Open Radar

 
Peter Steinberger28-Oct-2016 04:46 PM

This is also somewhat related to rdar://28999727 and the whole prefetching changes in iOS 10.
Peter Steinberger28-Oct-2016 04:45 PM

Area:
UIKit

Summary:
When asking a collection view for cellForItemAtIndexPath: it does only return cells that are visible and not all cells that are configured. This was fine before cell prefetching became a thing, as these two states basically were identical. Now however this is different.

Steps to Reproduce:
- Open the sample app
- Scroll through the list and notice the different cell colors
- Red means `cellForItemAtIndexPath:` did not return the cell, blue means it did.

Expected Results:
- All cells are blue

Actual Results:
- Depending on scroll speed, most of the non-initial cells are red.

Version:
iOS 10.1

Notes:
This is because in the collectionView:cellForItemAtIndexPath: delegate call I dispatch a configuration event. From what I’ve experienced, this is a pretty common approach to load data asynchronously. The issue arises when that data loads faster than the cell scrolls into the view.

I am aware of the fact that the documentation says `Returns the visible cell object at the specified index path.`, however to me this sounds like a bug. As mentioned above: Previously this was identical with everything that was configured via `collectionView:cellForItemAtIndexPath:` and due to the equal naming of these two methods they really should work hand in hand. So now that prefetching is a thing, the documentation and functionality on this should be changed to make these methods work like counterparts. `cellForItemAtIndexPath:` should return all cells that have been configured by `collectionView:cellForItemAtIndexPath:`.

Configuration:
iPhone 7 Plus

Attachments:
'CollectionViewTest.zip' was successfully uploaded.