ios - UICollectionView performance - _updateVisibleCellsNow

Question

I'm working on a custom UICollectionViewLayout that displays cells organized by day/week/month.

It is not scrolling smooth, and it looks like the lag is caused by [UICollectionView _updateVisibleCellsNow] being called on each rendering loop.

Performance is OK for < 30 items, but at around 100 or more, its terribly slow. Is this a limitation of UICollectionView and custom layouts, or am I not giving the view enough information to perform correctly?

Source here: https://github.com/oskarhagberg/calendarcollection

Layout: https://github.com/oskarhagberg/calendarcollection/blob/master/CalendarHeatMap/OHCalendarWeekLayout.m

Data source and delegate: https://github.com/oskarhagberg/calendarcollection/blob/master/CalendarHeatMap/OHCalendarView.m

Time Profile:

Update

Maybe its futile? Some testing with a plain UICollectionViewController with a UICollectionViewFlowLayout that is given approximately the same amount of cells/screen results in a similar time profile.

I feel that it should be able to handle ~100 simple opaque cells at a time without the jitter. Am I wrong?

Answer 1

Also don't forget to try to rasterize the layer of the cell:

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

I had 10 fps without that, and boom 55fps with! I'm not really familiar with GPU and compositing model, so what does it do exactly is not entirely clear to me, but basically it flatten the rendering of all subviews in only one bitmap (instead of one bitmap per subview?). Anyway I don't know if it has some cons, but it is dramatically faster!