SwiftData crashes at init

Question

Below is the minimum code that causes the crash. What I want to achieve: pass a default AchievementB to the view and then change (based on user input) and insert into achievements.

update: after a few attempts, it seems that i can't init the object before the view is appear, but i don't know why.

import SwiftData
import SwiftUI

@Model
class AchievementB: Identifiable {
  let id: UUID = UUID()

  init(id: UUID = UUID()) {
      self.id = id
  }
}

struct AchievementView: View {
  @State var options: AchievementB

  @Environment(\.modelContext) private var modelContext
  @Query var achievements: [AchievementB]
  
  var body: some View {
    Text("hello")
  }
}

#Preview {
  AchievementView(options: AchievementB())
    .modelContainer(for: AchievementB.self)
}

It doesn't crash for me. Did you set up the ModelContainer properly for the app itself? — Joakim Danielson
– Joakim Danielson, Commented May 28, 2024 at 8:17
that's all the code i have. what kind of setup are you referring to? — user1385809
– user1385809, Commented May 28, 2024 at 8:25
In your app code in <my app name>.swift you must also add a .modelContainer modifier to the WindowGroup — Joakim Danielson
– Joakim Danielson, Commented May 28, 2024 at 8:28

Sweeper · Accepted Answer · 2024-05-28 08:46:23Z

2

In your code, the AchievementB object is created before a model container exists. AchievementB() is executed first, before the .modelContainer(for: ...) modifier, which creates the model container.

Before you create any @Model objects, you should first create a model container, because the @Model objects needs a model container for their properties to work. When you get/set a property of a @Model object, the actual reading and writing of the data is delegated to the model container.

You should manually create a ModelContainer first, then add it to the view using the other overload of the modelContainer modifier.

#Preview {
    let config = ModelConfiguration(isStoredInMemoryOnly: true)
    let container = try! ModelContainer(for: AchievementB.self, configurations: config)
    return AchievementView(options: AchievementB())
        .modelContainer(container)
}

Note that you need to explicitly return because #Preview is not a view builder.

Side note: options should not be a @State if you are going to pass things to it. It should just be a regular let/var, or @Bindable if you want to get Bindings out of it.

answered May 28, 2024 at 8:46

Sweeper

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2 Comments

Joakim Danielson Over a year ago

With Xcode 15.4 this doesn't seem to be needed and having only AchievementView(options: AchievementB()).modelContainer(for: AchievementB.self, inMemory: true) in the #Preview macro works fine.

Sweeper Over a year ago

@JoakimDanielson Ah that's interesting. I (and probably the OP too) am on Xcode 15.3.

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