CoreData 可转换：自定义转换器永远不会被调用 - 使用 NSKeyedArchiver

Posted 2023-03-11

【中文标题】CoreData 可转换：自定义转换器永远不会被调用 - 使用 NSKeyedArchiver

【英文标题】：CoreData Transformable: Custom Transformer never gets called - NSKeyedArchiver is used instead

【发布时间】：2019-09-23 16:30:46

【问题描述】：

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，我得到nil，而第二次我得到Optional("String attribute")，看来转换工作正常。

但是，这是我的转换器类：

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)
    

CoreData 似乎使用NSKeyedArchiver/Unarchiver 而不是我的自定义类。可能是什么问题？

显然，这是我设置的一个测试项目，目的是弄清可转换属性的工作原理，并测试我正在处理的实际项目中面临的更大问题。

Configured project to investigate the issue

【参考方案1】：

您需要在使用之前注册您的 ValueTransformer。

ValueTransformer.setValueTransformer(MyTransformer(), forName: NSValueTransformerName(rawValue: "MyTransformer"))
let fetch: NSFetchRequest<Entity> = Entity.fetchRequest()

更多信息请参考docs。