SwiftUI. Preview with CoreData – populating, singleton and weird behaviour - swift

Question

I have a small app which can add and save arbitrary Events to CoreData.
And I'm struggling with preparing of preview data.
Correct behaviour
In preview window I see one text with content "Test event"
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
//init coredata
let container = NSPersistentContainer(name: "Model")
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
container.loadPersistentStores() { description, error in
if let error = error {
print("Can't load core data \(error.localizedDescription)")
}
}
//adding event
let event = Event(context: container.viewContext)
event.id = UUID()
event.name = "Test"
try! container.viewContext.save()
//getting events
let request = Event.fetchRequest()
let events = try! container.viewContext.fetch(request)
//view
return VStack {
ForEach(events, id: \.id) { event in
Text("\(event.id?.uuidString ?? "") \(event.name ?? "") fs")
}
}
}
But if I try to move logic to DataController, I get 7 Text blocks with the same name, but different UUIDs
class DataController {
static let preview = DataController()
private(set) var container: NSPersistentContainer
private init(inMemory: Bool = false) {
self.container = NSPersistentContainer(name: "Model")
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
container.loadPersistentStores() { description, error in
if let error = error {
fatalError("Can't load core data \(error.localizedDescription)")
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static let dataController = DataController.preview
static var previews: some View {
//adding event
let event = Event(context: dataController.container.viewContext)
event.id = UUID()
event.name = "Test event"
try! dataController.container.viewContext.save()
//getting events
let request = Event.fetchRequest()
let events = try! dataController.container.viewContext.fetch(request)
//view
return VStack {
ForEach(events, id: \.id) { event in
Text("\(event.id?.uuidString ?? "") \(event.name ?? "")")
}
}
}
}
What can cause such behaviour?
Model for Event was manually generated. The only thing I changed – commenting #objc(Event), because with this statement preview doesn't work at all.