NSBlockOperation or NSOperation with ALAsset Block to display photo-library images using ALAsset URL

Question

I am asking this question regarding my questions Display photolibrary images in an effectual way iPhone and Highly efficient UITableView \"cellForRowIndexPath\" method t

Answer 1

You have a line in your DisplayImages NSOperation subclass where you update the UI (DisplayImages.m line 54):

self.imageView.image = topicImage;

This operation queue is running on a background thread, and we know that you should only update the state of the UI on the main thread. Since updating the view of an image view is definitely updating the UI, this can be simply fixed by wrapping the call with:

dispatch_async(dispatch_get_main_queue(), ^{
   self.imageView.image = topicImage;
});

This puts an asynchronous call on the main queue to update the UIImageView with the image. It's asynchronous so your other tasks can be scheduled in the background, and it's safe as it is running on the main queue - which is the main thread.