How can I track/observe all changes within a subgraph?

Question

I have a NSManagedObjectContext in which I have a number of subclasses of NSManagedObjects such that some are containers for others. What I'd like to do is watch a top-level object to be notified of any changes to any of its properties, associations, or the properties/associations of any of the objects it contains.

Using the context's 'hasChanges' doesn't give me enough granularity. The objects 'isUpdated' method only applies to the given object (and not anything in its associations). Is there a convenient (perhaps, KVO-based) was I can observe changes in a context that are limited to a subgraph?

Ctn

Answer 1

You will want to listen for the NSManagedObjectContextObjectsDidChangeNotification to pick up all changes to your data model. This can be done using code like the following:

[[NSNotificationCenter defaultCenter] 
      addObserver:self 
         selector:@selector(handleDataModelChange:) 
             name:NSManagedObjectContextObjectsDidChangeNotification 
           object:myManagedObjectContext];

which will trigger -handleDataModelChange: on any changes to the myManagedObjectContext context.

Your -handleModelDataChange: method would look something like this:

- (void)handleDataModelChange:(NSNotification *)note
{
    NSSet *updatedObjects = [[note userInfo] objectForKey:NSUpdatedObjectsKey];
    NSSet *deletedObjects = [[note userInfo] objectForKey:NSDeletedObjectsKey];
    NSSet *insertedObjects = [[note userInfo] objectForKey:NSInsertedObjectsKey];

    // Do something in response to this
}

As you can see, the notification contains information on which managed objects were updated, deleted, and inserted. From that information, you should be able to act in response to your data model changes.

Decile

Thanks! I wasn't aware of that notification. It still seems like massive overkill to have to perform a search each time the notification is sent. That said, I'm not sure how it could be optimized even by Apple; fundamentally, it's still a graph traversal. – Ctn

If you use an NSPredicate to filter through these objects it is actually quite performant because everything is in memory. I use this solution in several applications for the iPhone and there has yet to be a performance bottleneck in this area. – Fantasia

Ah, good call on using an NSPredicate. Hadn't thought of that. – Ctn

You can also use notification NSManagedObjectContextDidSaveNotification to filter the changes after each save to the context. – Repertory

Careful: The objects in the NSNotification's userInfo dictionary are actually of type NSSet and NOT NSArray! – Hope

@racha - Good catch. I'd somehow screwed that up when copying code over. I've corrected the sample code in the answer. – Decile

You didn't mention this, but I want to point out for clarity that you can not issue a save to the object using this notification. It is posted during processPendingChanges, after the changes have been processed, but before it is safe to call save:. – Irs

@Harkonian - Yes, that's a good point. Something like this should be used by a controller to update the UI in response to changes in the underlying data model. You can use it to make some changes to the model, but trying to save in here somewhere would probably be a bad idea. – Decile

@BradLarson I simply use NSTimer to delay save by a few seconds. Also allows me to cancel save in case a new change comes along, and retry saving, so the saves are somewhat grouped. – Borderer

You could also dispatch_async the save to the main queue to delay the save. – Schilit

If you are using MagicalRecord // Listen to changes in ROOT SavingContext [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleDataModelChange:) name:NSManagedObjectContextObjectsDidChangeNotification object:[NSManagedObjectContext MR_rootSavingContext]]; – Byebye

Answer 2

here's a simple example in Swift:

    NotificationCenter.default.addObserver(forName: .NSManagedObjectContextObjectsDidChange, object: nil, queue: nil) { note in
        if let updated = note.userInfo?[NSUpdatedObjectsKey] as? Set<NSManagedObject>, updated.count > 0 {
            print("updated: \(updated)")
        }

        if let deleted = note.userInfo?[NSDeletedObjectsKey] as? Set<NSManagedObject>, deleted.count > 0 {
            print("deleted: \(deleted)")
        }

        if let inserted = note.userInfo?[NSInsertedObjectsKey] as? Set<NSManagedObject>, inserted.count > 0 {
            print("inserted: \(inserted)")
        }
    }

Fonsie

Typically one needs to keep track of the return value let observation = NSNotificationCenter.defaultCenter().addObserverForName(..){..} to be able to stop observing by calling NSNotificationCenter.defaultCenter().removeObserver(observation). – Prolonge

@DanBeaulieu, if you have used the blocked based version , NSNotificationCenter.defaultCenter().addObserverForName(..){‌​..}, you need to keep hold of the returned observation. – Prolonge

@LevLandau I'll undo my edit and read up, thanks for informing – Magnetometer

Answer 3

for me it's just lost following two func, maybe this save hours for someone

func controllerWillChangeContent(controller: NSFetchedResultsController) {
    tableView.beginUpdates()
}

func controllerDidChangeContent(controller: NSFetchedResultsController) {
    tableView.endUpdates()
}

Asta

This has nothing to do with the question. – Holmquist