Manage the number of active tasks in a background NSURLSession

Question

It seems that when you enqueue too many tasks (say, hundreds) on a background NSURLSession, it doesn't work well. How can you keep the number of enqueued tasks at a small fixed number, say 10?

Answer 1

In the class that is responsible for enqueueing tasks, have a ivar to track the active tasks, e.g.

// In MySessionWrapper.m

@interface MySessionWrapper () {
    NSMutableSet *activeTaskIds;
}
@end

When you enqueue a task, you add its ID to that set:

[activeTaskIds addObject:@([task taskIdentifier])]

When you get a didComplete callback, you remove the ID, and if the number of active tasks falls below your target, you add more tasks:

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    // other stuff
    [activeTaskIds removeObject:@([task taskIdentifier])]

    if ([activeTaskIds count] < NUMBER) {
        // add more tasks
    }
}

This system is working for me now.