#0  0x00000001948b89bc in +[NSManagedObjectContext __Multithreading_Violation_AllThatIsLeftToUsIsHonor__] ()
#1  0x0000000194853724 in -[_PFBatchFaultingArray managedObjectIDAtIndex:] ()
#2  0x0000000194853600 in -[_PFMutableProxyArray newArrayFromObjectIDs] ()
#3  0x0000000194853548 in -[_PFMutableProxyArray arrayFromObjectIDs] ()
#4  0x0000000194852638 in -[NSFetchedResultsController(PrivateMethods) _managedObjectContextDidSave:] ()
#5  0x00000001924e622c in __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ ()
#6  0x00000001924e5930 in _CFXRegistrationPost ()
#7  0x00000001924e56ac in ___CFXNotificationPost_block_invoke ()
#8  0x0000000192554b9c in -[_CFXNotificationRegistrar find:object:observer:enumerator:] ()
#9  0x0000000192427bf4 in _CFXNotificationPost ()
#10 0x0000000192f2e6bc in -[NSNotificationCenter postNotificationName:object:userInfo:] ()
#11 0x00000001948c5b48 in -[NSManagedObjectContext(_NSInternalNotificationHandling) _postContextDidSaveNotificationWithUserInfo:] ()
#12 0x0000000194850350 in -[NSManagedObjectContext(_NSInternalAdditions) _didSaveChanges] ()
#13 0x000000019483ca0c in -[NSManagedObjectContext save:] ()

var initializationError: NSError?

// The persistent container (local database) for the application.
lazy var persistentContainer: NSPersistentContainer = {

    let container = NSPersistentContainer(name: self.storeName)
    container.loadPersistentStores(completionHandler: { (storeDescription, error) in
        if let error = error as NSError? {
            self.initializationError = error // This will be displayed by the AppDelegate
        }
    })
    return container
}()

// The managed object context for the main thread of the application.
lazy var mainObjectContext: NSManagedObjectContext? = {

    // Get the main thread context.
    let context = self.persistentContainer.viewContext
    context.mergePolicy = NSOverwriteMergePolicy // In-memory properties overwrite datastore properties
    context.automaticallyMergesChangesFromParent = true // Merge all changes.

    return context
}()

// Return a managed object for background threads to create and modify objects without affecting the main thread until necessary.
lazy var backgroundObjectContext: NSManagedObjectContext? = {

    // Get a background thread context.
    let context = self.persistentContainer.newBackgroundContext()
    context.mergePolicy = NSOverwriteMergePolicy // In-memory properties overwrite datastore properties
    context.automaticallyMergesChangesFromParent = true // Merge all changes.

    return context

}()

// Synchronously save any pending object changes in a managed object context to the database persistent store.
class func saveAnyChangesSync(_ context: NSManagedObjectContext) -> NSError? {

    var error: NSError?

    // If the context has changes, save them.
    if context.hasChanges {

        do {
            try context.save()

        } catch let saveError as NSError {
            error = saveError
            NSLog("CoreData error saving context (saveError)")
        }
    }
    return error
}

// When initializing:
persistentContainer.viewContext.automaticallyMergesChangesFromParent = true
 persistentContainer.viewContext.mergePolicy = NSMergePolicy.mergeByPropertyStoreTrump
// And when saving:
persistentContainer.performBackgroundTask { moc in
    moc.automaticallyMergesChangesFromParent = true
    moc.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
    autoreleasepool {
        // ... do some nsmanagedobject loading/changing
    }
    do {
        try moc.save()
    } catch {}
}