如何使用Swift使用Core Data更新/保存和保留非标准（可转换）属性？

Question

I've constructed a very basic example to demonstrate the issue I'm having attempting to update a transformable type and get the change to persist between app restarts.

I have an entity of type Destination ...

import Foundation
import CoreData

class Destination: NSManagedObject {
    @NSManaged var name: String
    @NSManaged var location: Location
}

... that has a simple name attribute (of type String) and an attribute of type Location :

import Foundation

class Location: NSObject, NSCoding {
    var address: String
    var latitude: Double
    var longitude: Double

    required init?(coder aDecoder: NSCoder) {
        address = aDecoder.decodeObjectForKey("Address") as? String ?? ""
        latitude = aDecoder.decodeObjectForKey("Latitude") as? Double ?? 0.0
        longitude = aDecoder.decodeObjectForKey("Longitude") as? Double ?? 0.0
        super.init()
    }

    init(address: String, latitude: Double, longitude: Double) {
        self.address = address
        self.latitude = latitude
        self.longitude = longitude

        super.init()
    }

    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeObject(address, forKey: "Address")
        aCoder.encodeObject(latitude, forKey: "Latitude")
        aCoder.encodeObject(longitude, forKey: "Longitude")
    }
}

Location is configured as "transformable" in Core Data, because it has structure that none of the other basic types can handle.

Using Apple's boilerplate Core Data code, here's a view controller that simply does the following:

• Gets the necessary appDelegate / ManagedApplicationContext references
• Fetches a destination if one exists, creates one if not
• Prints the name and location.address of the destination
• Updates the name and location.address of the destination
• Saves changes to the ManagedObjectContext

When the app is run and re-run, only changes to the name will persist. Changes made to the location.address do not persist.

import UIKit
import CoreData

class ViewController: UIViewController {

    var appDelegate: AppDelegate!
    var context: NSManagedObjectContext!

    override func viewDidLoad() {
        super.viewDidLoad()
        updateDestination()
    }

    func updateDestination() {
        var destination: Destination
        appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        context = appDelegate.managedObjectContext

        if let dest = fetchOneDestination() {
            destination = dest
        }
        else {
            destination = create()!
        }
        print("destination named: \(destination.name), at: \(destination.location.address)")

        destination.name = "New name of place that will update and persist"
        destination.location.address = "123 main st (change that will never persist)"
        appDelegate.saveContext()
    }

    func create() -> Destination? {
        guard let newDestination = NSEntityDescription.insertNewObjectForEntityForName("Destination", inManagedObjectContext: context) as? Destination else {
            return nil
        }
        newDestination.name = "Original name of place that can be updated"
        newDestination.location = Location(address: "100 main st", latitude: 34.051145, longitude: -118.243595)
        return newDestination
    }

    func fetchOneDestination() -> Destination? {
        let request = NSFetchRequest()
        request.entity = NSEntityDescription.entityForName("Destination", inManagedObjectContext: context)
        do {
            let fetchResults = try context.executeFetchRequest(request)
            if fetchResults.count > 0 {
                if let dest = fetchResults[0] as? Destination {
                    return dest
                }
            }
        }
        catch {}
        return nil
    }
}

How can I make updates to the destination's location attribute persist?

Answer 1

Wain's answer is correct- it seems that the reference to the object needs to change in order for Core Data to persist the update. Changing a child attribute on the Location instance will not update that reference. Nor will it work to mutate and re-set the same object. Only when a new object reference is assigned will the change stick.

Here are some code examples:

This does NOT work:

let newLocation = destination.location
newLocation.address = "a new street that doesn't stick"
destination.location = newLocation

But this does:

destination.location = Location(address: "a new street that sticks", latitude: 34.051145, longitude: -118.243595)

This also works, providing that the Location class implements the copyWithZone method:

let newLocation = destination.location.copy() as! Location
newLocation.address = "another new street that sticks"
destination.location = newLocation

Answer 2

core data can't track the dirty state of that object because it doesn't know about its internals. Instead of mutating the object, create a copy, mutate that and then set the new object. It may work to mutate and then re-set the same object, not sure, haven't tested it.

You can check, just mutate the address and then ask the managed object if it has changes, if not then it won't save.