How do I tell a (managed) object to notify its KVOs that one of its properties needs to be recached?

Question

When we have an object who has a property that's generated based on other properties, usually we implement the +keyPathsForValuesAffecting{PropertyName} class method.

What I'm trying to do is basically the same thing for a property on my NSManagedObject, but traversing a relationship.

My model is simple; I have two Entities, App and Version (I'm creating an appcast-generating app). When the App's properties are changed, because I implemented the method above, the -appcast string is changed, and all bindings are updated appropriately.

However, when any properties on any of a specific App's Versions (to-many relationship) change, the -appcast property is not generated appropriately. I can haz fix/workaround?

Answer 1

This is a bit of a late answer, but I think it's a common situation, and the answer definitely isn't readily-apparent.

I generally watch for the managedObjectContext to change and then check if the any of the changed objects are ones that I want to look out for. So, in your NSManagedObject subclass:

// We need to register for the notification in both awakeFromFetch
// AND awakeFromInsert, since either one could be called, depending on
// if the object was previously-created or not
- (void)awakeFromFetch {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(managedObjectContextDidChange:) name: NSManagedObjectContextObjectsDidChangeNotification object:[self managedObjectContext]];
}

- (void)awakeFromInsert {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(managedObjectContextDidChange:) name: NSManagedObjectContextObjectsDidChangeNotification object:[self managedObjectContext]];
}

- (void)managedObjectContextDidChange:(NSNotification *)notification {
    // Get a set containing ALL objects which have been changed
    NSSet *insertedObjects = [[notification userInfo] objectForKey:NSInsertedObjectsKey];
    NSSet *updatedObjects = [[notification userInfo] objectForKey:NSUpdatedObjectsKey];
    NSSet *deletedObjects = [[notification userInfo] objectForKey:NSDeletedObjectsKey];

    NSSet *changedObjects = [insertedObjects setByAddingObjectsFromSet:updatedObjects];
    changedObjects = [changedObjects setByAddingObjectsFromSet:deletedObjects];

    if ([changedObjects intersectsSet:[self versions]]) {
        [self willChangeValueForKey:@"appCast"];
        [self didChangeValueForKey:@"appCast"];
    }
}

This is certainly not ideal from a performance perspective, since this notification is going to fire every time anything in your object graph changes, but I've found it to be the most straightforward way to accomplish this.