iOS: numberOfSectionsInCollectionView message sent to deallocated instance

Question

I have this code to manage a collectionview

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return  1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return [images count];
}

- (PhotoCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{

    PhotoCell *cell = [gridPhoto dequeueReusableCellWithReuseIdentifier:@"photocell" forIndexPath:indexPath];

    NSMutableDictionary *record = [images objectAtIndex:[indexPath row]];
    if ([record valueForKey:@"actualImage"]) {
        [cell.image setImage:[record valueForKey:@"actualImage"]];
        [cell.activity stopAnimating];
    } else {
        dispatch_async(imageQueue_, ^{
            NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:[record objectForKey:@"url"]]];
            dispatch_async(dispatch_get_main_queue(), ^{
                [record setValue:[UIImage imageWithData:imageData] forKey:@"actualImage"];
                [collectionView reloadItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];
            });
        });
    }

    return cell;
}

It work fine if the imageQueue_ finish its work and populate the entire collectionview; I have a problem when I exit from my view calling a back

- (IBAction)back:(id)sender{
    [self.navigationController popViewControllerAnimated:YES];
}

In this case if the collectionview is not entire populated of images I have this error when I do back action:

[PhotoGalleryViewController numberOfSectionsInCollectionView:]: message sent to deallocated instance 0x1590aab0

where is the problem?

Answer 1

A download is still in progress when you pop your view controller. So when the asynchronous callback is executed, your [collectionView reloadItemsAtIndexPaths...] gets called on a collectionView that no longer exists.

You should check that collectionView != nil on the first line of your callback block, and simply return; if it's nil:

dispatch_async(dispatch_get_main_queue(), ^{
    if (collectionView == nil)
        return;

    [record setValue:[UIImage imageWithData:imageData] forKey:@"actualImage"];
    [collectionView reloadItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];
});