How be notified only when CoreData object(s) change? - ios

Question

Each time I synchronise with my server, I refresh all fields of my CoreData objects.
Even if nothing changes I still receive NSManagedObjectContextObjectsDidChangeNotification elsewhere in my code.
Is this expected behaviour?
How can I prevent this and only be notified when something has actually changed?
When I add below code after refreshing an object, things work as I want. But why can't CoreData sort this out?
if (object.changedValues.count == 0)
{
[object.managedObjectContext refreshObject:object mergeChanges:NO];
}

Answer 1

Yes, even though values aren't changing new writes are still happening in Managed Object Context. So, NSManagedObjectContextObjectsDidChangeNotification are expected to be fired.
Consider using a NSFetchedResultsController as:
#interface CoreDataHelper : NSObject < NSFetchedResultsControllerDelegate >
#property (nonatomic, readonly) NSManagedObjectContext *context;
+ (instancetype)sharedInstance;
#end
#implementation
#synthesize context = _context;
+ (instancetype)sharedInstance{
static dispatch_once_t once;
static CoreDataHelper *sharedInstance;
dispatch_once(&once, ^{
sharedInstance = [self new];
});
return sharedInstance;
}
- (NSManagedObjectContext *)context{
if (_context != nil) {
return _context;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator]; // you know what goes in PSC here, right?
if (!coordinator) {
NSLOG(#"Argh! coordinator == nil");
return nil;
}
_context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_context setPersistentStoreCoordinator:coordinator];
return _context;
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller{
/*
This function will be invoked by the OS whenever a Managed Object changes.
Keep in mind that this will be invoked on the Managed Object Context Queue.
This is the place where your logic to be performed whenever a object changes needs to be.
*/
}
#end
// Now in your file
NSFetchedResultsController *fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[CoreDataHelper sharedInstance].context sectionNameKeyPath:nil cacheName:nil];
fetchedResultsController.delegate = [CoreDataHelper sharedInstance];
NSError *error;
if (![fetchedResultsController performFetch:&error]){
NSLOG(#"fetch error %#", error);
}
Now, since we have hooked up self as delegate of fetchedResultsController, self needs to conform to the NSFetchedResultsControllerDelegate protocol.
You choose to override any of the optional methods, most straight-forward being:
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller