Why is my core data not updating? - iphone

Question

I update core data in a background thread, like so:
entry.message = [self contentForNoteWithEDML:note.content];
entry.dataLastModified = [NSDate date];
[entry.managedObjectContext save:nil];
dispatch_async(dispatch_get_main_queue(), ^{
NSError *error;
if (![[self fetchedResultsController] performFetch:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
}
[self.tableView reloadData];
});
On each cell of the tableview, it displays a different entry from the fetchedResultsController. On the main thread, I do an NSLog in cellForRowAtIndexPath on the dataLastModified date, and the date doesn't change to the most recent value. If I close the app and run it again, it updates the contents of the cell and the dataLastModified date changes to the correct value.
It seems to be changing the data, as required, but my tableview isn't seeing the changes until the app is restarted. Any ideas why?
EDIT: Doing the NSLog in cellForRowAtIndexPath on a background thread gives the the correct data, but doing it on the main thread does not.
EDIT 2: How my background context works:
NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
[notificationCenter
addObserver:[AppDelegate applicationDelegate].coreDataManager
selector:#selector(mergeChanges:)
name:NSManagedObjectContextDidSaveNotification
object:[AppDelegate applicationDelegate].coreDataManager.managedObjectContext];
NSPersistentStoreCoordinator *journalDataPSC = [AppDelegate applicationDelegate].coreDataManager.managedObjectContext.persistentStoreCoordinator;
dispatch_queue_t addOrUpdateEntriesQueue = dispatch_queue_create("com.App.AddOrUpdateEntries", NULL);
dispatch_async(addOrUpdateEntriesQueue, ^{
NSManagedObjectContext *journalDataMOC = [[NSManagedObjectContext alloc] init];
[journalDataMOC setPersistentStoreCoordinator:journalDataPSC];
//Some code to get me an entry on this context
entry.message = [self contentForNoteWithEDML:note.content];
entry.dataLastModified = [NSDate date];
[entry.managedObjectContext save:nil];
[[NSNotificationCenter defaultCenter] removeObserver:[AppDelegate applicationDelegate].coreDataManager];
dispatch_async(dispatch_get_main_queue(), ^{
NSError *error;
if (![[self fetchedResultsController] performFetch:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
}
[self.tableView reloadData];
});
});
dispatch_release(addOrUpdateEntriesQueue);

Answer 1

Make sure you're using MOC's correctly, they are not thread-safe and can only be used in the thread they were created in. In this case, if you're doing it right enter.managedObjectContext is different from the MOC of the fetched results controller (which is in the main thread).
That means saves in the background do not necessarily get propagated to he main thread MOC. Make sure you handle NSManagedObjectContextDidSaveNotification in the main thread by adding an observer when you create the fetched results controller.
Looking at your code here are a few points that I notice:
You should register for the save notification thrown by the background MOC not the main thread MOC.
You should initialize your background MOC with initWithConcurrencyType: using NSPrivateQueueConcurrencyType
Once you're using NSPrivateQueueConcurrencyType it's much better to use performBlock: to make your changes and save instead of using low-level GCD dispatch methods.

Answer 2

Even though I don't know anything about your coreDataManager object your notification registration is wrong. The object you want to observe is your background managed object context journalDataMOC, not your coreDataManager.
So this should work, but notice that you have to move the registration inside your addOrUpdateEntriesQueue:
[notificationCenter
addObserver:[AppDelegate applicationDelegate].coreDataManager
selector:#selector(mergeChanges:)
name:NSManagedObjectContextDidSaveNotification
object:journalDataMOC];
But all in all you should use the CoreData API to do this work (like Engin said) as it is much cleaner. So remove all the GCD and notification stuff and use this snippet (not tested):
NSManagedObjectContext *mainContext = [[[AppDelegate applicationDelegate] coreDataManager] managedObjectContext];
NSManagedObjectContext *journalDataMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[journalDataMOC setParentContext:mainContext];
[journaDataMOC performBlock:^{
//Some code to get me an entry on this context
entry.message = [self contentForNoteWithEDML:note.content];
entry.dataLastModified = [NSDate date];
[journalDataMOC save:nil];
[NSOperationQueue mainQueue] addOperationWithBlock:^{
NSError *error;
[mainContext save:&error];
if (![[self fetchedResultsController] performFetch:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
}
[self.tableView reloadData];
}];
}];
When the journalDataMOC saves it will "push" the changes up to the paren context (mainContext).
Note that your [[[AppDelegate applicationDelegate] coreDataManager] managedObjectContext] has to be initialized with type NSMainQueueConcurrencyType. Also note that you have to save your mainContext here or at some time in the future to persist your changes to the database.