UICollectionview Scrolling choppy when loading cells

Question

I have a gallery in my app utilizing a UICollectionView. The cells are approximately 70,70 size. I am using ALAssets from the ALAssetLibrary in the gallery which I have stored in a list.

I am using the usual pattern for populating the cells:

-(UICollectionViewCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {    mycell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];   mycell.imageView.image = [[UIImage imageWithCGImage:[alassetList objectAtIndex:indexpath.row] thumbnail]];   return mycell; } 

My gallery is scrolling choppy. I don't understand why this is. I've tried adding a NSCache to cache the thumbnail images (thinking maybe creating the images was expensive) but this did not help for the performance.

I would expect the UI to be as buttery as the stock app.

I am now suspecting it may be something in the UICollectionViewCell prepareForReuse that may be holding up the dequeueReusableCellWithReuseIdentifier method but using instruments I was not able to find this.

Any other thing that may be be causing this? Is there a "faster" way to prepare the UICollectionViewCell or to dequeue them in a faster fashion?

Answer 1

So anybody having scrolling issues should do this

add these 2 lines after your dequeue

cell.layer.shouldRasterize = YES; cell.layer.rasterizationScale = [UIScreen mainScreen].scale; 

Answer 2

I would assume the "choppyness" is coming from the UIImage allocation, not anything with the dequeueReusableCellWithReuseIdentifier method. I'd try doing the image allocation on a background thread and see if that makes things a little more buttery.

-(UICollectionViewCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {   mycell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void) {      // Load image on a non-ui-blocking thread     UIImage *image = [[UIImage imageWithCGImage:[alassetList objectAtIndex:indexpath.row] thumbnail]];      dispatch_sync(dispatch_get_main_queue(), ^(void) {         // Assign image back on the main thread         mycell.imageView.image = image;     });   });    return mycell; } 

More details can be found here: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/Introduction/Introduction.html