Does NSURLConnection Block the Main/UI Thread

Question

I am downloading images in table view cells as they scroll onto the screen. For UX reasons, I start downloading the images in - (UITableViewCell *)tableView:(UITableVi

Answer 1

If you mean "does NSURLConnection execute on the main thread?", then yes, I believe that's the case. The connection is opened and the delegate methods are executed on the main thread. I haven't found any documentation to suggest otherwise, and you can verify it by debugging.

I think your supposition that the UITableView scrolling blocks the NSURLConnection callbacks in the main run loop is correct.

You've already posted one solution, spawning a thread for your selector. Another alternative would be to execute your downloads as NSOperations, which has a couple of benefits:

• If you force the operations to run concurrently (see Dave Dribin's excellent post on this), you can limit the number of simultaneous downloads, which might be desirable if you have a very large number of images in your table. You say your downloads occur "almost instantly", but that may not be the case if your user is on a slow connection and your table contains a lot of images.
• You can cancel all the operations if the user does something that makes the image downloads irrelevant, like executing another search.

Dave Dribin's approach, which I use, forces the connections to execute back on the main thread, but that probably isn't necessary for your purposes--you could use your current approach of calling back to the main thread after your images download.