Core Data对象数组获取及特定列表关联单词筛选求助

Hey Jeroen! Great work building an app to support kids with dyslexia—super impactful stuff. Let's fix those two Core Data issues you're running into:

问题1：将Core Data对象获取到数组中 #

You're actually already doing this correctly in your code, but let's clarify the two main reliable ways to get Core Data objects into an array:

1. Using NSFetchedResultsController (your current approach)
   The fetchedObjects property of NSFetchedResultsController returns an optional array of your fetched managed objects. You're properly unwrapping it here:

   if let fetchedObjects = fetchResultController.fetchedObjects {
       woordeninlijst = fetchedObjects
   }
   

   This is perfect for table views because the FRC automatically syncs your UI with Core Data changes—you're already using its delegate methods to handle inserts/deletes, which is exactly the right move.

2. Direct fetch with NSManagedObjectContext
   If you ever need to fetch objects without an FRC (for one-off operations), you can call the fetch(_:) method directly on the context:

   do {
       let woorden = try context.fetch(fetchRequest)
       woordeninlijst = woorden
   } catch {
       print("Fetch error: \(error)")
   }
   

   Stick with the FRC for your table view though—it takes care of all the heavy lifting for UI updates.

问题2：过滤获取特定列表的单词集合 #

The missing piece here is adding a predicate to your fetch request to filter WoordMO objects where their zitInLijst relationship matches the current lijst you're viewing. Here's how to implement this:

Step 1: Add the filter predicate to your fetch request #

In viewDidLoad(), right after setting the sort descriptor, add this line to target only words linked to the current list:

let fetchRequest: NSFetchRequest<WoordMO> = WoordMO.fetchRequest()
let sortDescriptor = NSSortDescriptor(key: "naam", ascending: true)
fetchRequest.sortDescriptors = [sortDescriptor]

// Add this predicate to filter words for the active list
let predicate = NSPredicate(format: "zitInLijst == %@", lijst)
fetchRequest.predicate = predicate

The predicate zitInLijst == %@ tells Core Data to only fetch WoordMO objects where their zitInLijst property is exactly the lijst object passed to this view controller.

Step 2: Verify your Core Data relationship setup #

Double-check your model to ensure the relationships are correctly configured:

• LijstMO should have a to-many relationship to WoordMO (e.g., named woorden)
• WoordMO should have a to-one relationship back to LijstMO (named zitInLijst, with the inverse set to woorden)
  You're already setting woord.zitInLijst = lijst when saving new words, so that part is spot-on!

Step 3: Optional: Handle list switching (if needed) #

If you ever need to switch the list being displayed, you can update the fetch request's predicate and re-fetch:

func updateFetchRequest(for newLijst: LijstMO) {
    guard let fetchRequest = fetchResultController.fetchRequest as? NSFetchRequest<WoordMO> else { return }
    fetchRequest.predicate = NSPredicate(format: "zitInLijst == %@", newLijst)
    do {
        try fetchResultController.performFetch()
        if let fetchedObjects = fetchResultController.fetchedObjects {
            woordeninlijst = fetchedObjects
        }
        tableView.reloadData()
    } catch {
        print("Re-fetch error: \(error)")
    }
}

Updated viewDidLoad() snippet #

Here's how your full updated viewDidLoad will look with the filter added:

override func viewDidLoad() {
    super.viewDidLoad()
    title = lijst.naam
    self.woordTextField.delegate = self

    let fetchRequest: NSFetchRequest<WoordMO> = WoordMO.fetchRequest()
    let sortDescriptor = NSSortDescriptor(key: "naam", ascending: true)
    fetchRequest.sortDescriptors = [sortDescriptor]
    
    // Filter words to only those in the current list
    let predicate = NSPredicate(format: "zitInLijst == %@", lijst)
    fetchRequest.predicate = predicate

    if let appDelegate = (UIApplication.shared.delegate as? AppDelegate) {
        let context = appDelegate.persistentContainer.viewContext
        fetchResultController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
        fetchResultController.delegate = self

        do {
            try fetchResultController.performFetch()
            if let fetchedObjects = fetchResultController.fetchedObjects {
                woordeninlijst = fetchedObjects
            }
        } catch {
            print(error)
        }
    }

    self.navigationItem.rightBarButtonItem = self.editButtonItem
}

That should resolve your issue—now only words linked to the active lijst will be fetched and displayed in your table view.

内容的提问来源于stack exchange，提问作者Jeroen De Bruin