I offer the option for my app users to add some sample data to the app, in order not to see an empty view at first launch. To do so, I call a method inside a specific struct:

struct SampleData {
  public func addSampleData(context: NSManagedObjectContext) {
    let sampleItem1 = Item(context: context)
    sampleItem1.id = UUID()
  //many more properties and items here...
  try? context.save()
  }
}

That's one usage of this struct: to populate persistent Core Data.

On the other side, on my DataController class, a make a call to to this method too, but in this case in order for my Previews to have data to be displayed. This is why I use a "context" argument for the "addSampleData" call: for "preview" content, I try to use a in-Memory not persisting instance of DataController container, while on "ordinary" (not in-Memory) common use, I try to persist data. Here my planned DataController class to do so:

final class DataController: ObservableObject {

  static let shared = DataController()

  //For previews...
  static var preview: DataController = {
    let result = DataController(inMemory: true)
    let viewContext = result.container.viewContext

    let sampleData = SampleData()
    sampleData.addSampleData(context: viewContext)

    do {
      try viewContext.save()
    } catch {
     //Error handling
    }

    return result
  }()

  let container = NSPersistentCloudKitContainer(name: "MyModel")

  init(inMemory: Bool = false) {
    if inMemory {
        container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
    }

    container.loadPersistentStores { description, error in
     //Error handling
      }

      self.container.viewContext.automaticallyMergesChangesFromParent = true
      self.container.viewContext.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
    }
  }
}

For my" Item" object I use an extension to fetch request one of this "sample-not persisted" items, for previews:

extension Item {
  static var sample: Item {
    let context = DataController.preview.container.viewContext
    let fetchRequest: NSFetchRequest<Item> = Item.fetchRequest()
    fetchRequest.fetchLimit = 1
    let results = try? context.fetch(fetchRequest)
    return (results?.first!)!
  }
}

Both previews for me as developer, and sample data import for app users, work perfectly, independently. But the problem now is that if I display a preview while using XCode, data are added AGAIN to the persistent store, resulting in a increasing amount of persisted data, each time I open ANY preview of the app.

I've cheked lots of tutorials and posts about this kind of Core Data preview usage, and I think that there is something in my code which is making this "copying" of planned-in-memory-sample-data to the persistent-data.

What am I doing wrong? Any suggestions? Thank you very much.