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I would like to determine if an attribute changed in core data and update my UI. The attribute may change as the result of a background fetch running in a managed object context of type NSPrivateQueueConcurrencyType.

I added a listener: [myCoreDataEntity addObserver:self forKeyPath:myCoreDataAttribute options:NSKeyValueObservingOptionNew context:nil]

But the event never fires. Any idea why? The object is changed in another context - could this be the reason? (When a save: is done on the parent context, nothing still fires).

I can use manual KVO, but since the object has not been saved yet to the parent context, refreshing the UI does not work since it is point at the context in NSMainQueueConcurrencyType when the object was changed in a context associated with NSPrivateQueueConcurrencyType

Paul Heller
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2 Answers2

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You're not getting notifications because you're observing the wrong object. The NSEntityDescription never changes at run time. It's a representation of the way the entity was defined in the data model. Instances of NSManagedObject that use the entity description can and do change at run time, though. If you want to know whether an attribute changed on a specific managed object, you need to observe that specific object.

If you need to get notified any time any managed object changes the value for that attribute, your best option is to write a custom setter for that attribute and handle it there. You might also find NSManagedObjectContextObjectsDidChangeNotification useful, but that will fire for any attribute change.

Tom Harrington
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  • Thanks, Tom. My description was not clear enough. I wrote myCoreDataEntity, but meant to write myManagedObject. Shame on me! The events don't fire for NSManagedObject property updates either. A custom setter is a good way to go. Thanks for the tip. – Paul Heller Jan 08 '13 at 15:51
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Assuming that myCoreDataEntity in your example is a managed object, the issue is that automatic external change notifications are [disabled by Core Data for managed objects] for modeled properties1:

NSManagedObject disables automatic key-value observing (KVO) change notifications for modeled properties, and the primitive accessor methods do not invoke the access and change notification methods. For unmodeled properties, on OS X v10.4 Core Data also disables automatic KVO; on OS X v10.5 and later, Core Data adopts to NSObject’s behavior.

You can turn them on for specific properties, or for all properties in your managed object subclass or in a category on the managed object subclass:

Single property:

- (BOOL) automaticallyNotifiesObserversFoMyCoreDataAttribute {
    return YES;
}

All properties (Not recommended):

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)theKey {
    return YES;
}

The reason it does not send out automatic change notifications is primarily performance. Automatic change notifications do add some overhead, though on recent hardware it's fairly minor, even when working with thousands of objects. As always, profile to see what works for you.

quellish
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  • This is not very clear but although it is correct that NSManagedObject disables automatic KVO, in NSManagedObject's dynamic methods for modeled properties it manually calls willChangeValueForKey, didChangeValueForKey so using KVO on modeled properties works fine as is. More info in this answer: https://stackoverflow.com/a/3728567/259521 – malhal Mar 02 '20 at 12:26