Question

I am creating a application which displays 8 thumbnails per page and it can have n pages. Each of these thumbnails are UIViews and are added to UIScrollView. However i have implemented Paging using the Apple sample code.

The prob:

1. Each thumbnail(UIView) takes 150
   millisecs to be created and added to
   scroll view
2. Hence for 3 pages it takes awful
   huge time to be created and added to
   the UI Scrollview.
3. At this point the scroll view is not very respsonsive and it is very jerky and gives a bad user experience
4. How can i create the thumbnails and add them to UIScrollview without affecting the touch responsiveness? I want them to run independent of the main thread which is resposible for handling touch events (i suppose).

Also i would like to mention that when a thumbnail is created i trigger a Async download of the image and the delegate method is called when download is complete.

Let me know the the options i have to make this more responsive and update UI without affecting the touch operations. The page control works fine with lazy loading of the grid of thumbnails.

TIA,

Praveen S

Answer 1

Grand Central Dispatch is easy to use for background loading. But GCD is only for after iOS4. If you have to support iOS3, performSelectorInBackground/performSelectorOnMainThread or NSOperationQueue are helpful.

And, be careful almost UIKit classes are not thread-safe except drawing to a graphics context. For example, UIScrollView is not thread-safe, UIImage imageNamed: is not thread-safe, but UIImage imageWithContentsOfFile: is thread-safe.

dispatch_queue_t mainQueue = dispatch_get_main_queue();
dispatch_queue_t concurrentQueue =
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_async(concurrentQueue, ^{

    dispatch_apply([thumbnails count], concurrentQueue, ^(size_t index) {

        Thumbnail *thumbnail = [thumbnails objectAtIndex:index];
        thumbnail.image = [UIImage imageWithContentsOfFile:thumbnail.url];

        dispatch_sync(mainQueue, ^{

            /* update UIScrollView using thumbnail. It is safe because this block is on main thread. */

        });
    }

    /* dispatch_apply waits until all blocks are done */

    dispatch_async(mainQueue, ^{
        /* do for all done. */
    });
}