ios - 我怎样才能使用 Core Data 和 Swift 获取非标准(可转换)属性来更新/保存和持久化？

Question

我构建了一个非常基本的示例来演示我在尝试更新可转换类型并让更改在应用重启之间持续存在时遇到的问题。

我有一个类型为Destination的实体...

import Foundation
import CoreData

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

...具有一个简单的名称属性(字符串类型)和一个Location类型的属性:

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 在 Core Data 中被配置为“可转换的”，因为它具有其他基本类型都无法处理的结构。

使用 Apple 的样板 Core Data 代码，这是一个仅执行以下操作的 View Controller :

• 获取必要的 appDelegate/ManagedApplicationContext 引用
• 如果存在则获取目标，如果不存在则创建目标
• 打印目的地的名称和位置。地址
• 更新目的地的名称和位置。地址
• 保存对 ManagedObjectContext 的更改

当应用程序运行并重新运行时，只会保留对名称的更改。对 location.address 所做的更改不会保留。

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
    }
}

如何使目的地位置属性的更新持久化？

Answer 1

Wain 的回答是正确的——似乎需要更改对对象的引用才能使 Core Data 持久更新。更改 Location 实例的子属性不会更新该引用。改变和重新设置同一个对象也不起作用。只有分配了新的对象引用时，更改才会生效。

下面是一些代码示例:

这不有效:

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

但是这样做:

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

这也有效，前提是 Location 类实现了 copyWithZone 方法:

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