@pawan is correct, that would be simplest way to implement SDWebImage. It's basically a category of UIImage, so simply use the category methods (setImageWithURL).
A couple of other points since you asked;
In your code, you should be setting the coverImage.image on the main thread, not in the background.
It's good practice to check if the Cell is visible, or there is little point in displaying the content.
It's good practice to create a weak reference to the Collection view when accessing the cell to avoid circular references
so your code ) might be written as follows if you were insistent on doing it the way you have it (Using the category is the best and simplest way)
__weak myCollectionViewController *weakSelf = self;
SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadWithURL:[NSURL URLWithString:coverURL] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
} completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) {
dispatch_async(dispatch_get_main_queue(), ^{
if ([weakSelf.collectionView.indexPathsForVisibleItems containsObject:indexPath]) {
RRRBigItemCell *cell = (RRRBigItemCell *)[weakSelf.collectionView cellForItemAtIndexPath:indexPath];
RRRImageView *coverView = (RRRImageView *)[cell viewWithTag:COVER_TAG];
coverView.image = image;
};
});
}];