How to set up a SwiftData + SwiftUI app with complex models?

Question

I am a beginner in Swift. I am trying to build a simple DnD Character sheet app. Here is my simplified character model.

@Model
final class Character {
    var name: String
    var hitDice: Dice

    init(name: String, hitDice: Dice) {
        self.name = name
        self.hitDice = hitDice
    }

}

class Dice {
    var number: Int
    var sides: Int

    init(_ number: Int, _ sides: Int) {
        self.number = number
        self.sides = sides
    }

}

When I build the app, I get this error: No exact matches in call to instance method 'setValue'. This does not only happen with classes. It is also the case for dictionaries. Seems like any non-primitive type. Could this have something to do with how these data type map for underlying coredata/sqlite data types?

Then I turn Dice into Model

@Model
final class Dice {
     // ...
}

The error goes away but I get this mysterious breakpoint

@Transient
private var _$backingData: any SwiftData.BackingData<Dice> = Dice.createBackingData()

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

static var schemaMetadata: [SwiftData.Schema.PropertyMetadata] {
  return [
    SwiftData.Schema.PropertyMetadata(name: "number", keypath: \Dice.number, defaultValue: nil, metadata: nil),
    SwiftData.Schema.PropertyMetadata(name: "sides", keypath: \Dice.sides, defaultValue: nil, metadata: nil)
  ]
}

init(backingData: any SwiftData.BackingData<Dice>) {
  _number = _SwiftDataNoType()
  _sides = _SwiftDataNoType()
  self.persistentBackingData = backingData
}

@Transient
private let _$observationRegistrar = Observation.ObservationRegistrar()

struct _SwiftDataNoType {
}

My app

import SwiftUI
import SwiftData

@main
struct CharacterSheetApp: App {
    let modelContainer: ModelContainer
    
    init() {
        do {
            modelContainer = try ModelContainer(for: Character.self)
        } catch {
            fatalError("Could not initialize ModelContainer")
        }
    }

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(modelContainer)
    }
}

What am I doing wrong?

Answer 1

You have declared your Character as an @Model - Which means that all the properties of that class need to be able to be stored in Swift Data. Simple types, like strings and integers aren't a problem. But SwiftData doesn't know how to store a Dice object.

When you add @Model to Dice then that object too can be stored. It can't be stored directly in the Character object, but a Dice object can be created and the Character object can hold a reference to the related Dice - Which is why the error goes away.

I tried your code and once I added the @Model to Dice I was able to create and fetch Character without any errors or exceptions.

One note on your data design, it may be a bit excessive to create an object to store Dice - You could probably just store a String and parse that to create dice when required. e.g. Just store "2d20" or whatever.

This is my code:

import Foundation
import SwiftData

@Model
final class Character {
    var name: String
    var hitDice: Dice

    init(name: String, hitDice: Dice) {
        self.name = name
        self.hitDice = hitDice
    }

}

@Model
class Dice {
    var number: Int
    var sides: Int

    init(_ number: Int, _ sides: Int) {
        self.number = number
        self.sides = sides
    }
}


struct ContentView: View {
    @Environment(\.modelContext) private var context
    @Query var allCharacters: [Character]
    var body: some View {
        VStack {
            List {
                ForEach(allCharacters) { char in
                    HStack {
                        Text("Name: \(char.name)")
                        Text("Dice: \(char.hitDice.number)d\(char.hitDice.sides)")
                    }
                }
                    
            }
            Button( action: {
                
                let char = Character(name: "Bob", hitDice: Dice(2, 20))
                context.insert(char)
                do {
                    try context.save()
                } catch {
                    print(error)
                }
            }) {
                Text("Make bob")
            }
        }
        .padding()
    }
}

struct CharacterSheet: App {
    let modelContainer: ModelContainer
    
    init() {
        do {
            modelContainer = try ModelContainer(for: Character.self)
        } catch {
            fatalError("Could not initialize ModelContainer")
        }
    }

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(modelContainer)
    }
}