Updating Core Data Database iniciated of publisher - swiftui

Question

I would like to use Contacts in an app. The user should select them and I will store the name and the id in the Core Data database.
When the Contact gets edited from the iOS Contacts app I would like to update the Core Data entities. When doing this I get the purple warning: "This method should not be called on the main thread as it may lead to UI unresponsiveness." (see comment in code) when fetching the contacts.
How to use the right threads to do it how it should be done? Im not really familiar with the best practices. Do I have to go back to the main thread for UI related things or is this not necessary here because of the #FetchRequest in other parts of the app where the saved contacts from Core Data are used?
class KontakteUpdater{
func update(moc: NSManagedObjectContext){
var contacts: [CNContact] = []
let key = [CNContactGivenNameKey,CNContactFamilyNameKey] as [CNKeyDescriptor]
let Kontakt_request = CNContactFetchRequest(keysToFetch: key)
// get contacts from the store
do{
try CNContactStore().enumerateContacts(with: Kontakt_request, usingBlock: { (contact, stoppingPointer) in // here with warning
contacts.append(contact)
})
}
catch
{
print(error)
}
// fetch currently saved contacts from core data
var kontakteUpdate: [Kontakt] = []
let request = NSFetchRequest<Kontakt>(entityName: "Kontakt")
do{
kontakteUpdate = try moc.fetch(request)
}
catch let error{
print("Error fetching. \(error)")
}
// update saved contacts in core data
for k in kontakteUpdate{
if let c = contacts.first(where: {contact in
contact.identifier == k.id
}){
k.vorname = c.givenName
k.nachname = c.familyName
}
}
do{
try moc.save()
}
catch{
print("\(error) - Saving didn't succeed")
}
}
}
Here is the entry point of the sample app:
import SwiftUI
import CoreData
#main
struct testApp: App {
let pub = NotificationCenter.default
.publisher(for: NSNotification.Name.CNContactStoreDidChange)
let kontakteUpdater = KontakteUpdater()
let persistenceContainer = PersistenceController.shared
var body: some Scene {
WindowGroup {
ContentView()
.onReceive(pub, perform: {output in
kontakteUpdater.update(moc: persistenceContainer.container.viewContext)
})
.environment(\.managedObjectContext, persistenceContainer.container.viewContext)
}
}
}