data is deleted the following method is invoked in the shared data model, resulting in the fetchedResultsController calling its delegate (i simply assert this by adding some debugging print statements to all the delegate's methods).
func deleteJournal(journal: Journal) {
// delete all posts attached to journal
deletePostsForJournal(journal)
// delete journal
managedObjectContext.deleteObject(journal)
saveManagedObjectContext()
}
func deletePostsForJournal(journal: Journal) {
// delete all content attached to posts of journal
deleteContentForJournal(journal)
// delete all posts attached to journal
performBatchDeleteForJournal(journal, deleteEntityName: "Post", predicateFormat: "journal == %#")
}
func deleteContentForJournal(journal: Journal) {
// delete all content attached to posts of journal
performBatchDeleteForJournal(journal, deleteEntityName: "Content", predicateFormat: "post.journal == %#")
// delete all images attached to posts of journal
performBatchDeleteForJournal(journal, deleteEntityName: "Image", predicateFormat: "post.journal == %#")
}
private func performBatchDeleteForJournal(journal: Journal, deleteEntityName entityName: String, predicateFormat: String) {
let fetchRequest = NSFetchRequest(entityName: entityName)
fetchRequest.predicate = NSPredicate(format: predicateFormat, journal)
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try persistentStoreCoordinator.executeRequest(deleteRequest, withContext: managedObjectContext)
} catch let error as NSError {
print("Could not execute batch delete fetch request \(error), \(error.userInfo)")
}
saveManagedObjectContext()
}
However, when I update the model by inserting a new journal using the insert method below, the fetchedResultsController is not being called.
func createJournal(name: String) {
// creates or returns (if already existing) a journal from the managed object context
journalForName(name)
saveManagedObjectContext()
}
func journalForName(name: String) -> Journal {
// configures fetch request
let fetchRequest = NSFetchRequest()
let entityDescription = NSEntityDescription.entityForName("Journal", inManagedObjectContext: managedObjectContext)
fetchRequest.entity = entityDescription
// configures predicate to match posts with date
fetchRequest.predicate = NSPredicate(format: "name == %#", name)
// executes fetch request
do {
let result = try self.managedObjectContext.executeFetchRequest(fetchRequest)
if let journal = result.first as? Journal {
return journal
}
} catch let error as NSError {
print("Could not execute fetch request from managedObjectContext \(error), \(error.userInfo)")
}
// creates a new journal since one doesn't exist with this name
let journal = Journal(entity: entityDescription!, insertIntoManagedObjectContext: managedObjectContext)
journal.name = name
return journal
}
The journalForName method is implemented to only create a new Journal if one with the same name doesn't exist already.
The insertion works and is updated in the model since the created Journal appears if you restart the application. I'd appreciate any help in solving this issue! Thanks.
Related
I have a simple entity with identifier (constraint) and name fields. In ViewController I try to add data to the database without worrying about duplicates and there really is no duplicate data in the database, but when I try to get records, I get twice as many of them. As I found out, this only happens when I try to write something to the database, and regardless of whether the attempt was successful, the data goes to my NSFetchRequestResult. What is the reason for this behavior?
DB content now:
ViewController:
class ViewController: UIViewController {
let moc = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
override func viewDidLoad() {
super.viewDidLoad()
//if comment this loop I won't get duplicates
for i in 0...5 {
let ent = Entity(context: moc)
ent.identifier = Int16(i)
ent.name = "Username"
try? moc.save() //if the code is not executed for the first time, then the attempt is unsuccessful due to identifier constraint
}
let fetch = NSFetchRequest<NSFetchRequestResult>(entityName: "Entity")
let fetchedEntities = try? moc.fetch(fetch) as? [Entity]
print(fetchedEntities!.count) // Output: 12 (actually only 6 records in the db)
}
}
Change your code where you create the objects to
for i in 0...5 {
let ent = Entity(context: moc)
ent.identifier = Int16(i)
ent.name = "Username"
}
do {
try moc.save()
} catch {
moc.reset()
}
This way you will remove the faulty (duplicate) objects from the context
I am having difficulties updating the collection view, after saving new entity object to persisentContainer. I have to reload the application to see the new data. Ive read similar questions, but they don't appear to be having the same issue that i'm having, since they are able to call reloadData with out any problem.
Code Overflow
popalert requesting user name
save info into core data entity
insert new section in collection view
Basically simplified scheme of what i'm trying to achieve. However, after i save the record, and i attempt to insert a new section with performBatchupdate i get same number of records. I've looked at the values through the cycle and im stuck between two probable causes. The first, my fetch request is not getting new data. Or, the context is not being saved until the application restarts. On restart the new data is loaded, however this is not the case when i push it to background and reopen it.
Below is general code. Looking for any suggestions, or to let me know what i'm missing.
class CoreDataUpdateCollectionView {
var name: String!
var persistentContainer: NSPersistentContainer!
var users: Users! // Entity fetch request
// users = fetchRequestController
have also used fetchRequestController directly
func save(entity: Entity) {
entity.name = name
save(context: persistentContainer)
insertSection()
}
func save(context: NSPersistentContainer) {
// have also attempted performAndWait
// assuming that it would run code after block was completed
context.viewContext.perform {
try context.viewContext.save()
} catch {
print(error)
}
}
// I've attempted to wrap this around a DispatchQueue.main.async{}
// still not reloading since the issue appears to be coming from the persistent store
func insertSection() {
collectionView.performBatchUpdates {
// replaced with, collectionView.reloadData() nothing appears to happen in view
collectionView.insertSection()
} , { finished in
// toggled on and off
collectionView.reloadData()
}
}
// DataSource Delegate
func collectionView(_ collectionView: UICollectionView, numOfItemsIn section: Int) -> {
// users retur the same number
return users.count
}
// fetch request controller
lazy var fetchUserController: NSFetchRequestResult<User> = {
let fetchRequest: NSFetchRequest<User> = User.fetchRequest()
fetchRequest.returnsObjectsAsFaults = false
fetchRequest.sortDescriptors = [
NSSortDescriptor(key: "name", ascending: true)
]
fetchRequest.relationshipKeyPathsForPrefetching = [
"friends"
]
let controller = NSFetchedResultsController(
fetchRequest: fetchRequest,
managedObjectContext: persistentContainer.viewContext, sectionNameKeyPath: "name",
cacheName: nil
)
controller.delegate = self
do {
try controller.performFetch()
} catch {
fatalError("###\(#function): Failed to performFetch: \(error)")
}
return controller
}()
}
I'm trying to get an example project using CoreData and QueryGenerationTokens working. The essence of the project is to be committing changes to a background context on a timer (emulating changes coming down from a server) that shouldn't be displayed until an action is taken on the UI (say, a button press).
Currently, I have changes being saved on the background context (an entity is being added every 5s and saved) and they are automatically coming into the view context (as expected, .automaticallyMergesChangesFromParent is set to true). Where things go wrong, I am pinning the view context before any of these changes happen to the current query generation token. I would expect the view to not update with the background items being added, but it is updating with them. So it seems the query generation tokens are having no effect?
Some of the possible issues I've thought of:
the only example I've found from Apple doesn't show them using it with a fetched results controller (I'm using #FetchRequest in SwiftUI, which I'm almost entirely certain is essentially the same), so that may have an effect?
.automaticallyMergeChangesFromParent shouldn't be used and I should try a merge policy, but that doesn't seem to work either and conceptually, it seems the query generation tokens should work with this and pin to the generation no matter the merging.
Code for view - handles loading data from view context
// Environment object before fetch request necessary
// Passed in wherever main view is instantiated through .environment()
#Environment(\.managedObjectContext) var managedObjectContext
// Acts as fetched results controller, loading data automatically into items upon the managedObjectContext updating
// ExampleCoreDataEntity.retrieveItemsFetchRequest() is an extension method on the entity to easily get a fetch request for the type with sorting
#FetchRequest(fetchRequest: ExampleCoreDataEntity.retrieveItemsFetchRequest()) var items: FetchedResults<ExampleCoreDataEntity>
var body: some View {
NavigationView {
// Button to refresh and bring in changes
Button(
action: {
do {
try self.managedObjectContext.setQueryGenerationFrom(.current)
self.managedObjectContext.refreshAllObjects()
} catch {
print(error.localizedDescription)
}
},
label: { Image(systemName: "arrow.clockwise") }
)
// Creates a table of items sorted by the entity itself (entities conform to Hashable)
List(self.items, id: \.self) { item in
Text(item.name ?? "")
}
}
}
Code in SceneDelegate (where a SwiftUI application starts up) where I also initialize what is needed for CoreData:
// Setup and pass in environment of managed object context to main view
// via extension on persistent container that sets up CoreData stack
let managedObjectContext = NSPersistentContainer.shared.viewContext
do {
try managedObjectContext.setQueryGenerationFrom(.current)
} catch {
print(error.localizedDescription)
}
let view = MainView().environment(\.managedObjectContext, managedObjectContext)
// Setup background adding
timer = Timer.scheduledTimer(timeInterval: 5, target: self, selector: #selector(backgroundCode), userInfo: nil, repeats: true)
// Setup window and pass in main view
let window = UIWindow(windowScene: windowScene)
window.rootViewController = UIHostingController(rootView: view)
Function adding data in the background:
#objc func backgroundCode() {
ExampleCoreDataEntity.create(names: ["background object"], in: backgroundContext, shouldSave: true)
}
Setup of NSPersistentContainer:
extension NSPersistentContainer {
private struct SharedContainerStorage {
static let container: NSPersistentContainer = {
let container = NSPersistentContainer(name: "Core_Data_Exploration")
container.loadPersistentStores { (description, error) in
guard error == nil else {
assertionFailure("CoreData: Unresolved error \(error!.localizedDescription)")
return
}
container.viewContext.automaticallyMergesChangesFromParent = true
}
return container
}()
}
static var shared: NSPersistentContainer {
return SharedContainerStorage.container
}
}
Create/Read/Update/Delete functions on the entity:
extension ExampleCoreDataEntity {
static func retrieveItemsFetchRequest() -> NSFetchRequest<ExampleCoreDataEntity> {
let request: NSFetchRequest<ExampleCoreDataEntity> = ExampleCoreDataEntity.fetchRequest()
request.sortDescriptors = [NSSortDescriptor(keyPath: \ExampleCoreDataEntity.creationDate, ascending: false)]
return request
}
static func create(names: [String], in context: NSManagedObjectContext, shouldSave save: Bool = false) {
context.perform {
names.forEach { name in
let item = ExampleCoreDataEntity(context: context)
item.name = name
item.creationDate = Date()
item.identifier = UUID()
}
do {
if save {
try context.save()
}
} catch {
// print error
}
}
}
func delete(in context: NSManagedObjectContext, shouldSave save: Bool = false) {
context.perform {
let name = self.name ?? "an item"
context.delete(context.object(with: self.objectID))
do {
if save {
try context.save()
}
} catch {
// print error
}
}
}
}
The issue was container.viewContext.automaticallyMergesChangesFromParent = true
That property cannot be set to true while working with query generation tokens. I came back to this issue and found this in the header of NSManagedObjectContext documented above automaticallyMergesChangesFromParent:
Setting this property to YES when the context is pinned to a non-current query generation is not supported.
The general flow of getting it to work is the following:
setting the query generation token to .current
calling .refreshAllObjects() on the view context
calling .performFetch() on the fetched results controller
This last part goes against the code I put in the original question which used #FetchRequest - currently, I can't figure out a way that doesn't seem extremely hacky to make it manually refetch. To get around this, I made an intermediate store class containing a FetchedResultsController that adopts its delegate protocol. That store also adopts ObservableObject which allows a SwiftUI view to listen to its changes when calling objectWillChange.send() within the ObservableObject adopting store.
In the documentation you linked to in the question you will see it says:
"Calling save(), reset(), mergeChangesFromContextDidSaveNotification:, or mergeChangesFromRemoteContextSave(:intoContexts:) on any pinned context will automatically advance it to the most recent version for the operation and then reset its query generation to currentQueryGenerationToken."
The reason you are seeing the changes from the background save is automaticallyMergesChangesFromParent is just convenience for mergeChangesFromContextDidSaveNotification so your generation is advancing.
FYI here is another sample project uses query generations - Synchronizing a Local Store to the Cloud
And here is the relevant code:
/*
See LICENSE folder for this sample’s licensing information.
Abstract:
A class to set up the Core Data stack, observe Core Data notifications, process persistent history, and deduplicate tags.
*/
import Foundation
import CoreData
// MARK: - Core Data Stack
/**
Core Data stack setup including history processing.
*/
class CoreDataStack {
/**
A persistent container that can load cloud-backed and non-cloud stores.
*/
lazy var persistentContainer: NSPersistentContainer = {
// Create a container that can load CloudKit-backed stores
let container = NSPersistentCloudKitContainer(name: "CoreDataCloudKitDemo")
// Enable history tracking and remote notifications
guard let description = container.persistentStoreDescriptions.first else {
fatalError("###\(#function): Failed to retrieve a persistent store description.")
}
description.setOption(true as NSNumber, forKey: NSPersistentHistoryTrackingKey)
description.setOption(true as NSNumber, forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)
container.loadPersistentStores(completionHandler: { (_, error) in
guard let error = error as NSError? else { return }
fatalError("###\(#function): Failed to load persistent stores:\(error)")
})
container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
container.viewContext.transactionAuthor = appTransactionAuthorName
// Pin the viewContext to the current generation token and set it to keep itself up to date with local changes.
container.viewContext.automaticallyMergesChangesFromParent = true
do {
try container.viewContext.setQueryGenerationFrom(.current)
} catch {
fatalError("###\(#function): Failed to pin viewContext to the current generation:\(error)")
}
// Observe Core Data remote change notifications.
NotificationCenter.default.addObserver(
self, selector: #selector(type(of: self).storeRemoteChange(_:)),
name: .NSPersistentStoreRemoteChange, object: container)
return container
}()
/**
Track the last history token processed for a store, and write its value to file.
The historyQueue reads the token when executing operations, and updates it after processing is complete.
*/
private var lastHistoryToken: NSPersistentHistoryToken? = nil {
didSet {
guard let token = lastHistoryToken,
let data = try? NSKeyedArchiver.archivedData( withRootObject: token, requiringSecureCoding: true) else { return }
do {
try data.write(to: tokenFile)
} catch {
print("###\(#function): Failed to write token data. Error = \(error)")
}
}
}
/**
The file URL for persisting the persistent history token.
*/
private lazy var tokenFile: URL = {
let url = NSPersistentContainer.defaultDirectoryURL().appendingPathComponent("CoreDataCloudKitDemo", isDirectory: true)
if !FileManager.default.fileExists(atPath: url.path) {
do {
try FileManager.default.createDirectory(at: url, withIntermediateDirectories: true, attributes: nil)
} catch {
print("###\(#function): Failed to create persistent container URL. Error = \(error)")
}
}
return url.appendingPathComponent("token.data", isDirectory: false)
}()
/**
An operation queue for handling history processing tasks: watching changes, deduplicating tags, and triggering UI updates if needed.
*/
private lazy var historyQueue: OperationQueue = {
let queue = OperationQueue()
queue.maxConcurrentOperationCount = 1
return queue
}()
/**
The URL of the thumbnail folder.
*/
static var attachmentFolder: URL = {
var url = NSPersistentContainer.defaultDirectoryURL().appendingPathComponent("CoreDataCloudKitDemo", isDirectory: true)
url = url.appendingPathComponent("attachments", isDirectory: true)
// Create it if it doesn’t exist.
if !FileManager.default.fileExists(atPath: url.path) {
do {
try FileManager.default.createDirectory(at: url, withIntermediateDirectories: true, attributes: nil)
} catch {
print("###\(#function): Failed to create thumbnail folder URL: \(error)")
}
}
return url
}()
init() {
// Load the last token from the token file.
if let tokenData = try? Data(contentsOf: tokenFile) {
do {
lastHistoryToken = try NSKeyedUnarchiver.unarchivedObject(ofClass: NSPersistentHistoryToken.self, from: tokenData)
} catch {
print("###\(#function): Failed to unarchive NSPersistentHistoryToken. Error = \(error)")
}
}
}
}
// MARK: - Notifications
extension CoreDataStack {
/**
Handle remote store change notifications (.NSPersistentStoreRemoteChange).
*/
#objc
func storeRemoteChange(_ notification: Notification) {
print("###\(#function): Merging changes from the other persistent store coordinator.")
// Process persistent history to merge changes from other coordinators.
historyQueue.addOperation {
self.processPersistentHistory()
}
}
}
/**
Custom notifications in this sample.
*/
extension Notification.Name {
static let didFindRelevantTransactions = Notification.Name("didFindRelevantTransactions")
}
// MARK: - Persistent history processing
extension CoreDataStack {
/**
Process persistent history, posting any relevant transactions to the current view.
*/
func processPersistentHistory() {
let taskContext = persistentContainer.newBackgroundContext()
taskContext.performAndWait {
// Fetch history received from outside the app since the last token
let historyFetchRequest = NSPersistentHistoryTransaction.fetchRequest!
historyFetchRequest.predicate = NSPredicate(format: "author != %#", appTransactionAuthorName)
let request = NSPersistentHistoryChangeRequest.fetchHistory(after: lastHistoryToken)
request.fetchRequest = historyFetchRequest
let result = (try? taskContext.execute(request)) as? NSPersistentHistoryResult
guard let transactions = result?.result as? [NSPersistentHistoryTransaction],
!transactions.isEmpty
else { return }
// Post transactions relevant to the current view.
DispatchQueue.main.async {
NotificationCenter.default.post(name: .didFindRelevantTransactions, object: self, userInfo: ["transactions": transactions])
}
// Deduplicate the new tags.
var newTagObjectIDs = [NSManagedObjectID]()
let tagEntityName = Tag.entity().name
for transaction in transactions where transaction.changes != nil {
for change in transaction.changes!
where change.changedObjectID.entity.name == tagEntityName && change.changeType == .insert {
newTagObjectIDs.append(change.changedObjectID)
}
}
if !newTagObjectIDs.isEmpty {
deduplicateAndWait(tagObjectIDs: newTagObjectIDs)
}
// Update the history token using the last transaction.
lastHistoryToken = transactions.last!.token
}
}
}
// MARK: - Deduplicate tags
extension CoreDataStack {
/**
Deduplicate tags with the same name by processing the persistent history, one tag at a time, on the historyQueue.
All peers should eventually reach the same result with no coordination or communication.
*/
private func deduplicateAndWait(tagObjectIDs: [NSManagedObjectID]) {
// Make any store changes on a background context
let taskContext = persistentContainer.backgroundContext()
// Use performAndWait because each step relies on the sequence. Since historyQueue runs in the background, waiting won’t block the main queue.
taskContext.performAndWait {
tagObjectIDs.forEach { tagObjectID in
self.deduplicate(tagObjectID: tagObjectID, performingContext: taskContext)
}
// Save the background context to trigger a notification and merge the result into the viewContext.
taskContext.save(with: .deduplicate)
}
}
/**
Deduplicate a single tag.
*/
private func deduplicate(tagObjectID: NSManagedObjectID, performingContext: NSManagedObjectContext) {
guard let tag = performingContext.object(with: tagObjectID) as? Tag,
let tagName = tag.name else {
fatalError("###\(#function): Failed to retrieve a valid tag with ID: \(tagObjectID)")
}
// Fetch all tags with the same name, sorted by uuid
let fetchRequest: NSFetchRequest<Tag> = Tag.fetchRequest()
fetchRequest.sortDescriptors = [NSSortDescriptor(key: Schema.Tag.uuid.rawValue, ascending: true)]
fetchRequest.predicate = NSPredicate(format: "\(Schema.Tag.name.rawValue) == %#", tagName)
// Return if there are no duplicates.
guard var duplicatedTags = try? performingContext.fetch(fetchRequest), duplicatedTags.count > 1 else {
return
}
print("###\(#function): Deduplicating tag with name: \(tagName), count: \(duplicatedTags.count)")
// Pick the first tag as the winner.
let winner = duplicatedTags.first!
duplicatedTags.removeFirst()
remove(duplicatedTags: duplicatedTags, winner: winner, performingContext: performingContext)
}
/**
Remove duplicate tags from their respective posts, replacing them with the winner.
*/
private func remove(duplicatedTags: [Tag], winner: Tag, performingContext: NSManagedObjectContext) {
duplicatedTags.forEach { tag in
defer { performingContext.delete(tag) }
guard let posts = tag.posts else { return }
for case let post as Post in posts {
if let mutableTags: NSMutableSet = post.tags?.mutableCopy() as? NSMutableSet {
if mutableTags.contains(tag) {
mutableTags.remove(tag)
mutableTags.add(winner)
}
}
}
}
}
}
I am using Swift 4 to build a single view iOS 11 application that has a UITableViewController that is also defined as a delegate for a NSFetchedResultsController.
class MyTVC: UITableViewController, NSFetchedResultsControllerDeleagate {
var container:NSPersistentContainer? =
(UIApplication.shared.delegate as? AppDelegate)?.persistentContainer
var frc : NSFetchedResultsController<Student>?
override func viewDidLoad() {
container?.performBackgroundTask { context in
// adds 100 dummy records in background
for i in 1...100 {
let student = Student(context: context)
student.name = "student \(i)"
}
try? context.save() // this works because count is printed below
if let count = try? context.count(for: Student.fetchRequest()) {
print("Number of students in core data: \(count)") // prints 100
}
} // end of background inserting.
// now defining frc:
if let context = container?.viewContext {
let request:NSFetchRequest<Student> = Student.fetchRequest()
request.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
frc = NSFetchedResultsController<Student> (
fetchRequest: request,
managedObjectContext: context,
sectionNameKeyPath: nil,
cacheName: nil )
try? frc?.performFetch() // this works and I get no errors
tableView.reloadData()
frc.delegate = self
} // end of frc definition
}
}
If I add one row of Student using the viewContext, the frc will fire the required methods to show it in the tableView. However, the 100 dummy rows are not shown. In fact, If I try to tell the tableview to reload after the insertion is done, my app starts to behave weirdly and becomes buggy, and does not do what it should do (i.e: does not delete rows, does not edit, etc).
But If I restart my app, without calling the dummy insertion, I can see the 100 rows inserted from the previous run.
The only problem is that I can't call tableView.reloadData() from the background thread, so I tried to do this:
// after printing the count, I did this:
DispatchQueue.main.async { [weak self] in
self?.tableView.reloadData() // causes UI to behave weirdly
}
then I tried to call viewContext.perform to reload the table view in the proper thread
func viewDidLoad() {
// code for inserting 100 dummy rows in background thread
// code for defining frc and setting self as delegate
if let context = container?.viewContext {
context.perform { [weak self] in
self?.tableView.reloadData() // that also causes UI to behave weirdly
}
}
}
How can tell my tableview to reload and display the 100 dummy rows in a thread-safe manner?
override func viewDidLoad() {
super.viewDidLoad()
//Always need your delegate for the UI to be set before calling the UI's delegate functions.
frc.delegate = self
//First we can grab any already stored values.
goFetch()
//This chunk just saves. I would consider putting it into a separate function such as "goSave()" and then call that from an event handler.
container?.performBackgroundTask { context in
//We are in a different queue than the main queue, hence "backgroundTask".
for i in 1...100 {
let student = Student(context: context)
student.name = "student \(i)"
}
try? context.save() // this works because count is printed below
if let count = try? context.count(for: Student.fetchRequest()) {
print("Number of students in core data: \(count)") // prints 100
}
//Now that we are done saving its ok to fetch again.
goFetch()
}
//goFetch(); Your other code was running here would start executing before the backgroundTask is done. bad idea.
//The reason it works if you restart the app because that data you didn't let finish saving is persisted
//So the second time Even though its saving another 100 in another queue there were still at least 100 records to fetch at time of fetch.
}
func goFetch() {
if let context = container?.viewContext {
let request:NSFetchRequest<Student> = Student.fetchRequest()
request.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
frc = NSFetchedResultsController<Student> (
fetchRequest: request,
managedObjectContext: context,
sectionNameKeyPath: nil,
cacheName: nil )
try? frc?.performFetch()
//Now that records are both stored and fetched its safe for our delegate to access the data on the main thread.
//To me it would make sense to do a tableView reload everytime data is fetched so I placed this inside o `goFetch()`
DispatchQueue.main.async { [weak self] in
self?.tableView.reloadData()
}
}
}
After a lot of reading about the NSFetchedResultsController and the NSPersistentContainer and finally finding an important piece of information here at SO I think I have a working example.
My code is slightly different since I used a project I had for this. Anyway here is what I did:
In my view controller I had a property for my container
private var persistentContainer = NSPersistentContainer(name: coreDataModelName)
And in viewDidLoad I loaded the persistent store and created my 100 records.
persistentContainer.loadPersistentStores { persistentStoreDescription, error in
if let error = error {
print("Unable to add Persistent Store [\(error)][\(error.localizedDescription)]")
} else {
self.createFakeNotes() // Here 100 elements get created
DispatchQueue.main.async {
self.setupView() // other stuff, not relevant
self.fetchNotes() // fetch using fetch result controller
self.tableView.reloadData()
}
}
}
Below is createFakeNotes() where I use a separate context for inserting the elements in a background thread, this code is pretty much taken from Apple's Core Data programming guide but to make the UI being updated I needed to set automaticallyMergesChangesFromParent to true which I found out in this SO answer
I also delete old notes first to make the testing easier.
private func createFakeNotes() {
let deleteRequest = NSBatchDeleteRequest(fetchRequest: Note.fetchRequest())
do {
try persistentContainer.persistentStoreCoordinator.execute(deleteRequest, with: persistentContainer.viewContext)
} catch {
print("Delete error [\(error)]")
return
}
let privateContext = persistentContainer.newBackgroundContext()
privateContext.automaticallyMergesChangesFromParent = true //Important!!!
privateContext.perform {
let createDate = Date()
for i in 1...100 {
let note = Note(context: privateContext)
note.title = String(format: "Title %2d", i)
note.contents = "Content"
note.createdAt = createDate
note.updatedAt = createDate
}
do {
try privateContext.save()
do {
try self.persistentContainer.viewContext.save()
} catch {
print("Fail saving main context [\(error.localizedDescription)")
}
} catch {
print("Fail saving private context [\(error.localizedDescription)")
}
}
}
You should fetch your data by calling it from viewwillappear and then try to reload your tableview.
override func viewWillAppear(_ animated: Bool) {
getdata()
tableView.reloadData()
}
func getdata() {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
do{
persons = try context.fetch(Person.fetchRequest())
}
catch {
print("fetching failed")
}
}
I'm trying to insert a default record into my core data entity while the tableview first-time loaded and checked there's no data in the entity.
The data inserted just fine , but the reloadData() didn't work, after navigate to other view and navigate back to the view the data appears. no matter the reloadData() in or out of the .save() method.
override func viewDidLoad() {
let cateContext = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
let categoryRequest = NSFetchRequest(entityName: "Category")
categoryArray = (try! cateContext.executeFetchRequest(categoryRequest)) as! [Category]
if categoryArray.count == 0 {
let category = NSEntityDescription.insertNewObjectForEntityForName("Category", inManagedObjectContext: cateContext) as! Category
category.sno = "1"
category.category = "General"
category.locate = false
do {
try cateContext.save()
self.categoryTableView.reloadData()
} catch let saveError as NSError {
print("Saving Error : \(saveError.localizedDescription)")
}
}
//self.categoryTableView.reloadData()
}
If your are calling self.categoryTableView.reloadData() in viewDidLoad() method it will not reload your tableView twice. You need to call self.categoryTableView.reloadData() after you have checked if entity existed again.