CoreData - Store an array of Tranformable objects

Question

I have an entity that has a traformable property. It is an array of custom object, Reminder that confirms to NSSecureCoding.

@objc(Reminder)
public class Reminder: NSObject, NSSecureCoding {
    public static var supportsSecureCoding: Bool = true
    
    
    public var date: Date
    public var isOn: Bool
    
    public  init(date: Date, isOn: Bool) {
        self.date = date
        self.isOn = isOn
    }
    
    struct Keys {
        static var date: String = "date"
        static let isOn: String = "isOn"
    }
    
    public func encode(with aCoder: NSCoder) {
        aCoder.encode(date as NSDate,forKey: Keys.date)
        aCoder.encode(isOn,forKey: Keys.isOn)
    }
    
    required public init?(coder aDecoder: NSCoder) {
        guard let date = aDecoder.decodeObject(of: NSDate.self, forKey: Keys.date) as Date? else {
            return nil
        }
        
        self.date = date
        self.isOn = aDecoder.decodeBool(forKey: Keys.isOn)
    }
}

---

And the following code is my NSSecureUnarchiveFromDataTransformer.

class ReminderDataTransformer: NSSecureUnarchiveFromDataTransformer {

    override class func allowsReverseTransformation() -> Bool {
        return true
    }

    override class func transformedValueClass() -> AnyClass {
        return Reminder.self
    }

    override class var allowedTopLevelClasses: [AnyClass] {
        return [Reminder.self]
    }

    override func transformedValue(_ value: Any?) -> Any? {
        guard let data = value as? Data else {
            fatalError("Wrong data type: value must be a Data object; received \(type(of: value))")
        }
        return super.transformedValue(data)
    }

    override func reverseTransformedValue(_ value: Any?) -> Any? {
        guard let reminder = value as? [Reminder] else {
            fatalError("Wrong data type: value must be a Reminder object; received \(type(of: value))")
        }
        return super.reverseTransformedValue(reminder)
    }
}



extension NSValueTransformerName {
    static let reminderToDataTransformer = NSValueTransformerName(rawValue: "ReminderToDataTransformer")
}

---

I have registered ReminderDataTransformer using the following code before initializing NSPersistantContainer.

ValueTransformer.setValueTransformer(ReminderDataTransformer(), forName: .reminderToDataTransformer)

---

I have used ReminderToDataTransformer as Transformer in xCode's Data Model Inspector.

---

But it does not work because of the following error that occurs when saving entities.

 [error] error: SQLCore dispatchRequest: exception handling request: <NSSQLSaveChangesRequestContext: 0x282ec0780> , <shared NSSecureUnarchiveFromData transformer> threw while encoding a value. with userInfo of (null)
CoreData: error: SQLCore dispatchRequest: exception handling request: <NSSQLSaveChangesRequestContext: 0x282ec0780> , <shared NSSecureUnarchiveFromData transformer> threw while encoding a value. with userInfo of (null)
2020-12-31 21:44:09.300394+0100 ReminderApp[26406:6247995] [error] error: -executeRequest: encountered exception = <shared NSSecureUnarchiveFromData transformer> threw while encoding a value. with userInfo = (null)

Update

There is no exception. When launching the app for the second time, this error is logged in console.

[error] fault: exception raised during multi-threaded fetch <shared NSSecureUnarchiveFromData transformer> threw while decoding a value. ({
    NSUnderlyingError = "Error Domain=NSCocoaErrorDomain Code=4864 \"value for key 'root' was of unexpected class 'NSArray (0x1fa392238) [/System/Library/Frameworks/CoreFoundation.framework]'. Allowed classes are '{(\n    \"Reminder (0x100fb6920) [/private/var/containers/Bundle/Application/306C3F0B-75AA-4A2D-A934-260B2EB63313/ReminderApp]\”\n)}’.\” UserInfo={NSDebugDescription=value for key 'root' was of unexpected class 'NSArray (0x1fa392238) [/System/Library/Frameworks/CoreFoundation.framework]'. 

---

I guess I am unable to correctly encode / decode Reminder's array as It works if I change the code to store Reminder instead of [Reminder].

Just to be clear, I can store Reminder, but not [Reminder].

---

How to store [Reminder] as Transformable?

---

Answer 1

If you are going to unarchive an array you have to add NSArray to the top level classes, this is how I understand the error message

override class var allowedTopLevelClasses: [AnyClass] {
    return [NSArray.self, Reminder.self]
}

---

By the way, instead of a transformable consider to encode the array to JSON and save it as a simple String. The conversion can be performed by a computed property.

The class can be a struct with dramatically less code

public struct Reminder : Codable {
    public var date: Date
    public var isOn: Bool
}

In the NSManagedObject subclass create a String Attribute

@NSManaged public var cdReminders : String

and a computed property

var reminders : [Reminder] {
    get {
       return (try? JSONDecoder().decode([Reminder].self, from: Data(cdReminders.utf8))) ?? []
    }
    set {
       do {
           let reminderData = try JSONEncoder().encode(newValue)
           cdReminders = String(data: reminderData, encoding:.utf8)!
       } catch { cdReminders = "" }
    }
}