Question

I’ve attempted to add the TopSongs parser and Core Data files into my application, and it now builds succesfully, with no errors or warning messages. However, as soon as the app loads, it crashes, giving the following reason:

UPDATE: I’ve got it all working, but my TableView doesn’t show any data, and the app doesn’t respond to the following breakpoints.

Thanks.

UPDATE: Here’s the new code that doesn’t respond to the breakpoints.

- (NSInteger)numberOfSectionsInTableView:(UITableView *)table {
    return [[fetchedResultsController sections] count];
}

- (NSInteger)tableView:(UITableView *)table numberOfRowsInSection:(NSInteger)section {
    id <NSFetchedResultsSectionInfo> sectionInfo = [[fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (void)viewDidUnload {
    [super viewDidUnload];
    self.tableView = nil;
    [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];
    [self.tableView reloadData]; 
}

- (UITableViewCell *)tableView:(UITableView *)table cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *kCellIdentifier = @"SongCell";
    UITableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:kCellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:kCellIdentifier] autorelease];
        cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
        cell.textLabel.font = [UIFont boldSystemFontOfSize:14];
    }
    Incident *incident = [fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = [NSString stringWithFormat:NSLocalizedString(@"#%d %@", @"#%d %@"), incident.title];
    return cell;
}

- (void)tableView:(UITableView *)table didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    [table deselectRowAtIndexPath:indexPath animated:YES];
    self.detailController.incident = [fetchedResultsController objectAtIndexPath:indexPath];
    [self.navigationController pushViewController:self.detailController animated:YES];
}

UPDATE: Here’s the code where all instances of fetch are found.

- (Category *)categoryWithName:(NSString *)name {
    NSTimeInterval before = [NSDate timeIntervalSinceReferenceDate];
#ifdef USE_CACHING
    // check cache
    CacheNode *cacheNode = [cache objectForKey:name];
    if (cacheNode != nil) {
        // cache hit, update access counter
        cacheNode.accessCounter = accessCounter++;
        Category *category = (Category *)[managedObjectContext objectWithID:cacheNode.objectID];
        totalCacheHitCost += ([NSDate timeIntervalSinceReferenceDate] - before);
        cacheHitCount++;
        return category;
    }
#endif
    // cache missed, fetch from store - if not found in store there is no category object for the name and we must create one
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:self.categoryEntityDescription];
    NSPredicate *predicate = [self.categoryNamePredicateTemplate predicateWithSubstitutionVariables:[NSDictionary dictionaryWithObject:name forKey:kCategoryNameSubstitutionVariable]];
    [fetchRequest setPredicate:predicate];
    NSError *error = nil;
    NSArray *fetchResults = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
    [fetchRequest release];
    NSAssert1(fetchResults != nil, @"Unhandled error executing fetch request in import thread: %@", [error localizedDescription]);

    Category *category = nil;
    if ([fetchResults count] > 0) {
        // get category from fetch
        category = [fetchResults objectAtIndex:0];
    } else if ([fetchResults count] == 0) {
        // category not in store, must create a new category object 
        category = [[Category alloc] initWithEntity:self.categoryEntityDescription insertIntoManagedObjectContext:managedObjectContext];
        category.name = name;
        [category autorelease];
    }
#ifdef USE_CACHING
    // add to cache
    // first check to see if cache is full
    if ([cache count] >= cacheSize) {
        // evict least recently used (LRU) item from cache
        NSUInteger oldestAccessCount = UINT_MAX;
        NSString *key = nil, *keyOfOldestCacheNode = nil;
        for (key in cache) {
            CacheNode *tmpNode = [cache objectForKey:key];
            if (tmpNode.accessCounter < oldestAccessCount) {
                oldestAccessCount = tmpNode.accessCounter;
                [keyOfOldestCacheNode release];
                keyOfOldestCacheNode = [key retain];
            }
        }
        // retain the cache node for reuse
        cacheNode = [[cache objectForKey:keyOfOldestCacheNode] retain];
        // remove from the cache
        [cache removeObjectForKey:keyOfOldestCacheNode];
    } else {
        // create a new cache node
        cacheNode = [[CacheNode alloc] init];
    }
    cacheNode.objectID = [category objectID];
    cacheNode.accessCounter = accessCounter++;
    [cache setObject:cacheNode forKey:name];
    [cacheNode release];
#endif
    totalCacheMissCost += ([NSDate timeIntervalSinceReferenceDate] - before);
    cacheMissCount++;
    return category;
}

And this one…

- (void)fetch {
    NSError *error = nil;
    BOOL success = [self.fetchedResultsController performFetch:&error];
    NSAssert2(success, @"Unhandled error performing fetch at SongsViewController.m, line %d: %@", __LINE__, [error localizedDescription]);
    [self.tableView reloadData];
}

- (NSFetchedResultsController *)fetchedResultsController {
    if (fetchedResultsController == nil) {
        NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
        [fetchRequest setEntity:[NSEntityDescription entityForName:@"Song" inManagedObjectContext:managedObjectContext]];
        NSArray *sortDescriptors = nil;
        NSString *sectionNameKeyPath = nil;
        if ([fetchSectioningControl selectedSegmentIndex] == 1) {
            sortDescriptors = [NSArray arrayWithObjects:[[[NSSortDescriptor alloc] initWithKey:@"category.name" ascending:YES] autorelease], [[[NSSortDescriptor alloc] initWithKey:@"rank" ascending:YES] autorelease], nil];
            sectionNameKeyPath = @"category.name";
        } else {
            sortDescriptors = [NSArray arrayWithObject:[[[NSSortDescriptor alloc] initWithKey:@"rank" ascending:YES] autorelease]];
        }
        [fetchRequest setSortDescriptors:sortDescriptors];
        fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:sectionNameKeyPath cacheName:@"SongsCache"];
    }    
    return fetchedResultsController;
} 

Answer 1

1. your extra caching is probably a waste of cycles as Core Data performs its own caching internally. I am willing to bet you are slowing things down rather than speeding them up, not to mention the additional memory you are consuming.

2. Where are you setting categoryEntityDescription? That is now shown in the code you posted. It is probably nil.

3. Why are you retaining an NSEntityDescription?!? They are already in memory because of Core Data and retaining them is a waste which could lead to issues if Core Data wants to release it at some point.

update

Your caching is definitely not coming from Apple’s code because they know that the cache is in Core Data.

As for the NSEntityDescription, again, do not retain the NSEntityDescription.

Are you 100% positive that the NSEntityDescription is not nil? Have you confirmed it in the debugger? Have you tested it with a freshly retrieved NSEntityDescription?

update

You need to learn to use the debugger as that will solve most of your coding issues. Put a breakpoint in this method and run your code in the debugger. Then when the execution stops on that break point you can inspect the values of the variables and learn what they are currently set to. That will confirm or deny your suspicions about what is and is not nil.

This error you are seeing happens when you fail to set the Entity in the NSFetchRequest which, based on your code, means that retained property is not being set before the code you have shown is being called.