Invalid update: invalid number of items in section 0. - swift

Question

Recently I got the following error:
Fatal Exception: NSInternalInconsistencyException
Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (13) must
be equal to the number of items contained in that section before the
update (12), plus or minus the number of items inserted or deleted
from that section (0 inserted, 0 deleted) and plus or minus the number
of items moved into or out of that section (0 moved in, 0 moved out).
The error occurs in the following code in my tvOS client:
let removedIndexPaths = removedIndexes.map({ IndexPath(row: $0, section: 0) })
let addedIndexPaths = addedIndexes.map({ IndexPath(row: $0, section: 0) })
let updatedIndexPaths = updatedIndexes.map({ IndexPath(row: $0, section: 0) })
self.collectionView?.performBatchUpdates({
self.collectionView?.deleteItems(at: removedIndexPaths)
self.collectionView?.insertItems(at: addedIndexPaths)
}, completion: { _ in
guard let collectionView = self.collectionView else {
return
}
for indexPath in updatedIndexPaths {
if let myCell = collectionView.cellForItem(at: indexPath) as? MyCollectionViewCell {
let item = self.dataManager.items[indexPath.row]
myCell.updateUI(item)
}
}
let collectionViewLayout = self.collectionViewLayoutForNumberOfItems(self.dataManager.items.count)
if collectionViewLayout.itemSize != self.collectionFlowLayout.itemSize {
collectionView.setCollectionViewLayout(collectionViewLayout, animated: false)
}
})
I am only using one section in my collection view:
override func numberOfSections(in collectionView: UICollectionView) -> Int {
return 1
}
I have checked out couple of posts on the same topic, but they have not solved my problem, my guess is that the problem is in the following two lines, but I am not sure:
self.collectionView?.deleteItems(at: removedIndexPaths)
self.collectionView?.insertItems(at: addedIndexPaths)
Please help.

Answer 1

The call to insertItems(at:) and deleteItems(at:) must be accompanied with change in the datasource as well.
So, before calling these APIs, you would want to change your datasource, i.e. add objects into it before calling insertItems and remove objects from it before calling deleteItems

Answer 2

Found a very nice article about UICollectionView invalid number of items crash problem - https://fangpenlin.com/posts/2016/04/29/uicollectionview-invalid-number-of-items-crash-issue/
The item count returned by collectionView(_:numberOfItemsInSection:) should be sync with the updates made inside the closure. With this idea in mind, it’s easy to solve, just add a property as the item count and update it inside performBatchUpdates closure
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)
}
and for the collectionView(_:numberOfItemsInSection:), instead of returning items.count, we return the property which is manually maintained by performBatchUpdates closure.
func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return itemCount
}