GCD cancel async block?

Question

I have regular code for loading images in table view cells

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        NSImage *image = file.image;
        dispatch_async(dispatch_get_main_queue(), ^{
            imageView.image = image;
        });
    });

The problem is when I scroll too fast I can see that last block fires several times on same imageView. And that looks really weird. Is there any way to cancel all previously scheduled operations for one imageView (lets say they all will have unique id) before scheduling new one?

I mean, I want to be sure that only last scheduled operation should be executed and all all previously scheduled should be dropped. Is that possible by means of Grand Central Dispatch? Or I have to add my own atomic flags to imageView objects and check whether flags are set before calling imageView.image = image;

Are the images really loading that slowly if you perform this all synchronously? — James Webster

Michał Banasiak Michał Banasiak · Accepted Answer · 2014-02-15T09:50:22

To archieve it using GCD you have to use your own atomic flag.

But there is a better solution where you have cancellation of tasks out of the box. It is a NSOperationQueue.

You can read everything you need under this link: http://www.raywenderlich.com/19788/how-to-use-nsoperations-and-nsoperationqueues

GCD cancel async block?

2 Answers