Open Radar

 
Area:
UIKit

Summary:
UICollectionView seems to have serious performance issues when using a ‘complex’ layout with many sections and and many supplementary/decoration views in each section.

UICollectionView blocks the main thread when calling -[UICollectionView performBatchUpdates:completion:] and performing changes.

I've attached a screenshot of time profiler in Instruments.
It looks as though most of the time is spent in UICollectionView’s internal classes, UICollectionViewData in particular, accessing NSDictionaries, probably using NSIndexPath as the key by looking at how much time is spent is -[NSIndexPath hash] and -[NSIndexPath isEqual:].

The issue is only present when using supplementary views.
When only showing items, -[UICollectionView performBatchUpdates:completion:] is very performant.

I'm excluding a problem in my own layout, given that I'm making good use of caches, never recomputing attributes that have already been computed. Moreover, Instruments clearly shows performance issues in UIKit's internal classes.

Steps to Reproduce:
1. Run the example project and start scrolling the app horizontally
2. As you scroll, new items will be added calling
-[UICollectionView performBatchUpdates:completion:]
3. Notice how the scroll is very laggy. This is because -[UICollectionView performBatchUpdates:completion:] blocks the main thread.

4. Now, stop the app and comment out the code in CollectionViewLayout.m from line 83 to line 95, which removes supplementary views.
5. As you scroll, new items will be added calling
-[UICollectionView performBatchUpdates:completion:]
6. Notice how the scroll is now very performant.

Expected Results:
-[UICollectionView performBatchUpdates:completion:] should be very performant, regardless of whether the UICollectionView is making use of supplementary/decoration views.

Actual Results:
-[UICollectionView performBatchUpdates:completion:] is very slow when supplementary/decoration views are being used.

Version:
iOS 10.2

Configuration:
iOS simulator, iPhone 6S

Attachments:
'UICollectionView performance.png' and 'RDAR - CollectionViewBatchUpdates.zip' were successfully uploaded.
https://www.dropbox.com/sh/uguxlfqxo6nybny/AACoVVRGzZlecrur29PguQESa?dl=0