Synchronizing Main and Background Core Data Threads (Swift 3)

Let’s say we have two different managedObjectContext (with one persistentStoreCoordinator). 

The first one is used across the app for most quick fetches:

var mainManagedObjectContext = NSManagedObjectContext(concurrencyType: NSManagedObjectContextConcurrencyType.mainQueueConcurrencyType)


        mainManagedObjectContext.persistentStoreCoordinator = coordinator



And the second, running in the background, for the queries that take a long time:

var backgroundManagedObjectContext = NSManagedObjectContext(concurrencyType: NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType)

        backgroundManagedObjectContext.persistentStoreCoordinator = coordinator


When they are initialized both contain the same data. However once changes are applied to one of them -in this example the main- like adding, changing or deleting rows, the other will not know -in this example the background/private one-.


Screen Shot 2016 11 27 at 13 19 12


Therefore if we add objects to the main managedObjectContext, when we fetch the backgroundManagedObjectContext we will not get them. 

How do we fix this? Well first we have to save the changes of the objectContext that was modified. We do so by adding once we’ve finalized the changes:

       //Commit changes

        do {



            print(“ERROR: Cound not save Core Data”)



At this point we have saved the changes in the Persistent Store. We still have to ask the backgroundObjectContext to get them from the persistent store thought. For that we will use the automatic notification “NSManagedObjectContextDidSave”.

In the class where the backgroundThread is contained we will add an observer for the notification (note that we’re passing as object the context that was modified):

//Handle changes in core data for background thread

        NotificationCenter.default.addObserver(self, selector: #selector(AppDelegate.handleMainCoreDataChangeInBackgroundManagedContext(notification:)), name: NSNotification.Name.NSManagedObjectContextDidSave, object: managedObjectContext)


And then we will add the function that will syncronize the backgroundThread, in this example: 

func handleMainCoreDataChangeInBackgroundManagedContext(notification: Notification){

        managedObjectContextBackgroundThread?.mergeChanges(fromContextDidSave: notification)


That’s it! Now when you query the background thread you’ll get the same information as you have in the main one. 

You can read more about NSManagedObjectContext in Apple Docs


Questions / comments? I’m at @MarcMasVi 





Note: using two contexts is very powerful when used in combination with dispatchqueues {

            //Query here in background context & save if applicable

            DispatchQueue.main.async {

                //Refresh UI & query main context & save if applicable




Are we (NSViewController view) the First Responder?

In some cases a class may want to know if it’s the first responder before triggering an action.

For instance if several loaded View Controller classes are listening for the same notification, how can they know which one should trigger the action? Well… they should ask their view. 

In this case a NSTableView class is asking if it’s currently the first responder:

        if (self.view.window?.firstResponder == self.tableView){



And that’s it 🙂

Questions / comments? I’m at @MarcMasVi 


Undo functionality in Core Data (Swift 3)

If you’re developing a Core Data based application for Mac, wouldn’t it be great if you could add undo support?

Well, turns out that is a couple of lines away. Follow the simple steps:

1. Make sure your managedObjectContext has an undo manager, without it Core Data can’t keep track of the changes:

         managedObjectContext?.undoManager = UndoManager() 

2. In the viewController where you should handle the undo operation just add:

      func undo(_ sender: AnyObject?){




        refreshTable() //If applicable (for instance if you don’t use bindings)

        re-fetch the information and reload the table. This is what this

        function does in my code



And that’s it, now you have full undo functionality! You’ll see how in the menus Undo no longer appears greyed out.

Questions / comments? I’m at @MarcMasVi 


Sorting Multi Dimensional Array (Swift 3)

Hello all, 

This one I’ve found specially useful when working with Core Data fetched arrays. 

Let’s say that we want to sort the result of a fetch request based on the value of the field “gold”, we would therefore do the following:

let dwarfGoldBags = … //Contains the result of a fetch request with one of the fields named “gold” and being an integer.


We will now proceed to sort the array and store it in sortedGoldArray:

let sortedGoldArray = dwarfGoldBags.sorted { ($0.goldas Int) > ($1.goldas Int) } 


If instead of core data you have a multi-dimensional array it would be as easy as:

let sortArray = originalArray.sorted { ($0.[0] as Int) > ($1.[0] as Int) } 


In this case we’re comparing the values stored in the first value of the subArrays. 

Questions / comments? I’m at @MarcMasVi