via #iosdev on irc.freenode.net: With the Parse SDK, a user had trouble displaying images that downloaded successfully.

Async Collection View Update

//  Problem: Collection View data does not appear after calling [UICollectionView reloadData] in [PFQuery findObjectsInBackgroundBlock:]
//  Symptoms: The correct number returns in numberOfItems: but cellForItem: is never called
//  Solution: Try performing collection view updates manually via performBatchUpdates: or reloadItemsAtIndexPaths: or reloadSectionsAtIndexSet:

- (void)queryForPhoto {
    NSLog(@"%s", __PRETTY_FUNCTION__);
    
    PFQuery *query = [PFQuery queryWithClassName:@"Image"];
    PFUser *user = [PFUser currentUser];
    [query whereKey:@"user" equalTo:user];
    [query orderByAscending:@"createdAt"];
    [query setCachePolicy:kPFCachePolicyNetworkOnly];
    
    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        if (!error) {
            // success
            NSLog(@"retrieved %lu photos.", (unsigned long)objects.count);
            self.photoArray = [NSMutableArray arrayWithCapacity:objects.count];

            for (PFObject *object in objects) {
                PFFile *thumbnail = [object objectForKey:@"thumbnail"];
                [thumbnail getDataInBackgroundWithBlock:^(NSData *data, NSError *error) {
                    if (!error) {
                        [self.photoArray addObject:[UIImage imageWithData:data]];
                        // [self.collectionView reloadData];
                        [self reloadCollection:self.collectionView];
                        NSLog(@"photoArray array size: %lu", (unsigned long)[self.photoArray count]);
                    } else {
                        NSLog(@"error: %@ %@", error, [error userInfo]);
                    }
                }];
            }
        } else {
            // details of the failure
            NSLog(@"error: %@ %@", error, [error userInfo]);
        }
    }];
}

- (void)reloadCollection:(UICollectionView*)collectionView {
    [UIView setAnimationsEnabled:NO];
    [collectionView performBatchUpdates:^{
        [collectionView reloadSections:[NSIndexSet indexSetWithIndex:0]];
    } completion:^(BOOL completed){
        [UIView setAnimationsEnabled:YES];
    }];
}