CoreData discard changes

ProfileViewController.swift

import UIKit

class ProfileViewController: UIViewController {

    /// First create a throwable child context for editing the Profile
    private lazy var writeContext: NSManagedObjectContext = {
        let result = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        result.automaticallyMergesChangesFromParent = true
        result.performAndWait {
            // Our main parent context
            result.parent = DataSource.shared.mainContext
        }
        return result
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        title = "My Profile"
        // Let's add Cancel and Save buttons
        navigationItem.leftBarButtonItem = UIBarButtonItem(barButtonSystemItem: .cancel, target: self, action: #selector(promptToCancel(_:)))
        navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .save, target: self, action: #selector(save(_:)))
    }

    /// Present an alert asking the user to confirm discarding his changes
    @objc func promptToCancel(_ sender: UIBarButtonItem?) {
        guard writeContext.hasChanges else {
            closeAndDiscard()
            return
        }

        let alert = UIAlertController(title: "Do you want to leave and discard your changes?", message: nil, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "Stay", style: .default, handler: nil))
        alert.addAction(UIAlertAction(title: "Discard", style: .destructive) { [weak self] _ in
            guard let self = self else { return }
            self.closeAndDiscard()
        })

        present(alert, animated: true, completion: nil)
    }

    // Cancel is confirmed:
    private func closeAndDiscard() {
        // Return our context to the previous state,
        // by discarding all unsaved changes
        writeContext.rollback()
        dismiss(animated: true, completion: nil)
    }

    @objc func save(_ sender: UIBarButtonItem?) {
        // No need to save if there are no changes
        guard writeContext.hasChanges else {
            dismiss(animated: true, completion: nil)
        }

        // Sync save our changes
        writeContext.performAndWait { [weak self] in
            guard let self = self else { return }

            do {
                try writeContext.save()
                self.didSaveAndClose()
            } catch {
                print("failed to sync save \(error)")
                // Show the error to the user
            }
        }
    }

    private func didSaveAndClose() {
        // Parent context should be observing these
        NotificationCenter.default.post(name: .NSManagedObjectContextDidSave, object: writeContext)
        dismiss(animated: true, completion: nil)
    }
}