Making your SwiftData fashions Codable – Donny Wals

In a earlier put up, I defined how one can make your NSManagedObject subclasses codable. This was a considerably tedious course of that includes a bunch of handbook work. Particularly as a result of essentially the most handy means I’ve discovered wasn’t all that handy. It is simple to neglect to set your managed object context in your decoder’s person information dictionary which might lead to failed saves in Core Information.

With SwiftData it is a lot simpler to outline mannequin objects so it is sensible to check out making SwiftData fashions Codable to see if it is higher than Core Information. Finally, SwiftData is a wrapper round Core Information which signifies that the @Mannequin macro will sooner or later generate managed objects, an object mannequin, and extra. On this put up, we’ll see if the @Mannequin macro may even make it simpler to make use of Codable with mannequin objects.

For those who favor studying by video, take a look at the video for this put up on YouTube:

Tip: when you’re not too acquainted with Codable or customized encoding and decoding of fashions, take a look at my put up sequence on the Codable protocol proper right here.

Defining a easy mannequin

On this put up I want to begin us off with a easy mannequin that is sufficiently small to not get complicated whereas nonetheless being consultant for a mannequin that you simply may outline in the true world. In my Sensible Core Information e book I make a variety of use of a Film object that I exploit to characterize a mannequin that I might load from The Film Database. For comfort, let’s simply go forward and use the a simplified model of that:

@Mannequin class Film {
  let originalTitle: String
  let releaseDate: Date

  init(originalTitle: String, releaseDate: Date) {
    self.originalTitle = originalTitle
    self.releaseDate = releaseDate
  }
}

The mannequin above is easy sufficient, it has solely two properties and for example the fundamentals of utilizing Codable with SwiftData we actually do not want something greater than that. So let’s transfer on and add Codable to our mannequin subsequent.

Marking a SwiftData mannequin as Codable

The best option to make any Swift class or struct Codable is to verify the entire object’s properties are Codable and having the compiler generate any and all boilerplate for us. Since each String and Date are Codable and people are the 2 properties on our mannequin, let’s examine what occurs after we make our SwiftData mannequin Codable:

// Sort 'Film' doesn't conform to protocol 'Decodable'
// Sort 'Film' doesn't conform to protocol 'Encodable'
@Mannequin class Film: Codable {
  let originalTitle: String
  let releaseDate: Date

  init(originalTitle: String, releaseDate: Date) {
    self.originalTitle = originalTitle
    self.releaseDate = releaseDate
  }
}

The compiler is telling us that our mannequin is not Codable. Nevertheless, if we take away the @Mannequin macro from our code we’re sure that our mannequin is Codable as a result of our code does compiler with out the @Mannequin macro.

So what’s taking place right here?

A macro in Swift expands and enriches our code by producing boilerplate or different code for us. We will proper click on on the @Mannequin macro and select develop macro to see what the @Mannequin macro expands our code into. You do not have to totally perceive or grasp the complete physique of code under. The purpose of displaying it’s to indicate you that the @Mannequin macro provides a variety of code, together with properties that do not conform to Codable.

@Mannequin class Film: Codable {
  @_PersistedProperty
  let originalTitle: String
  @_PersistedProperty
  let releaseDate: Date

  init(originalTitle: String, releaseDate: Date) {
    self.originalTitle = originalTitle
    self.releaseDate = releaseDate
  }

  @Transient
  non-public var _$backingData: any SwiftData.BackingData<Film> = Film.createBackingData()

  public var persistentBackingData: any SwiftData.BackingData<Film> {
    get {
      _$backingData
    }
    set {
      _$backingData = newValue
    }
  }

  static func schemaMetadata() -> [(String, AnyKeyPath, Any?, Any?)] {
    return [
      ("originalTitle", Movie.originalTitle, nil, nil),
      ("releaseDate", Movie.releaseDate, nil, nil)
    ]
  }

  required init(backingData: any SwiftData.BackingData<Film>) {
    self.persistentBackingData = backingData
  }

  @Transient
  non-public let _$observationRegistrar = Commentary.ObservationRegistrar()
}

extension Film: SwiftData.PersistentModel {
}

extension Film: Commentary.Observable {
}

If we apply Codable to our SwiftData mannequin, the protocol is not utilized to the small mannequin we have outlined. As a substitute, it is utilized to the totally expanded macro. Which means we’ve got a number of properties that do not conform to Codable which makes it not possible for the compiler to (on the time of scripting this) accurately infer what it’s that we wish to do.

We will repair this by writing our personal encoding and decoding logic for our mannequin.

Writing your encoding and decoding logic

For a whole overview of writing customized encoding and decoding logic to your fashions, take a look at this put up.

Let’s begin off by defining the CodingKeys enum that we’ll use for each our encoding and decoding logic:

@Mannequin class Film: Codable {
  enum CodingKeys: CodingKey {
    case originalTitle, releaseDate
  }

  // ...
}

These coding keys immediately observe the property names for our mannequin. We’ve to outline them as a result of we’re defining customized encoding and decoding logic.

The decoding init can look as follows:

required init(from decoder: Decoder) throws {
  let container = attempt decoder.container(keyedBy: CodingKeys.self)
  self.originalTitle = attempt container.decode(String.self, forKey: .originalTitle)
  self.releaseDate = attempt container.decode(Date.self, forKey: .releaseDate)
}

This initializer is fairly easy. We seize a container from the decoder, after which we ask the container to decode the properties we’re involved in utilizing our coding keys.

The encoding logic would look as follows:

func encode(to encoder: Encoder) throws {
  var container = encoder.container(keyedBy: CodingKeys.self)
  attempt container.encode(originalTitle, forKey: .originalTitle)
  attempt container.encode(releaseDate, forKey: .releaseDate)
}

With this initializer and encode(to:) operate in place, our mannequin is now totally Codable. Observe that when you’re solely grabbing knowledge from the community and which to decode that knowledge into SwiftData fashions you possibly can conform to Decodable as a substitute of Codable in an effort to skip having to jot down the encode(to:) methodology.

Let’s examine how we will really use our mannequin subsequent.

Decoding JSON right into a SwiftData mannequin

For essentially the most half, decoding your JSON knowledge right into a SwiftData mannequin might be comparatively striaghtforward. The important thing factor to remember is that you want to register all your decoded objects in your mannequin context after decoding them. This is an instance of how to do that:

let url = URL(string: "https://path.to.knowledge")!
let (knowledge, _) = attempt await URLSession.shared.knowledge(from: url)

// that is the precise decoding
let films = attempt! JSONDecoder().decode([Movie].self, from: knowledge)

// remember to register the decoded objects
for film in films {
  context.insert(film)
}

Making our mannequin Codable and dealing with it was easy sufficient. To wrap issues up, I might wish to discover how this method works with relationships.

Including relationships to our mannequin

First, let’s replace our mannequin object to have a relationship:

@Mannequin class Film: Codable {
  enum CodingKeys: CodingKey {
    case originalTitle, releaseDate, solid
  }

  let originalTitle: String
  let releaseDate: Date

  @Relationship([], deleteRule: .cascade)
  var solid: [Actor]

  init(originalTitle: String, releaseDate: Date, solid: [Actor]) {
    self.originalTitle = originalTitle
    self.releaseDate = releaseDate
    self.solid = solid
  }

  required init(from decoder: Decoder) throws {
    let container = attempt decoder.container(keyedBy: CodingKeys.self)
    self.originalTitle = attempt container.decode(String.self, forKey: .originalTitle)
    self.releaseDate = attempt container.decode(Date.self, forKey: .releaseDate)
    self.solid = attempt container.decode([Actor].self, forKey: .solid)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    attempt container.encode(originalTitle, forKey: .originalTitle)
    attempt container.encode(releaseDate, forKey: .releaseDate)
    attempt container.encode(solid, forKey: .solid)
  }
}

The Film object right here has gained a brand new property solid which is annotated with SwiftData’s @Relationship macro. Observe that the decode and encode logic does not get fancier than it must be. We simply decode and encode our solid property like we’d another property.

Let’s take a look at the definition of our Actor mannequin subsequent:

@Mannequin class Actor: Codable {
  enum CodingKeys: CodingKey {
    case title
  }

  let title: String

  @Relationship([], deleteRule: .nullify)
  let films: [Movie]

  init(title: String, films: [Movie]) {
    self.title = title
    self.films = films
  }

  required init(from decoder: Decoder) throws {
    let container = attempt decoder.container(keyedBy: CodingKeys.self)
    self.title = attempt container.decode(String.self, forKey: .title)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    attempt container.encode(title, forKey: .title)
  }
}

Our Actor defines a relationship again to our Film mannequin however we do not account for this in our encode and decode logic. The information we’re loading from an exterior supply would infinitely recurse from actor to film and again if actors would additionally maintain lists of their films within the knowledge we’re decoding. As a result of the supply knowledge does not include the inverse that we have outlined on our mannequin, we do not decode it. SwiftData will guarantee that our films property is populated as a result of we have outlined this property utilizing @Relationship.

When decoding our full API response, we needn’t replace the utilization code from earlier than. It seems to be like we do not have to explicitly insert our Actor situations into our mannequin context resulting from SwiftData’s dealing with of relationships which is sort of good.

With the code as it’s on this put up, we will encode and decode our SwiftData mannequin objects. No magic wanted!

In Abstract

All in all I’ve to say that I am a little bit unhappy that we did not get Codable help for SwiftData objects without cost. It is good that it is simpler to make SwiftData fashions Codable than it’s to make an NSManagedObject conform to Codable nevertheless it’s not too far off. We nonetheless need to guarantee that we affiliate our decoded mannequin with a context. It is just a bit bit simpler to do that in SwiftData than it’s in Core Information.

You probably have a unique method to make your SwiftData fashions Codable, or when you have questions on this put up be happy to attain out!