How to use swift 4 Codable in Core Data?

Swift 4.2:

Following casademora's solution,

guard let context = decoder.userInfo[.context] as? NSManagedObjectContext else { fatalError() }

should be

guard let context = decoder.userInfo[CodingUserInfoKey.context!] as? NSManagedObjectContext else { fatalError() }.

This prevents errors that Xcode falsely recognizes as array slice problems.

Edit: Use implicitly unwrapped optionals to remove the need to force unwrap .context every time it is being used.

You can use the Codable interface with CoreData objects to encode and decode data, however it's not as automatic as when used with plain old swift objects. Here's how you can implement JSON Decoding directly with Core Data objects:

First, you make your object implement Codable. This interface must be defined on the object, and not in an extension. You can also define your Coding Keys in this class.

class MyManagedObject: NSManagedObject, Codable {
    @NSManaged var property: String?

    enum CodingKeys: String, CodingKey {
       case property = "json_key"
    }
}

Next, you can define the init method. This must also be defined in the class method because the init method is required by the Decodable protocol.

required convenience init(from decoder: Decoder) throws {
}

However, the proper initializer for use with managed objects is:

NSManagedObject.init(entity: NSEntityDescription, into context: NSManagedObjectContext)

So, the secret here is to use the userInfo dictionary to pass in the proper context object into the initializer. To do this, you'll need to extend the CodingUserInfoKey struct with a new key:

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

Now, you can just as the decoder for the context:

required convenience init(from decoder: Decoder) throws {

    guard let context = decoder.userInfo[CodingUserInfoKey.context!] as? NSManagedObjectContext else { fatalError() }
    guard let entity = NSEntityDescription.entity(forEntityName: "MyManagedObject", in: context) else { fatalError() }

    self.init(entity: entity, in: context)

    let container = decoder.container(keyedBy: CodingKeys.self)
    self.property = container.decodeIfPresent(String.self, forKey: .property)
}

Now, when you set up the decoding for Managed Objects, you'll need to pass along the proper context object:

let data = //raw json data in Data object
let context = persistentContainer.newBackgroundContext()
let decoder = JSONDecoder()
decoder.userInfo[.context] = context

_ = try decoder.decode(MyManagedObject.self, from: data) //we'll get the value from another context using a fetch request later...

try context.save() //make sure to save your data once decoding is complete

To encode data, you'll need to do something similar using the encode protocol function.

As an alternative for those who would like to make use of XCode's modern approach to NSManagedObject file generation, I have created a DecoderWrapper class to expose a Decoder object which I then use within my object which conforms to a JSONDecoding protocol:

class DecoderWrapper: Decodable {

    let decoder:Decoder

    required init(from decoder:Decoder) throws {

        self.decoder = decoder
    }
}

protocol JSONDecoding {
     func decodeWith(_ decoder: Decoder) throws
}

extension JSONDecoding where Self:NSManagedObject {

    func decode(json:[String:Any]) throws {

        let data = try JSONSerialization.data(withJSONObject: json, options: [])
        let wrapper = try JSONDecoder().decode(DecoderWrapper.self, from: data)
        try decodeWith(wrapper.decoder)
    }
}

extension MyCoreDataClass: JSONDecoding {

    enum CodingKeys: String, CodingKey {
        case name // For example
    }

    func decodeWith(_ decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: CodingKeys.self)

        self.name = try container.decode(String.self, forKey: .name)
    }
}

This is probably only useful for models without any non-optional attributes, but it solves my problem of wanting to use Decodable but also manage relationships and persistence with Core Data without having to manually create all my classes / properties.

Edit: Example of it in use

If I have a json object:

let myjson = [ "name" : "Something" ]

I create the object in Core Data (force cast here for brevity):

let myObject = NSEntityDescription.insertNewObject(forEntityName: "MyCoreDataClass", into: myContext) as! MyCoreDataClass

And I use the extension to have the object decode the json:

do {
    try myObject.decode(json: myjson)
}
catch {
    // handle any error
}

Now myObject.name is "Something"

CoreData is its own persistence framework and, per its thorough documentation, you must use its designated initializers and follow a rather specific path to creating and storing objects with it.

You can still use Codable with it in limited ways just as you can use NSCoding, however.

One way is to decode an object (or a struct) with either of these protocols and transfer its properties into a new NSManagedObject instance you've created per Core Data's docs.

Another way (which is very common) is to use one of the protocols only for a non-standard object you want to store in a managed object's properties. By "non-standard", I mean anything thst doesn't conform to Core Data's standard attribute types as specified in your model. For example, NSColor can't be stored directly as a Managed Object property since it's not one of the basic attribute types CD supports. Instead, you can use NSKeyedArchiver to serialize the color into an NSData instance and store it as a Data property in the Managed Object. Reverse this process with NSKeyedUnarchiver. That's simplistic and there is a much better way to do this with Core Data (see Transient Attributes) but it illustrates my point.

You could also conceivably adopt Encodable (one of the two protocols that compose Codable - can you guess the name of the other?) to convert a Managed Object instance directly to JSON for sharing but you'd have to specify coding keys and your own custom encode implementation since it won't be auto-synthesized by the compiler with custom coding keys. In this case you'd want to specify only the keys (properties) you want to be included.

Hope this helps.