UICollectionViewData updateItemCounts Crash

Question

I'm getting crashes from deep inside Apple's layout code for a UICollectionViewFlowLayout and I have no idea how to address these (stack trace below). Any suggestions would be greatly appreciated.

Details:

• Unfortunately I can't reproduce the issue
• The issue isn't frequent (it happens in less than 0.1% of sessions) but it's our #1 crash

#0. Crashed: com.apple.main-thread
0  libsystem_platform.dylib       0x1879490d4 __bzero + 36
1  UIKit                          0x18e824e40 -[UICollectionViewData _updateItemCounts] + 544
2  UIKit                          0x18e824e40 -[UICollectionViewData _updateItemCounts] + 544
3  UIKit                          0x18e8e67e0 -[UICollectionViewData numberOfSections] + 28
4  UIKit                          0x18f086bc0 -[UICollectionViewFlowLayout _getSizingInfosWithExistingSizingDictionary:] + 612
5  UIKit                          0x18f088834 -[UICollectionViewFlowLayout _fetchItemsInfoForRect:] + 152
6  UIKit                          0x18e8e66b4 -[UICollectionViewFlowLayout prepareLayout] + 224
7  UIKit                          0x18e7cf574 -[UICollectionViewData _prepareToLoadData] + 164
8  UIKit                          0x18e7ceb5c -[UICollectionViewData validateLayoutInRect:] + 100
9  UIKit                          0x18e7ce55c -[UICollectionView layoutSubviews] + 212
10 UIKit                          0x18e76fa80 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1196
11 QuartzCore                     0x18bc1d9d8 -[CALayer layoutSublayers] + 148
12 QuartzCore                     0x18bc124cc CA::Layer::layout_if_needed(CA::Transaction*) + 292
13 QuartzCore                     0x18bc1238c CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 32
14 QuartzCore                     0x18bb8f3e0 CA::Context::commit_transaction(CA::Transaction*) + 252
15 QuartzCore                     0x18bbb6a68 CA::Transaction::commit() + 512
16 QuartzCore                     0x18bbb7488 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 120
17 CoreFoundation                 0x18886a0c0 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 32
18 CoreFoundation                 0x188867cf0 __CFRunLoopDoObservers + 372
19 CoreFoundation                 0x188868180 __CFRunLoopRun + 1024
20 CoreFoundation                 0x1887962b8 CFRunLoopRunSpecific + 444
21 GraphicsServices               0x18a24a198 GSEventRunModal + 180
22 UIKit                          0x18e7dd7fc -[UIApplication _run] + 684
23 UIKit                          0x18e7d8534 UIApplicationMain + 208
24 AppName Mobile                 0x1000333e8 main (main.m:16)
25 libdispatch.dylib              0x1877795b8 (Missing)

Answer 1

We finally found this one. It wasn't the performBatchUpdates issue reported on a lot of other cards. The root cause was one our data sources would return a negative number for item count. The SDK is using an unsigned int, so setting a negative rolled over, and would try to allocate billions of cells (34 GB of virtual memory and counting). It would eventually crash.

Answer 2

There is a rampant problem with item counts and performBatchUpdates. Specifically, you may run into discrepancies and an OS assert if these are not in sync. An assert is not a crash.

I do suspect that this is what you encountered here, and if so, this question may be a duplicate of:

• Nightmare with performBatchUpdates crash
• How to solve this CollectionView crash
• UICollectionView performBatchUpdates crash
• crashing at performBatchUpdates of collection view

---

Assert or not, a defensive approach is to guard the item count as excellently outlined in Fang-Pen Lin's^(†)UICollectionView invalid number of items crash problem and solution blog post:

func updateItems(updates: [ItemUpdate]) {
    collectionView.performBatchUpdates({
        for update in updates {
            switch update {
            case .Add(let index):
              collectionView.insertItemsAtIndexPaths([NSIndexPath(forItem: index, inSection: 0)])
                itemCount += 1
            case .Delete(let index):
              collectionView.deleteItemsAtIndexPaths([NSIndexPath(forItem: index, inSection: 0)])
                itemCount -= 1
            }
        }
    }, completion: nil)
}

^†Full Credit