Question

I need to know if it’s better to use NSOperation or Block to load a large number of image into a UIScrollView? I create all the Imageview and positioning each UIImageView in the right position into the scroll.

To avoid memory warning I choose to load the image once at time. My idea is to create a sort of queue and insert all the image to load in the queue. I have to use block or NSOperation to do this?

Answer 1

In tableView:cellForRowAtIndexPath:, you can use GCD (Grand Central Dispatch) to load the images asynchronously.

Like this:

static NSString *CellIdentifier = @"ImageCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                   reuseIdentifier:CellIdentifier] autorelease];
}
NSString *imagepath = //get path to image here
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
    UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
    dispatch_sync(dispatch_get_main_queue(), ^{
        [[cell imageView] setImage:image];
        [cell setNeedsLayout];
    });
});
return cell;

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Edit: For a better answer, watch WWDC ’12 Session 211 – “Building Concurrent User Interfaces on iOS” (thanks @sc0rp10n!)