Making your SwiftData fashions Codable – Donny Wals

In a earlier submit, 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 method I’ve discovered wasn’t all that handy. It is simple to overlook to set your managed object context in your decoder’s consumer information dictionary which might lead to failed saves in Core Knowledge.

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 Knowledge. In the end, SwiftData is a wrapper round Core Knowledge which signifies that the @Mannequin macro will in some unspecified time in the future generate managed objects, an object mannequin, and extra. On this submit, we’ll see if the @Mannequin macro can even make it simpler to make use of Codable with mannequin objects.

Tip: in the event you’re not too acquainted with Codable or customized encoding and decoding of fashions, try my submit sequence on the Codable protocol proper right here.

Defining a easy mannequin

On this submit 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 actual world. In my Sensible Core Knowledge guide I make a number of use of a Film object that I exploit to signify 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 straightforward sufficient, it has solely two properties and as an instance 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 approach 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 have a look at 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. Nonetheless, 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 occurring right here?

A macro in Swift expands and enriches our code by producing boilerplate or different code for us. We are able to proper click on on the @Mannequin macro and select broaden macro to see what the @Mannequin macro expands our code into. You do not have to completely perceive or grasp your complete physique of code under. The purpose of exhibiting it’s to indicate you that the @Mannequin macro provides a number 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 = Statement.ObservationRegistrar()
}

extension Film: SwiftData.PersistentModel {
}

extension Film: Statement.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 now have a number of properties that do not conform to Codable which makes it unattainable for the compiler to (on the time of scripting this) appropriately infer what it’s that we need to do.

We are able to repair this by writing our personal encoding and decoding logic for our mannequin.

Writing your encoding and decoding logic

For an entire overview of writing customized encoding and decoding logic to your fashions, try this submit.

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 straight comply with the property names for our mannequin. We have now 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 curious about 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:) perform in place, our mannequin is now totally Codable. Be aware that in the event you’re solely grabbing information from the community and which to decode that information into SwiftData fashions you’ll be able to conform to Decodable as an alternative of Codable as a way to skip having to put in writing the encode(to:) methodology.

Let’s have a look at how we are able to truly use our mannequin subsequent.

Decoding JSON right into a SwiftData mannequin

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

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

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

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

Making our mannequin Codable and dealing with it was easy sufficient. To wrap issues up, I might prefer 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, forged
  }

  let originalTitle: String
  let releaseDate: Date

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

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

  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.forged = attempt container.decode([Actor].self, forKey: .forged)
  }

  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(forged, forKey: .forged)
  }
}

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

Let us 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 motion pictures: [Movie]

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

  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 info we’re loading from an exterior supply would infinitely recurse from actor to film and again if actors would additionally maintain lists of their motion pictures within the information we’re decoding. As a result of the supply information would not comprise the inverse that we have outlined on our mannequin, we do not decode it. SwiftData will ensure that our motion pictures 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 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 submit, we are able to encode and decode our SwiftData mannequin objects. No magic wanted!

In Abstract

All in all I’ve to say that I am just a little unhappy that we did not get Codable help for SwiftData objects at no cost. It is good that it is simpler to make SwiftData fashions Codable than it’s to make an NSManagedObject conform to Codable but it surely’s not too far off. We nonetheless need to ensure 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 Knowledge.

If in case you have a distinct method to make your SwiftData fashions Codable, or you probably have questions on this submit be happy to attain out!