Codable Compatibility for NSManagedObjects

ManagedCodableProtocol.swift

/*

//Extended Protocol which allows decoding NSManagedObjects using swift4 Codable Protocol.
//Requires the convenience initializer implementation as follows

public final class City: NSManagedObject, ManagedObjectCodable {

  convenience public init(from decoder: Decoder) throws {
// Get Context from decoder
		guard let context = decoder.userInfo[.context] as? NSManagedObjectContext else { fatalError("No Managed Object Context!") }

// Get Entity from Self-class-name
		guard let entity = NSEntityDescription.entity(forEntityName: String(describing:type(of: self)), in: context) else { fatalError("No Managed Entity Description!") }

// Default init for NSManagedObjects
		self.init(entity: entity, insertInto: context)

// Protocol Method to parse data
  try self.loadData(from: decoder, keyType: CityCodingKeys.self)

// performing any property manipulations if required
		if self.populationRanking < 1 { self.populationRanking = Int16.max }

// State is an NSManagedObject relationship
		self.state = try decoder.container(keyedBy: CityCodingKeys.self).decode(State.self, forKey: Cities.CityCodingKeys.state)

	}

// And the City class would have an CodingKey compliant enumeration 
	enum CityCodingKeys: String, CodingKey {
		case state
		case name
		case modifiedAt = "modified_at"
		case ranking
		case id
  }
}

//Usage >> 
			let decoder = JSONDecoder()
			decoder.userInfo[.context] = <NSManagedObjectContext> 
			let parsedValue = try decoder.decode(City.self, from: cityResponseData)
*/

import Foundation
import CoreData

public extension CodingUserInfoKey {
	public static let context = CodingUserInfoKey(rawValue: "context")!
}

public protocol ManagedCodableProtocol: Codable { }

public extension ManagedCodableProtocol where Self: NSManagedObject {

	func loadData<Key>(from decoder: Decoder, keyType: Key.Type) throws where Key: CodingKey {
		let container = try decoder.container(keyedBy: Key.self)
    let attributes = self.entity.attributesByName
    
		for nameKey in container.allKeys {
			guard let subStrName = "\(nameKey)".split(separator: ".").last else { continue }
			let name = String(subStrName)
			guard let attr = attributes[name] else { continue }
			let attrType = attr.attributeType // NSAttributeType enumeration for the property type
			switch attrType {
			case .integer16AttributeType:
				let value = try container.decodeIfPresent(Int16.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .integer32AttributeType:
				let value = try container.decodeIfPresent(Int32.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .integer64AttributeType:
				let value = try container.decodeIfPresent(Int64.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .decimalAttributeType:
				let value = try container.decodeIfPresent(Decimal.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .doubleAttributeType:
				let value = try container.decodeIfPresent(Double.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .floatAttributeType:
				let value = try container.decodeIfPresent(Float.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .stringAttributeType:
				let value = try container.decodeIfPresent(String.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .booleanAttributeType:
				let value = try container.decodeIfPresent(Bool.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			case .dateAttributeType:
				let value = try container.decodeIfPresent(Date.self, forKey: nameKey)
				self.setValue(value, forKey: name)
			default:
				print("Undefined NameKey : \(nameKey)")
				break
			}
		}
	}
}

I think you've got a typo at link 6. Looks like you might have renamed your protocol.

Should be

public final class City: NSManagedObject, ManagedCodableProtocol {