How to encode something on a background thread and then post it on the main one iOS

Check out How to encode something on a background thread and then post it on the main one iOS by JR. Here is an excerpt:

// encode image on background thread
                           dispatch_async(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), ^{
                               NSData *imageData = result.namedArguments[@"image_data"];
                               UIImage *image = [UIImage...

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