DispatchQueue.global(qos: .background).async {
  let context = NSManagedObjectContext.getMainContext() // made up method
  context.fetch(/* ... */)
}

DispatchQueue.global(qos: .background).async {
  let context = NSManagedObjectContext.createPrivateContextAndAssignParent() // made up method
  context.performAndWait {
    context.fetch(/* ... */)
  }
}

DispatchQueue.global(qos: .background).async {
  NSManagedObjectContext.getMainContext()
  context.performAndWait {
    context.fetch(/* ... */)
  }
}