Swift 5中Core Data向NSSet添加数据的问题（宝可梦场景）

Let's break down your two Core Data issues and fix them step by step:

1. Pokemon Instance is Nil in fetchMoreDetails #

Root Cause #

The main problem here is Core Data thread safety violations:

• You're passing a Pokemon instance from the main queue's view context to a background network thread in fetchMoreDetails. Core Data managed objects are bound to their context's queue—accessing them outside that queue causes them to become invalid (appear as nil or trigger faults).
• There's also a potential context sync timing issue: after saving the private context in downloadPokemonsFromServer, the view context's fetched results controller might not immediately update, leading you to access stale objects.

Fix #

Instead of passing the Pokemon instance directly, pass its thread-safe objectID. Then fetch the corresponding object in the private context of fetchMoreDetails, and always perform Core Data operations within the context's queue using perform:

Updated fetchMoreDetails Method

func fetchMoreDetails(pokemonObjectID: NSManagedObjectID, urlString: String, completion: @escaping ()->()) {
    guard let url = URL(string: urlString) else { 
        completion()
        return 
    }
    let privateContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
    privateContext.parent = CoreDataManager.shared.persistentContainer.viewContext
    
    // Perform all Core Data work on the private context's queue
    privateContext.perform {
        URLSession.shared.dataTask(with: url) { (data, response, error) in
            if let err = error {
                print("Unable to get more details for pokemon", err)
                completion()
                return
            }
            guard let data = data else { 
                completion()
                return 
            }
            let decoder = JSONDecoder()
            decoder.keyDecodingStrategy = .convertFromSnakeCase
            do {
                let pokemonDetailJSON = try decoder.decode(PokemonDetailJSON.self, from: data)
                // Fetch the Pokemon instance using the objectID in the private context
                guard let pokemon = privateContext.object(with: pokemonObjectID) as? Pokemon else {
                    print("Failed to fetch Pokemon from objectID")
                    completion()
                    return
                }
                
                pokemonDetailJSON.types.forEach { (nestedType) in
                    let type = Type(context: privateContext)
                    type.name = nestedType.type.name
                    // Add type to Pokemon's types relation (fix for issue 2)
                    let mutableTypes = pokemon.mutableSetValue(forKey: "types")
                    mutableTypes.add(type)
                }
                
                try privateContext.save()
                // Save parent context on its own queue
                privateContext.parent?.perform {
                    try? privateContext.parent?.save()
                    completion()
                }
            } catch let err {
                print("Unable to decode pokemon more details", err)
                completion()
            }
        }.resume()
    }
}

Updated handleRefresh in ViewController

@objc func handleRefresh() {
    print("DDDDD")
    Service.shared.downloadPokemonsFromServer {
        // Refresh fetched results to get latest data after save
        do {
            try self.pokemonController.performFetch()
        } catch let err {
            print("Failed to refetch pokemons: \(err)")
        }
        
        self.pokemonController.fetchedObjects?.forEach({ (pokemon) in
            print(pokemon.name)
            // Pass objectID instead of the managed object instance
            Service.shared.fetchMoreDetails(pokemonObjectID: pokemon.objectID, urlString: pokemon.url ?? "") {
                print(pokemon.abilities?.count)
            }
        })
        self.tableView.refreshControl?.endRefreshing()
    }
}

2. Can't Add Type to pokemon.types NSSet #

Root Cause #

NSSet is an immutable collection—calling pokemon.types?.adding(type) creates a new set but doesn't update the pokemon.types property. Additionally, you need to ensure the operation happens on the correct context's queue.

Fix #

Use Core Data's built-in mutableSetValue(forKey:) method to get a mutable collection for the relation, then add your Type instance directly. This handles the Core Data relation management properly:

// Inside fetchMoreDetails, after creating the Type instance:
let mutableTypes = pokemon.mutableSetValue(forKey: "types")
mutableTypes.add(type)

This modifies the underlying relation directly, and changes will be saved when you persist the private context.

Additional Best Practices #

• Always wrap Core Data operations in context.perform { ... } when working with background contexts to ensure thread safety.
• When saving a child context, save the parent context within its own queue using parent?.perform { ... } to avoid cross-thread issues.
• For fetched results controllers, call performFetch() after context saves to ensure the UI reflects the latest data.

内容的提问来源于stack exchange，提问作者Luis Ramirez