Core Data: Error when deleting/adding objects

Question

Sometimes my app crashes when I want to update my Core Data file by downloading and parsing a json file. I get the following error:

CoreData: error: Serious application error. Exception was caught during Core Data change processing. This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification. -[__NSCFSet addObject:]: attempt to insert nil with userInfo (null)

Does it matter where I save the NSManagedObjectContext within an iteration if I change properties during iteration?

here is my code:

- (void) updateData
{           
    dispatch_queue_t serialdQueue;
    serialdQueue = dispatch_queue_create("update", NULL);
    dispatch_async(serialdQueue, ^{
        [self method1];
    });
    dispatch_async(serialdQueue, ^{
        [self method2];
    });
    dispatch_async(serialdQueue, ^{
        [self method3];
    });
    dispatch_async(serialdQueue, ^{
        [self method4];
    });
    dispatch_async(serialdQueue, ^{
        [self method5];
    });

}

-(void)method1
{        
    //DOWNLOAD JSON FILE
}

-(void)method2 //here i add objects to the core data file
{        
    @try {

        for (NSDictionary *jsonActivity in [json objectForKey:@"Activities"]) {  //ITERATE THROUGH JSON ACTIVITY ARRAY

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

            NSEntityDescription *entity = [NSEntityDescription entityForName:@"Activity" inManagedObjectContext:self.managedObjectContext];
            [request setEntity:entity];

            NSPredicate *searchFilter = [NSPredicate predicateWithFormat:@"title == %@", [jsonActivity objectForKey:@"title"]]; // CHECK IF OBJECT FROM JSON FILE ALREADY EXISTS...
            [request setPredicate:searchFilter];
            NSError *error = nil;

            NSArray *results = [self.managedObjectContext executeFetchRequest:request error:&error];
            if (error) {
                NSLog(@"Error %@", error);
                abort();
            }

            if ([results count] == 0) { // ADD NEW ACTIVITY IF OLD LIST DOESNT CONTAIN IT

                Activity *activity = [NSEntityDescription insertNewObjectForEntityForName:@"Activity" inManagedObjectContext:self.managedObjectContext];

                activity.title = [jsonActivity objectForKey:@"title"];

                activity.remove = [NSNumber numberWithBool:NO]; // REMOVE FLAG = NO BECAUSE NEW OBJECTS AREN'T REMOVED

            } else  {

                Activity *activity = (Activity*) [results objectAtIndex:0];
                activity.remove = [NSNumber numberWithBool:NO]; // IF OBJECT ALREADY EXISTS IT SHOULD BE OBTAINED
            }

            AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
            [appDelegate saveContext]; // SAVE MO CONTEXT
        }

    }   @catch (NSException *exception) {

        NSLog(@"Exception: %@", exception);
    }
}

-(void)method3 // DELETE OLD OBJECTS 
{   
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Activity" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entityDescription];

    NSPredicate *searchFilter = [NSPredicate predicateWithFormat:@"remove == %@", [NSNumber numberWithBool:YES]];
    [fetchRequest setPredicate:searchFilter];

    NSArray *objectsToDelete = [[NSArray alloc] init];

    NSError *error = nil;
    objectsToDelete = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
    if (error) {
        NSLog(@"Error %@", error);
        abort();
    }

    for (Activity *activity in objectsToDelete) { // DELETE OBJECTS WITH THE PROPERTY REMOVE = YES

        [self.managedObjectContext deleteObject:activity]; // DELETE ACTIVITY
    }

    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    [appDelegate saveContext]; // SAVE MO CONTEXT
}

-(void)method4 // CHANGE THE REMOVE PROPERTY TO YES OF ALL OBJECTS
{        
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Activity" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entityDescription];

    NSArray *objects = [[NSArray alloc] init];

    NSError *error = nil;
    objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
    if (error) {
        NSLog(@"Error %@", error);
        abort();
    }

    for (Activity *activity in objects) {

        activity.remove = [NSNumber numberWithBool:YES];
    }

    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    [appDelegate saveContext]; // SAVE MO CONTEXT

    NSLog(@"End Update");
}

-(void)method5 //UPDATE UI
{
    //UI UPDATES
}

Answer 1

You shouldn't be accessing your managed object context on serialQueue. Take a look at the Concurrency section of the NSManagedObjectContext documentation.

If in your code your context is using NSPrivateQueueConcurrencyType or NSMainQueueConcurrencyType concurrency type, you can use one of the block-based methods to ensure you're on the right queue:

//asyncrhonous
[self.managedObjectContext performBlock:^{
    //do stuff with the context
}];

//syncrhonous
[self.managedObjectContext performBlockAndWait:^{
    //do stuff with the context
}];