How to make SwiftData models conform to Codable 관련

SwiftData by Example

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How to make SwiftData models conform to Codable | SwiftData by Example

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How to make SwiftData models conform to Codable

Updated for Xcode 15

You can make any SwiftData model Codable, but you need to add the conformance by hand. If your model has relationships that you want to be encoded and decoded, those relationships must also conform to Codable.

I'll give you a trivial sample first, then show a more complex example with a relationship.

First, the simple example. This creates a Movie model with a single string property, so encoding and decoding it is just a matter of reading and writing that one value:

@Model
class Movie: Codable {
    enum CodingKeys: CodingKey {
        case name
    }

    var name: String

    init(name: String) {
        self.name = name
    }

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

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

As you can see, making the model Codable follows the standard steps you would use elsewhere:

1. Defining your CodingKeys enum, listing all the properties you want to load and save.
2. Adding an init(from:) initializer that knows how to read all the values from your container.
3. Adding an encode(to:) method that knows how to write all the values to a container.

Now you can go ahead and use it as normal. So, encoding a Movie would look like this:

if let data = try? JSONEncoder().encode(movies) {
    print(String(decoding: data, as: UTF8.self))
}

And if you wanted to decode data, you can do so either from local data:

do {
    guard let url = Bundle.main.url(forResource: "movies", withExtension: "json") else {
        fatalError("Failed to find movies.json")
    }

    let data = try Data(contentsOf: url)
    let movies = try JSONDecoder().decode([Movie].self, from: data)

    for movie in movies {
        modelContext.insert(movie)
    }
} catch {
    print("Failed to load movies.")
}

Or from a remote server:

do {
    let url = URL(string: "https://hws.dev/movies.json")!
    let (data, _) = try await URLSession.shared.data(from: url)
    let movies = try JSONDecoder().decode([Movie].self, from: data)

    for movie in movies {
        modelContext.insert(movie)
    }
} catch {
    print("Failed to load movies.")
}

Important

Both those samples insert the data from JSON into a model context, so they are stored in SwiftData. However, whether or not you choose to insert the data, the very act of creating a Movie instance requires that you load that model as part of your model container, e.g. modelContainer(for: Movie.self).

That bears repeating because it's critical: Even if you're creating your model instances from JSON, you must still have that model type registered with your model container. Failure to do so will trigger a crash.

There's no magic to it, and honestly I'm a bit surprised Apple couldn't find a way to add Codable conformance for all models out of the box.

Working with more complex models is really just more of the same. For example, in the code below there's an Author model that has many Book objects inside it, and because both conform to Codable they work great:

@Model
class Author: Codable {
    enum CodingKeys: CodingKey {
        case firstName, lastName, books
    }

    var firstName: String
    var lastName: String
    var books: [Book]

    init(firstName: String, lastName: String, books: [Book]) {
        self.firstName = firstName
        self.lastName = lastName
        self.books = books
    }

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        firstName = try container.decode(String.self, forKey: .firstName)
        lastName = try container.decode(String.self, forKey: .lastName)
        books = try container.decode([Book].self, forKey: .books)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(firstName, forKey: .firstName)
        try container.encode(lastName, forKey: .lastName)
        try container.encode(books, forKey: .books)
    }
}

@Model
class Book: Codable {
    enum CodingKeys: CodingKey {
        case title, genre
    }

    var title: String
    var genre: String

    init(title: String, genre: String) {
        self.title = title
        self.genre = genre
    }

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

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

As you can see, adding more properties and relationships is just a matter of adding more of the same Encoder and Decoder functionality – it's not much fun, but at least it works.

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이찬희 (MarkiiimarK)

Never Stop Learning.