How do I correctly set up asynchronous image downloading within a UICollectionView that uses a custom cell?

Question 1

After several days the issue was my images were far too large. I had to resize them and this instantly solved my issue.

I literally narrowed things down and checked my images to find they were not being resized by the method I thought was resizing them. This is why I need to get myself used to testing.

I learnt a lot about GCD and caching in the past few days but this issue could have been solved much earlier.

Question 2

You can make use of the internal cache used by NSURLConnection for this.

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

    //Standard code for initialisation.
    NSURL *url; //The image URL goes here.
        NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:5.0]; //timeout can be adjusted

        [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError)
         {
             if (!connectionError)
             {
                 UIImage *image = [UIImage imageWithData:data];

                 //Add image as subview here.
             }
         }];

    .
    .
    return cell;

}

Question 3

This is for a table view, but same concept basically. I had the same issue you were having. I had to check for a cached image, if not, retrieve it from a server. The main thing to watch out for is when you retrieve the image back, you have to update it in the collection view on the main thread. You also want to check if the cell is still visible on the screen. Here is my code as an example. teamMember is a dictionary and @"avatar" is the key which contains the URL of the user's image. TeamCommitsCell is my custom cell.

// if  user has an avatar
    if (![teamMember[@"avatar"] isEqualToString:@""]) {
        // check for cached image, use if it exists
        UIImage *cachedImage = [self.imageCache objectForKey:teamMember[@"avatar"]];
        if (cachedImage) {
            cell.memberImage.image = cachedImage;
        }
        //else  retrieve the image from server
        else {
            NSURL *imageURL = [NSURL URLWithString:teamMember[@"avatar"]];

            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
                NSData *imageData = [NSData dataWithContentsOfURL:imageURL];
                // if valid data, create UIImage
                if (imageData) {
                    UIImage *image = [UIImage imageWithData:imageData];
                    // if valid image, update in tableview asynch
                    if (image) {
                        dispatch_async(dispatch_get_main_queue(), ^{
                            TeamCommitsCell *updateCell = (id)[tableView cellForRowAtIndexPath:indexPath];
                            // if valid cell, display image and add to cache
                            if (updateCell) {
                                updateCell.memberImage.image = image;
                                [self.imageCache setObject:image forKey:teamMember[@"avatar"]];
                            }
                        });
                    }
                }
            });
        }
    }

Question 4

NSURLCache is iOS's solution to caching retrieved data, including images. In your AppDelegate, initialize the shared cache via:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    NSURLCache *cache = [[NSURLCache alloc] initWithMemoryCapacity:8 * 1024 * 1024
                                                      diskCapacity:20 * 1024 * 1024
                                                          diskPath:nil];
    [NSURLCache setSharedURLCache:cache];
    return YES;
}

-(void)applicationDidReceiveMemoryWarning:(UIApplication *)application {
    [[NSURLCache sharedURLCache] removeAllCachedResponses];
}

Then use AFNetworking's UIImageView category to set the image using:

[imageView setImageWithURL:myImagesURL placeholderImage:nil];

This has proven to load images the second time around incredibly faster. If you are worried about loading images faster for the first time, you will have to create a way to determine when and how many images you want to load ahead of time. It is very common to load data using paging. If you are using paging and still are having trouble, consider using AFNetworking's:

- (void)setImageWithURLRequest:(NSURLRequest *)urlRequest
              placeholderImage:(UIImage *)placeholderImage
                       success:(void (^)(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image))success
                       failure:(void (^)(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error))failure;

This way you can create an array of UIImages and using this method to return the images for each cell before dequeuing the cell. So in this case you would have two parallel arrays; one holding your data and the other holding corresponding UIImages. Memory management will eventually get out of hand so keep that in mind. If someone scrolls quickly to the bottom of the available cells, there is honestly not much else you can do since the data depends on the network connection of the user.