【SwiftData】バックグラウンドで @Modelactor を singleton で使う 👉 https://android.benigumo.com/20240717/background-modelactor/

BackgroundModelActor.swift

import SwiftUI
import SwiftData

// https://forums.developer.apple.com/forums/thread/736226

struct TestModelActorView: View {

  @Environment(\.modelContext) private var modelContext

//  private var dataSource: DataSource {
//    // ModelActor
//    DataSource(modelContainer: modelContext.container)
//  }

  @State private var count = 0

  @State private var task: Task<Void, Error>?

  var body: some View {
    VStack {
      Text("\(count)")
      Button("fetch") {
        task = Task {
          //let items = await dataSource.fetch()
          //let items = await DataSource(modelContainer: modelContext.container).fetch()
          let items = await DataSource.shared.fetch()
          count = items.count
        }
      }

      Button("cancel") {
        task?.cancel()
      }
    }
    .buttonStyle(.borderedProminent)
//    .onAppear { // main
//      DataSource.createInstance(modelContainer: modelContext.container)
//    }
    .task { // not main
      await DataSource.createInstance(modelContainer: modelContext.container)
    }
  }
}

@ModelActor
actor DataSource {
  nonisolated(unsafe) private(set) static var shared: DataSource!
//
//  static func createInstance(modelContainer: ModelContainer) {
//    print("createInstance()", Thread.current) // main
//    shared = DataSource(modelContainer: modelContainer)
//  }
//

  static func createInstance(modelContainer: ModelContainer) async {
    print("createInstance()", Thread.current) // not main
    shared = DataSource(modelContainer: modelContainer)
  }

  func fetch() async -> [Item] {
    //print("fetch()", Thread.current) // depends on creating instance
    do {
      let items = try modelContext.fetch(FetchDescriptor<Item>())

      //let items: [Item] = []
      //try await Task.sleep(for: .seconds(3))

      return items
    } catch {
      if Task.isCancelled {
        print("canceled")
      }
      return []
    }
  }
}

extension Item: @unchecked Sendable {}


#Preview {
  TestModelActorView()
    .dataContainer()
    .padding()
}