Use Codable to serialize custom class containing reference loop - swift

Question

I am trying to serialize a custom class containing a reference loop and got it working using NSCoding:
import Foundation
class Person: NSObject, NSCoding {
let name: String
weak var parent: Person?
var child: Person?
init(name: String) {
self.name = name
}
required init(coder aDecoder: NSCoder) {
self.name = aDecoder.decodeObject(forKey: "name") as? String ?? ""
self.parent = aDecoder.decodeObject(forKey: "parent") as? Person
self.child = aDecoder.decodeObject(forKey: "child") as? Person
}
func encode(with aCoder: NSCoder) {
aCoder.encode(name, forKey: "name")
aCoder.encode(parent, forKey: "parent")
aCoder.encode(child, forKey: "child")
}
}
let per = Person(name: "Per")
let linda = Person(name: "Linda")
linda.child = per
per.parent = linda
var people = [Person]()
people.append(linda)
people.append(per)
let encodedData = NSKeyedArchiver.archivedData(withRootObject: people)
let myPeopleList = NSKeyedUnarchiver.unarchiveObject(with: encodedData) as! [Person]
myPeopleList.forEach({
print("\($0.name)\n\t Child: \($0.child?.name ?? "nil")\n\t Parent: \($0.parent?.name ?? "nil")"
)})
// Linda
// Child: Per
// Parent: nil
// Per
// Child: nil
// Parent: Linda
No I want to do the same using Codable:
import Foundation
class Person: Codable {
let name: String
weak var parent: Person?
var child: Person?
init(name: String) {
self.name = name
}
}
let per = Person(name: "Per")
let linda = Person(name: "Linda")
linda.child = per
per.parent = linda
var people = [Person]()
people.append(linda)
people.append(per)
let archiver = NSKeyedArchiver()
try archiver.encodeEncodable(people, forKey: NSKeyedArchiveRootObjectKey)
But I get the error:
error: Execution was interrupted, reason: EXC_BAD_ACCESS
During the last line. I assume it has to do with the reference loop, because it works if I comment out the line:
per.parent = linda
So can we use Codable to serialize reference loops? If so, how?

Answer 1

You can choose which properties are serialized by overriding Coding Keys (from here)
e.g. in your case within the class:
enum CodingKeys: String, CodingKey
{
case name
case child
}
Only the keys included here should be saved, so no child->parent loop. This does however mean the connection will only exist in one direction when loading, so you will have to re-connect them when loaded.
FWIW, if you're dealing with 2-way relationships you will be much better off using a database rather than using NSKeyedArchiver for persistence.