ios - Caching an Image and UICollectionView

Question

I'm pretty new to Objective-C so hopefully this all makes sense. I've downloaded images from a server and have displayed them in an image view in the collectionView cellForItemAtIndexPath: method. The issue I'm facing is that the images don't appear to be caching. It looks like each time a cell is being reused, the associated image from the server is being re-downloaded.

In my viewDidLoad method I'm creating a NSMutableDictionary:

imageDictionary = [[NSMutableDictionary alloc]initWithCapacity:50.0];

From reading the documentation and looking at answers to similar questions I thought that this, plus the following code would be enough. I've been at this for a couple of days and I know there is something I'm missing or a concept I'm not grasping.

#pragma mark - UICollectionView Data Source
- (NSInteger)collectionView:(UICollectionView *)view numberOfItemsInSection:(NSInteger)section;{
    NSLog(@"Begin retrieving photos");
    return [self.photos count];
}

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

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath;{
    CollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"MY_CELL" forIndexPath:indexPath];
    cell.imageView.image = nil;

    if (cell.imageView.image == nil) {
    dispatch_queue_t downloadQueue = dispatch_queue_create("image downloader", NULL);
    dispatch_async(downloadQueue, ^{
        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[[self.photos objectAtIndex:indexPath.row] objectForKey:@"fullimage"]]];
        UIImage *image = [UIImage imageWithData:data];
        [imageDictionary setObject:image forKey:@"Image"];

        dispatch_async(dispatch_get_main_queue(), ^{
            cell.imageView.image = [imageDictionary objectForKey:@"Image"];
            [cell setNeedsDisplay];
        });
    });
    }
    return cell;
}

Any help would be greatly appreciated. Thanks in advance.

Answer 1

@yuf - thanks again for the direction. NSCache seems to be getting me the results I was after. Here is the code that is working properly. If anyone has a similar issue, you can compare the following to my original question.

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath;{
    CollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"MY_CELL" forIndexPath:indexPath];

    NSString *imageName = [[self.photos objectAtIndex:indexPath.row] objectForKey:@"fullimage"];
    UIImage *image = [imageCache objectForKey:imageName];

    if(image){

        cell.imageView.image = image;
    }

    else{

    cell.imageView.image = nil;

    dispatch_queue_t downloadQueue = dispatch_queue_create("image downloader", NULL);
    dispatch_async(downloadQueue, ^{

        NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:[[self.photos objectAtIndex:indexPath.row] objectForKey:@"fullimage"]]];
        UIImage *image = [UIImage imageWithData:data];

        dispatch_async(dispatch_get_main_queue(), ^{

            cell.imageView.image = image;

        });

        [imageCache setObject:image forKey:imageName];
    });
    }

    return cell;
}