NSFetchedResultsController / CoreData with multiple threads

iThoms · May 2012

Hi!

I'm currently developping an application which uses CoreData and a NSFectchedResultsController. This application only contains one UITableView which uses a NSFetchedResultsController.

1/ When the application is launched, another thread is detached. In this new thread, a WS call permits to retrieve data from a web server. After the WS call, I store data in my CoreData DB with another NSManagedObjectContext (Apple's best practice: Another thread => Another context). I have to delete all objects of this entity before saving new objects. I merge this other context with the main context via mergeChangesFromContextDidSaveNotification.


// Data Manager (in another thread)
        NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
        [[NSNotificationCenter defaultCenter]   addObserver:self
                                                selector:@selector(contextDidSave:)
                                                name:NSManagedObjectContextDidSaveNotification
                                                object:context];
        [context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
        [context setPersistentStoreCoordinator:[self getPersistentStoreCoordinator]];

        ...

        for (NSManagedObject * obj in objects) 
        {
            [context deleteObject:obj];
        }

        ...

        for(NSDictionary *serverObj in serverObjects)
        {
            objAd =  [NSEntityDescription
                      insertNewObjectForEntityForName:@\"MyEntity\" 
                      inManagedObjectContext:context];
            ...
        }

        [context save:&error];

        [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:context];
        [context release];

        ...

        - (void)contextDidSave:(NSNotification *)notification
        {

            SEL selector = @selector(mergeChangesFromContextDidSaveNotification:); 
            [[self getContext] performSelectorOnMainThread:selector withObject:notification waitUntilDone:YES];
        }

        - (NSManagedObjectContext *) getContext
        {
            return [(AppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
        }

        - (NSPersistentStoreCoordinator *) getPersistentStoreCoordinator
        {
            return [(AppDelegate *)[[UIApplication sharedApplication] delegate] persistentStoreCoordinator];
        }

2/ Here is my NSFectchedResultsController's getter:


// UIView
    - (NSFetchedResultsController*) offersFRC {

        if (offersFRC == nil) 
        {
            NSManagedObjectContext *l_ManagedObjectContext = [[DataManager sharedDataManager] getContext];

            NSFetchRequest *l_FetchRequest = [[NSFetchRequest alloc] init];

            NSEntityDescription *l_Entity = [NSEntityDescription entityForName:@\"MyEntity\" inManagedObjectContext:l_ManagedObjectContext];
            [l_FetchRequest setEntity:l_Entity];

            [l_FetchRequest setFetchBatchSize:5];

            NSNumber *sortType = [self.searchCriterions objectForKey:@\"sortType\"];
            NSSortDescriptor *l_SortDescriptor = [[NSSortDescriptor alloc] initWithKey:[Constants getFieldNameBySortType:sortType] ascending:[Constants isAscendingBySortType:sortType]];
            [l_FetchRequest setSortDescriptors:[NSArray arrayWithObjects:l_SortDescriptor, nil]];
            [l_SortDescriptor release];

            NSFetchedResultsController *l_FetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:l_FetchRequest                                                                                                      managedObjectContext:l_ManagedObjectContext 
                sectionNameKeyPath:nil                                                                                                     
                cacheName:nil];
            [l_FetchRequest release];

            [self setOffersFRC:l_FetchedResultsController];
            [l_FetchedResultsController release],l_FetchedResultsController = nil;

            [self.offersFRC setDelegate:self];
        }

        return offersFRC;
    }

3/ I've got the following error when the application is launched:


2012-02-29 11:56:09.119 Nanopost[1996:207] *** Terminating app due to uncaught exception      'NSObjectInaccessibleException', reason: 'CoreData could not fulfill a fault for '0x5c3c760 <x-coredata://E176B0A1-275B-4332-9231-49FD88238C2B/Ads/p231>''
    *** Call stack at first throw:
    (
    0   CoreFoundation                      0x02bfe919 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x02e595de objc_exception_throw + 47
    2   CoreData                            0x028b833f _PFFaultHandlerLookupRow + 1407
    3   CoreData                            0x028b5ee3 _PF_FulfillDeferredFault + 499
    4   CoreData                            0x028b9f3f _sharedIMPL_pvfk_core + 95
    5   CoreData                            0x0292a010 _PF_Handler_Public_GetProperty + 160
    6   Foundation                          0x02442c4f -[NSSortDescriptor compareObject:toObject:] + 128
    7   CoreData                            0x0297db5e +[NSFetchedResultsController(PrivateMethods) _insertIndexForObject:inArray:lowIdx:highIdx:sortDescriptors:] + 286
    8   CoreData                            0x0297e1b2 -[NSFetchedResultsController(PrivateMethods) _postprocessInsertedObjects:] + 402
    9   CoreData                            0x029841bc -[NSFetchedResultsController(PrivateMethods) _managedObjectContextDidChange:] + 1804
    10  Foundation                          0x02380c1d _nsnote_callback + 145
    11  CoreFoundation                      0x02bd6cf9 __CFXNotificationPost_old + 745
    12  CoreFoundation                      0x02b5611a _CFXNotificationPostNotification + 186
    13  Foundation                          0x023767c2 -[NSNotificationCenter postNotificationName:object:userInfo:] + 134
    14  CoreData                            0x028c0519 -[NSManagedObjectContext(_NSInternalNotificationHandling) _postObjectsDidChangeNotificationWithUserInfo:] + 89
    15  CoreData                            0x028f802b -[NSManagedObjectContext mergeChangesFromContextDidSaveNotification:] + 1579
    16  Foundation                          0x02395e9a __NSThreadPerformPerform + 251
    17  CoreFoundation                      0x02bdfd7f __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
    18  CoreFoundation                      0x02b3e2cb __CFRunLoopDoSources0 + 571
    19  CoreFoundation                      0x02b3d7c6 __CFRunLoopRun + 470
    20  CoreFoundation                      0x02b3d280 CFRunLoopRunSpecific + 208
    21  CoreFoundation                      0x02b3d1a1 CFRunLoopRunInMode + 97
    22  GraphicsServices                    0x031e62c8 GSEventRunModal + 217
    23  GraphicsServices                    0x031e638d GSEventRun + 115
    24  UIKit                               0x0063cb58 UIApplicationMain + 1160
    25  Nanopost                            0x0000230a main + 170
    26  Nanopost                            0x00002255 start + 53
    )
    terminate called after throwing an instance of '_NSCoreDataException'

Important Notes:

- It crashes only on iOS4
- controllerWillChangeContent is the last function called in my code before application's crash. controllerDidChangeContent / didChangeObject/ didChangeSection are not called.
- When I comment [l_FetchRequest setFetchBatchSize:5] => No more crashes
- When I add a [context save:&error] after objects deletion and before new objects insertion => No more crashes
- When I use [l_FetchRequest setFetchBatchSize:24] => Crashes
- When I use [l_FetchRequest setFetchBatchSize:25] => No more crashes

I have spent a lot of time trying to understand this issue so Thank you very much in advance for your answers! :)

Thomas

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NSFetchedResultsController / CoreData with multiple threads