UICollectionView reloads data without calling reloadData() - ios

Question

I have structured my app using the MVVM pattern, the datasource of the collectionView gets data from the viewModel.
In viewModel I have a closure which gets called after updating the data, it passes IndexSet of sections and [IndexPath] of items which collectionView should insert. However I get the crash everytime after calling the insert method with error:
'Invalid update: invalid number of sections. The number of sections contained in the collection view after the update (11) must be equal to the number of sections contained in the collection view before the update (11), plus or minus the number of sections inserted or deleted (11 inserted, 0 deleted).'
I understand what this error means, however I noticed that this is the order in which the methods are called
1. viewDidLoad()
2. numberOfSections(in collectionView: UICollectionView) // returns 0
3. update?(insertedSectionsSet, insertedRows) // the closure gets called in viewModel, insertedSectionsSet has 11 elements, that's correct
4. numberOfSections(in collectionView: UICollectionView) // returns 11, that's correct but it should be called after next point
5. viewModel.update = { [weak self] sections, items in
DispatchQueue.main.async {
self?.collectionView.performBatchUpdates({
self?.collectionView.insertSections(sections) // crash
self?.collectionView.insertItems(at: items)
}, completion: nil)
}
}
As you can see in the code below, I added prints before calling each methods and you can clearly see that numberOfSections gets called after calling the closure and before performing it. It makes absolutely no sense to me why is it happening. I believe that the cause of crash lies in calling the numberOfSections before inserting them, because then it expects 22 sections after inserting.
update
numberofsections
closure
numberofsections
Code:
class PlaylistsDataSource: NSObject, UICollectionViewDataSource {
...
func numberOfSections(in collectionView: UICollectionView) -> Int {
print("numberofsections")
return viewModel.numberOfSections
}
...
}
class PlaylistsMasterViewController: UIViewController {
...
viewDidLoad() {
viewModel.update = { [weak self] sections, items in
DispatchQueue.main.async {
print("closure")
self?.collectionView.performBatchUpdates({
self?.collectionView.insertSections(sections)
self?.collectionView.insertItems(at: items)
}, completion: nil)
}
}
}
...
}
class PlaylistsMasterViewModel {
private var sectionIndexes = [String]()
private var playlistsDictionary = [String: [AWPlaylist]]()
var update: ((IndexSet, [IndexPath]) -> Void)?
var numberOfSections: Int {
return sectionIndexes.count
}
EDIT: Added more code
extension PlaylistsMasterViewModel {
func fetchPlaylists() {
repo.getAllPlaylists(from: [.iTunes]) { [weak self] result in
switch result {
case .success(let playlists):
self?.sortIncoming(playlists: playlists)
case .failure(let error):
print(error.localizedDescription)
}
}
}
private func sortIncoming(playlists: [AWPlaylist]) {
var insertedPlaylists = [(key: String, list: AWPlaylist)]()
var insertedIndexes = [String]()
func insertNewSection(playlist: AWPlaylist, key: String) {
insertedIndexes.append(key)
playlistsDictionary.updateValue([playlist], forKey: key)
}
func insertNewRow(playlist: AWPlaylist, key: String) {
guard var value = playlistsDictionary[key] else {
print("Oh shit")
return
}
value.append(playlist)
value.sort(by: SortingPredicates.Playlist.nameAscending)
playlistsDictionary.updateValue(value, forKey: key)
insertedPlaylists.append((key, playlist))
}
for list in playlists {
let name = list.localizedName.uppercased().trimmingCharacters(in: CharacterSet.whitespaces)
guard let firstCharacter = name.first else { return }
let firstLetter = String(firstCharacter)
let key: String
if CharacterSet.english.contains(firstLetter.unicodeScalars.first!) {
key = firstLetter
} else if CharacterSet.numbers.contains(firstLetter.unicodeScalars.first!) {
key = "#"
} else {
key = "?"
}
if playlistsDictionary[key] == nil {
insertNewSection(playlist: list, key: key)
} else {
insertNewRow(playlist: list, key: key)
}
}
sectionIndexes.append(contentsOf: insertedIndexes)
sectionIndexes.sort(by: { $0 < $1 })
let insertedSections = insertedIndexes.compactMap { index -> Int? in
guard let sectionIndex = self.sectionIndexes.firstIndex(of: index) else {
return nil
}
return sectionIndex
}
let insertedSectionsSet = IndexSet(insertedSections)
let insertedRows = insertedPlaylists.compactMap { tuple -> IndexPath? in
if let section = self.sectionIndexes.firstIndex(of: tuple.key) {
if let row = self.playlistsDictionary[tuple.key]?.firstIndex(where: { $0.equals(tuple.list) }) {
return IndexPath(row: row, section: section)
}
}
return nil
}
print("update")
update?(insertedSectionsSet, insertedRows)
}