NSURLSession task execution order

Question

I am adding multiple tasks to a NSURLSession (background mode) , in an order. I have kept HTTPMaximumConnectionsPerHost = 1. However, I am seeing that the uploads are taken up in a random order i.e, after the 1st item may be 5th item is picked up and then the 3rd item etc - the upload does not happen in the order I had provided to NSURLSession. Is there a way to order the uploads exactly the way it is added?

Answer 1

We do not ensure your execution tasks will be executed in order with the configuration of HTTPMaximumConnectionsPerHost = 1 because it just guarantees one task being executed at a time. In term of executing tasks synchronously in order, you can use NSOperationQueue and NSOperation to add the dependency between operations.

NSMutableArray *operations = [NSMutableArray array];
    NSArray *urls = @[];
    NSURLSession *urlSession = [NSURLSession sharedSession];
    for (int i = 0;i < urls.count;i++) {
        NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
            NSURLSessionDataTask *task = [urlSession dataTaskWithURL:[NSURL URLWithString:urls[i]] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            }];
            [task resume];
        }];
        i > 0 ? [operation addDependency:operations[i - 1]] : 0;
        [operations addObject:operation];
    }
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    queue.maxConcurrentOperationCount = 1;
    [queue addOperations:operations waitUntilFinished:YES];

Another solution is use dispatch semaphore of GCD.

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    NSArray *urls = @[];
    NSURLSession *urlSession = [NSURLSession sharedSession];
    for (NSString *url in urls) {
        NSURLSessionDataTask *task = [urlSession dataTaskWithURL:[NSURL URLWithString:url] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            dispatch_semaphore_signal(semaphore);                  // signal when done
        }];
        [task resume];
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); // wait for signal before continuing
    }
    //Do s.t after all tasks finished