// Within the init: Create a background context child of main context
   backgroundContext = NSManagedObjectContext(concurrencyType: .

// MARK:  - Batch processing in the background
extension CoreDataStack{
    typealias Batch=(workerContext: NSManagedObjectContext) -> ()

    func performBackgroundBatchOperation(batch: Batch){

        backgroundContext.performBlock(){
            batch(workerContext: self.backgroundContext)

            // Save it to the parent context, so normal saving
            // can work
            do{
                try self.backgroundContext.save()
            }catch{
                fatalError("Error while saving backgroundContext: \(error)")
            }
        }
    }
}