So far, here's are the code snippets that almost work for NSFetchedResultsController + UICollectionView, based on the information provided
- https://developer.apple.com/videos/play/wwdc2018/225 (Starting at 36:50)
- https://stackoverflow.com/questions/38597804/how-to-order-moves-inserts-deletes-and-updates-in-a-uicollectionview-performb/53271562#53271562
Please note that, there are 2 [BlockOperation], as reloadItems and moveItem doesn't play well within single performBatchUpdates. Based on the workaround proposed in the video, we have to call reloadItems in a separate performBatchUpdates.
We also do not follow 100% methods (Perform reloadItems typed performBatchUpdates first, followed by insert/ move/ delete typed performBatchUpdates) proposed in the video.
This is because we notice that it doesn't work well even for simple case. Some strange behaviour including reloadItems will cause duplicated cell UI to be shown on screen. The "almost" work method we found are
- Perform performBatchUpdates for insert, move and delete
- At completion handler of performBatchUpdates, perform another performBatchUpdates for reloadItems
NSFetchedResultsController + UICollectionView integration
private var blockOperations: [BlockOperation] = []
// reloadItems and moveItem do not play well together. We are using the following workaround proposed at
// https://developer.apple.com/videos/play/wwdc2018/225/
private var blockUpdateOperations: [BlockOperation] = []
extension DashboardViewController: NSFetchedResultsControllerDelegate {
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
if type == NSFetchedResultsChangeType.insert {
print(">> insert")
blockOperations.append(
BlockOperation(block: { [weak self] in
if let self = self {
self.collectionView!.insertItems(at: [newIndexPath!])
}
})
)
}
else if type == NSFetchedResultsChangeType.update {
print(">> update")
blockUpdateOperations.append(
BlockOperation(block: { [weak self] in
if let self = self, let indexPath = indexPath {
self.collectionView.reloadItems(at: [indexPath])
}
})
)
}
else if type == NSFetchedResultsChangeType.move {
print(">> move")
blockOperations.append(
BlockOperation(block: { [weak self] in
if let self = self, let newIndexPath = newIndexPath, let indexPath = indexPath {
self.collectionView.moveItem(at: indexPath, to: newIndexPath)
}
})
)
}
else if type == NSFetchedResultsChangeType.delete {
print(">> delete")
blockOperations.append(
BlockOperation(block: { [weak self] in
if let self = self {
self.collectionView!.deleteItems(at: [indexPath!])
}
})
)
}
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {
if type == NSFetchedResultsChangeType.insert {
print(">> section insert")
blockOperations.append(
BlockOperation(block: { [weak self] in
if let self = self {
self.collectionView!.insertSections(IndexSet(integer: sectionIndex))
}
})
)
}
else if type == NSFetchedResultsChangeType.update {
print(">> section update")
blockOperations.append(
BlockOperation(block: { [weak self] in
if let self = self {
self.collectionView!.reloadSections(IndexSet(integer: sectionIndex))
}
})
)
}
else if type == NSFetchedResultsChangeType.delete {
print(">> section delete")
blockOperations.append(
BlockOperation(block: { [weak self] in
if let self = self {
self.collectionView!.deleteSections(IndexSet(integer: sectionIndex))
}
})
)
}
}
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
if blockOperations.isEmpty {
performBatchUpdatesForUpdateOperations()
} else {
collectionView.performBatchUpdates({ [weak self] () -> Void in
guard let self = self else { return }
for operation: BlockOperation in self.blockOperations {
operation.start()
}
self.blockOperations.removeAll(keepingCapacity: false)
}, completion: { [weak self] (finished) -> Void in
print("blockOperations completed")
guard let self = self else { return }
self.performBatchUpdatesForUpdateOperations()
})
}
}
private func performBatchUpdatesForUpdateOperations() {
if blockUpdateOperations.isEmpty {
return
}
collectionView.performBatchUpdates({ [weak self] () -> Void in
guard let self = self else { return }
for operation: BlockOperation in self.blockUpdateOperations {
operation.start()
}
self.blockUpdateOperations.removeAll(keepingCapacity: false)
}, completion: { [weak self] (finished) -> Void in
print("blockUpdateOperations completed")
guard let self = self else { return }
})
}
}
The above way, works "almost" well when no "section" operations involved.
If there is item moved operation between different section, the following logging will be printed
blockOperations completed
>> move
blockOperations completed
>> move
blockOperations completed
However, if there are item being moved and section being added/ removed, the following logging will be printed
>> section delete
>> move
>> section insert
>> move
This means the completion handler block is not executed! Does anyone know why it is so, and how I can workaround with this issue? Thanks.