Observing NSArrayController changes: detect insertion or deletion

Question

I have an NSArrayController that is bound to an entity "Address" in my Core Data model. I want to monitor which new objects are inserted into this Address entity, or which existing objects are deleted. I'm trying to do this through KVO, but I'm not sure how I should go about this.

In my awakeFromNib, I add the view controller class as the observer for "arrangedObjects":

[self.addressArrayController addObserver:self
                                    forKeyPath:@"arrangedObjects"
                                       options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
                                       context:@"Address"];

and I do get the observer notifications in this:

- (void)observeValueForKeyPath:(NSString *)keyPath
                  ofObject:(id)object
                    change:(NSDictionary *)change
                   context:(void *)context {

   if( object == self.addressArrayController) {
      // what do do here?
   }
}

The problem is that the change dictionary is always nil (due to some long-standing Apple bug, I believe), so I have no idea which object was added or deleted, or even something was added or deleted! And I do need the exact object that was added or deleted, ideally without having to traverse all the objects for this entity and trying to figure out based on the object's timestamp or any other criterion.

What is the best way to do this?

In this case, you mean that the `new` and `old` lists are `NSNull`, right? I'm not whether this is a bug, but it's certainly a behavior that is long-standing (10.4 at least) and is under-documented. However, even if the dictionaries were not empty, they'd be useless for this purpose, since I've never seen a notification on `arrangedObjects` that wasn't `NSKeyValueChangeSetting`, for which `new` and `old` are just copies of the array, so you'd have to do a full difference traversal anyway. Since you're using Core Data, have you tried `NSManagedObjectContextObjectsDidChangeNotification`? — gaige, May 23 '14 at 08:30
Thanks. It makes a lot sense to use the context notifications in this case. — Z S, May 23 '14 at 18:31

score 4 · Accepted Answer · edited May 23 '17 at 10:32

Adapted from my answer here, which lays out the code for what gaige suggests in the comments.

Register for NSManagedObjectContextObjectsDidChangeNotification:

[[NSNotificationCenter defaultCenter] addObserver: self
                                         selector: @selector(mocDidChangeNotification:)
                              name:NSManagedObjectContextObjectsDidChangeNotification
                                           object: nil];

And parse the userInfo dictionary in the corresponding method:

- (void)mocDidChangeNotification:(NSNotification *)notification
{
    NSManagedObjectContext* savedContext = [notification object];

    // Ignore change notifications for anything but the mainQueue MOC
    if (savedContext != self.managedObjectContext) {
        return;
    }

    // Ignore updates -- lots of noise from maintaining user-irrelevant data

    // Log insertion
    for (NSManagedObject* insertedObject in 
           [notification.userInfo valueForKeyPath:NSInsertedObjectsKey])
    {
        if ([insertedObject isKindOfClass:[Address class]] )
        {
            NSLog(@"Inserted Address");
        }
    }   

    // Log deletion
    for (NSManagedObject* deletedObject in 
           [notification.userInfo valueForKeyPath:NSDeletedObjectsKey])
    {
        if ([deletedObject isKindOfClass:[Address class]] )
        {
            NSLog(@"Deleted Address");
        }
    }

}

How would this work if the model is not stored in CoreData? – Etan Nov 13 '15 at 10:08 — Etan, Nov 13 '15 at 10:08

Observing NSArrayController changes: detect insertion or deletion

1 Answers1