How to save to managed object context in a background thread in Core Data

Question

I have an app which takes about a minute to set up, so when the user taps ‘Start New Game’ I want to show an activity spinner while data is loaded into Core Data.

I understand I have to do this on a background thread so I can update the UI on the main thread, but I don’t know how to save the managed context in the background thread. Here is what I have so far:

func startNewGame() {
  initiateProgressIndicator() // start the spinner and 'please wait' message

  DispatchQueue.global(qos: .background).async {
    self.coreDataStack.importDefaultData() // set up the database for the new game

    DispatchQueue.main.async {
        stopProgressIndicator()

        // Transition to the next screen
        let vc: IntroViewController = self.storyboard?.instantiateViewController(withIdentifier: "IntroScreen") as! IntroViewController
        vc.rootVCReference = self
        vc.coreDataStack = self.coreDataStack
        self.present(vc, animated:true, completion:nil)
    }
}

In importDefaultData() I need to save multiple times but it crashes when I try to do so. I understand now it's because I'm trying to access the main context from a background thread. Here is the basic structure of the function:

func importDefaultData() {
    // import data into Core Data here, code not shown for brevity

    saveContext()

    // import more data into Core Data here, code not shown for brevity

    saveContext()

    // import final data here

    saveContext()
}

From what I have read I think I need to create another managed object context for the background thread, but I am unclear on how to go about this and how to fit it into my current code. I also do not know how to make sure the data is saved correctly before I move on to the next screen. I have no experience with multithreading so any help would be appreciated.

Edit: regarding being marked as duplicate, the suggested topic is 8 years old and doesn't contain any answers. I was hoping for a more up to date answer with perhaps a brief code example in Swift.

Answer 1

I have solved this after reading the helpful comments from @user1046037 (thanks!) and researching how to use context.perform { } and context.performAndWait { }. I will post my code below in case it benefits anyone else, as I couldn't find any examples on SO in swift:

initiateProgressIndicator() // start the spinner and 'please wait' message

let childContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
childContext.parent = coreDataStack.managedContext

// Create a background task
childContext.perform {
  // Perform tasks in a background queue
  self.coreDataStack.importDefaultData(childContext: childContext) // set up the database for the new game

  do {
    // Saves the tasks done in the background to the child context
    try childContext.save()

    // Performs a task in the main queue and wait until this tasks finishes
    self.coreDataStack.managedContext.performAndWait {
        do {
            // Saves the data from the child to the main context
            try self.coreDataStack.managedContext.save()

            self.activitySpinner.stopAnimating()

            // Transition to the next screen
            let vc: IntroViewController = self.storyboard?.instantiateViewController(withIdentifier: "IntroScreen") as! IntroViewController
            vc.rootVCReference = self
            vc.coreDataStack = self.coreDataStack
            self.present(vc, animated:true, completion:nil)

        } catch {
            fatalError("Failure to save context: \(error)")
        }
    }
  } catch {
    fatalError("Failure to save context: \(error)")
  }
}

In my CoreDataStack class:

func importDefaultData(childContext: NSManagedObjectContext) {
  // import data into Core Data here, code not shown for brevity

  saveChildContext(childContext: childContext)

  // import more data into Core Data here, code not shown for brevity

  saveChildContext(childContext: childContext)

  // import final data here

  saveChildContext(childContext: childContext)
}

func saveChildContext(childContext: NSManagedObjectContext) {
    guard childContext.hasChanges else {
        return
    }

    do {
        try childContext.save()
    } catch {
        let nserror = error as NSError
    }
}

If this approach is not best practise or anyone can see a way to improve it, I'd appreciate your thoughts. I should add that I found the following link very helpful: https://marcosantadev.com/coredata_crud_concurrency_swift_1