我创建了一个简单的 CoreData 模型，其中包含一个具有自定义可转换属性的实体：
import Foundation
import CoreData
extension Entity {
@nonobjc public class func fetchRequest() -> NSFetchRequest<Entity> {
return NSFetchRequest<Entity>(entityName: "Entity")
}
@NSManaged public var attribute: String?
@NSManaged public var title: String?
}
然后，在 AppDelegate 中，我尝试获取并保存单个实体：
import UIKit
import CoreData
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
private lazy var context: NSManagedObjectContext = {
return persistentContainer.newBackgroundContext()
}()
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
let fetch: NSFetchRequest<Entity> = Entity.fetchRequest()
context.perform {
print(try! fetch.execute().first?.attribute)
let entity = NSEntityDescription.insertNewObject(forEntityName: "Entity", into: self.context) as! Entity
entity.title = "Entity Title"
entity.attribute = "String attribute"
try! self.context.save()
}
return true
}
// MARK: UISceneSession Lifecycle
func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
// Called when a new scene session is being created.
// Use this method to select a configuration to create the new scene with.
return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
}
// MARK: - Core Data stack
lazy var persistentContainer: NSPersistentContainer = {
/*
The persistent container for the application. This implementation
creates and returns a container, having loaded the store for the
application to it. This property is optional since there are legitimate
error conditions that could cause the creation of the store to fail.
*/
let container = NSPersistentContainer(name: "Europe")
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
/*
Typical reasons for an error here include:
* The parent directory does not exist, cannot be created, or disallows writing.
* The persistent store is not accessible, due to permissions or data protection when the device is locked.
* The device is out of space.
* The store could not be migrated to the current model version.
Check the error message to determine what the actual problem was.
*/
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
return container
}()
// MARK: - Core Data Saving support
func saveContext () {
let context = persistentContainer.viewContext
if context.hasChanges {
do {
try context.save()
} catch {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
}
}
}
我第一次运行应用程序时得到nil，而第二次得到Optional("String attribute")，这似乎转换工作正常。
但是，这是我的变压器类：
import Foundation
/// Never Called!
@objcMembers public final class MyTransformer: ValueTransformer {
public override init() {
super.init()
fatalError("Nevern gets executed")
}
public override class func transformedValueClass() -> AnyClass {
fatalError("Nevern gets executed")
print("transformedValueClass")
return NSData.self
}
public override class func allowsReverseTransformation() -> Bool {
fatalError("Nevern gets executed")
print("allowsReverseTransformation")
return true
}
public override func transformedValue(_ value: Any?) -> Any? {
fatalError("Nevern gets executed")
print("transformedValue")
guard let string = value as? String else {return nil}
return string.data(using: .utf8)
}
public override func reverseTransformedValue(_ value: Any?) -> Any? {
fatalError("Nevern gets executed")
print("reverseTransformedValue")
guard let data = value as? Data else {return nil}
return String(data: data, encoding: .utf8)
}
}
似乎 CoreData 使用NSKeyedArchiver/Unarchiver而不是我的自定义类。可能是什么问题呢？
显然，这是我设置的一个测试项目，目的是弄清楚可转换属性是如何工作的，并测试我正在处理的真实项目中面临的一个更大的问题。