KVO not called when merging changes from background context - ios

Question

I've subclassed NSManagedObject to add some properties that deal with ordering the object's relationship sets, this ordering is then stored in a property. I use KVO to determine when the original set changes and nil the stored property so that it orders using the updated set's data.
However the issue I'm having is that the changes to these sets are performed in a background context and then merged back into the main context which updates the NSManagedObject subclass but without firing the KVO observers that I've set up. Which in turns means that I don't nil the stored property.
How can I tell when an NSManagedObject subclass has been updated via a merge?
Below is my KVO structure inside the subclass:
#pragma mark - Awake
- (void)awakeFromFetch
{
[super awakeFromFetch];
/*-------------------------------*/
[self addObserver:self
forKeyPath:#"comments"
options:NSKeyValueObservingOptionNew
context:NULL];
}
#pragma mark - KVO
- (void) observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
if ([object isEqual:self])
{
if ([keyPath isEqualToString:#"comments"])
{
_orderedComments = nil;
}
}
}
#pragma mark - OrderedComments
- (NSArray *)orderedComments
{
if (!_orderedComments)
{
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"dateCreated"
ascending:NO];
NSArray *sortDescriptors = #[sortDescriptor];
_orderedComments = [self.comments sortedArrayUsingDescriptors:sortDescriptors];
}
return _orderedComments;
}
Additional information:
I'm using the parent-child pattern for controlling my NSManagedObjectContexts so when I create the main context:
_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
and in each NSOperation context I create the child/background context:
_localManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];
[_localManagedObjectContext setParentContext:[CDSServiceManager managedObjectContext]];
[_localManagedObjectContext setUndoManager:nil];
[_localManagedObjectContext setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
(CDSServiceManager is a helper class to allow access to the main context)
I use the following to control merging:
- (void) saveLocalContextChangesToMainContextWithSuccess:(void (^)(void))success
failure:(void (^)(NSError *error))failure
{
NSError *localManagedObjectContentSaveError = nil;
if (![self.localManagedObjectContext save:&localManagedObjectContentSaveError])
{
DLog(#"Error to saving local context: %#", [localManagedObjectContentSaveError userInfo]);
if (failure)
{
failure(localManagedObjectContentSaveError);
}
}
else
{
[[CDSServiceManager managedObjectContext] performBlock:^
{
NSError *mergeSaveError = nil;
if (![[CDSServiceManager managedObjectContext] save:&mergeSaveError])
{
DLog(#"Error saving main context: %#", [mergeSaveError userInfo]);
if (failure)
{
failure(mergeSaveError);
}
}
else
{
if (success)
{
success();
}
}
}];
}
}
This is called from the NSOperation subclass.