a code segment from http://jeffreysambells.com/2013/03/01/asynchronous-operations-in-ios-with-grand-central-dispatch

gcd.m

NSArray *images = @[@"http://example.com/image1.png",
                 @"http://example.com/image2.png",
                 @"http://example.com/image3.png",
                 @"http://example.com/image4.png"];
 
dispatch_queue_t imageQueue = dispatch_queue_create("Image Queue",NULL);
 
for (NSString *urlString in images) {
    dispatch_async(imageQueue, ^{
 
        NSURL *url = [NSURL URLWithString:urlString];
        NSData *imageData = [NSData dataWithContentsOfURL:url];
        UIImage *image = [UIImage imageWithData:imageData];
 
        NSUInteger imageIndex = [images indexOfObject:urlString];
        UIImageView *imageVIew = (UIImageView *)[self.view viewWithTag:imageIndex];
 
        if (!imageView) return;
 
        dispatch_async(dispatch_get_main_queue(), ^{
            // Update the UI
            [imageVIew setImage:image];
        });
 
    }); 
}
 
// Continue doing other stuff while images load

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