Problem

Sometimes, you'll work with a very big UICollectionView, ex: A cell contains a UICollectionView, a cell contains a UITableView, a cell contains a bundle UIView and a lot of Auto Layout constraint.

Especially, in that cell contains UIImageView, when you scroll on the screen, it's laggy.

How to solve that

Determine issues

Why does it happen?

• Rendering UIImage on UIImageView is take time.
• Draw the Image when scrolling is take time.
• Auto Layout is take time.
• Draw UIImageView is take time.
• Caching on disk is take time.

How I solve that?

• For rendering UIImage on UIImageView, firstly, avoid use native setUIImage method or setImageWithURL of AFNetworking now. Why?. Look at SDWebImage and see what happen :)
• For drawing the Image when scrolling, use below method when setup cell. objc cell.layer.shouldRasterize = YES; cell.layer.rasterizationScale = [UIScreen mainScreen].scale;
• For Auto Layout, it is a problem on iOS 8, make the cell have auto layout scroll very laggy. So this is solution: Override 1 method in UICollectionViewCell class. objc - (UICollectionViewLayoutAttributes *)preferredLayoutAttributesFittingAttributes:(UICollectionViewLayoutAttributes *)layoutAttributes { return layoutAttributes; }
• For the problem of UIImageView, some properties of it will decrease the performance, ex: clipToBounds
• Writting and Reading on Disk is taked more time than Memory, therefore, take care when working with Caching.