* Our layer, retained. Under iOS < 9, when collection views are removed from the hierarchy,
* their layers may be deallocated and become dangling pointers. This puts the collection view
* into a very dangerous state where pretty much any call will crash it. So we manually retain our layer.
*
* You should never access this, and it will be nil under iOS >= 9.
*/
CALayer *_retainedLayer;
/**
* If YES, the `UICollectionView` will reload its data on next layout pass so we should not forward any updates to it.
* Rationale:
* In `reloadData`, a collection view invalidates its data and marks itself as needing reload, and waits until `layoutSubviews` to requery its data source.
* This can lead to data inconsistency problems.
* Say you have an empty collection view. You call `reloadData`, then immediately insert an item into your data source and call `insertItemsAtIndexPaths:[0,0]`.
* You will get an assertion failure saying `Invalid number of items in section 0.
* The number of items after the update (1) must be equal to the number of items before the update (1) plus or minus the items added and removed (1 added, 0 removed).`
* The collection view never queried your data source before the update to see that it actually had 0 items.
|| _asyncDataSourceFlags.asyncDataSourceNodeForItemAtIndexPath, @"Data source must implement collectionView:nodeForItemAtIndexPath: or collectionView:nodeBlockForItemAtIndexPath:");