Downloading a large number of images (1500+) efficiently with AFNetworking 2

Question

I am building an app that downloads a large number of images, sometimes 1500-5000 images depending on what the user requests. To do this, I am using AFNetworking 2. At first, I was just looping through all of my URLs and then making a request for each one.

for (NSString *url in urls) {
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    AFHTTPRequestOperation *requestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    requestOperation.responseSerializer = [AFImageResponseSerializer serializer];
    [requestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        completion(responseObject);
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        failure(error);
    }];
    [requestOperation setDownloadProgressBlock:^(NSUInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {
        double percentDone = (double)totalBytesRead / (double)totalBytesExpectedToRead;
        progress(percentDone);
    }];
    [requestOperation start];
}

But, after I got to about 900 downloads/requests I would start to get the following error:

The request timed out

I am assuming this error came directly from AFNetworking.

What is the best and most efficient way to make a large number of download requests like this without timing out? Should I be using dispatch_group to batch the requests as outlined here?

Or, should I use a recursive method that will download one image at a time, and only start the next request once the first one finishes?

Answer 1

Try this

NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
[request setTimeoutInterval:600];  /* 10 minutes */

But it would be best solution if you just download an archive of images and then unpack it.