Updating selected collectionview indexes after responding to PHPhotoLibraryChangeObserver event - ios

Question

I am making a multiple selection feature for my collection view which shows photos from the user's library. I keep track of the selected indexPaths in an array and I want to update them in case a photo library change observer event happens in the middle of selecting cells. for example, if a user has selected indexes 3 and 4 and a change observer event removes indexes 1 and 2 from the collection view, selected indexes should change to 1 and 2.
I am trying to do it manually using these functions:
fileprivate func removeIndicesFromSelections(indicesToRemove:IndexSet){
var itemToRemove: Int?
for (_, removeableIndex) in indicesToRemove.map({$0}).enumerated() {
itemToRemove = nil
for (itemIndex,indexPath) in selectedIndices.enumerated() {
//deduct 1 from indices after the deletion index
if (indexPath.item > removeableIndex) && (indexPath.item > 0) {
selectedIndices[itemIndex] = IndexPath(item: indexPath.item - 1, section: 0)
} else if indexPath.item == removeableIndex {
itemToRemove = itemIndex
}
}
if let remove = itemToRemove {
selectedIndices.remove(at: remove)
disableDeleteButtonIfNeeded()
}
}
}
fileprivate func moveSelectedIndicesAfterInsertion (insertedIndices:IndexSet){
for (_, insertedIndex) in insertedIndices.map({$0}).enumerated() {
for (itemIndex,indexPath) in selectedIndices.enumerated() {
//add 1 to indices after the insertion index
if (indexPath.item >= insertedIndex) {
selectedIndices[itemIndex] = IndexPath(item: indexPath.item + 1, section: 0)
}
}
}
}
However, these are getting more complicated than I expected and I keep finding bugs in them. Is there any better way to handle this situation (such as any built in collection view capabilities) or I just have to come up with my own functions like above?

Answer 1

You're on the right path, but you should store a reference to what object the user actually selected, not where they selected it (since that can change).
In this case, you should keep a reference to the selected photos' identifiers (see docs) and then you can determine what cell/index-path should be selected. You can compare your selection array against your image datasource to determine what the most up-to-date index path is.

Answer 2

There is a solution provided by Apple. You can find more information in official documentation page:
Bacically you want to adopt PHPhotoLibraryChangeObserver and implement the following function:
func photoLibraryDidChange(_ changeInstance: PHChange) {
guard let collectionView = self.collectionView else { return }
// Change notifications may be made on a background queue.
// Re-dispatch to the main queue to update the UI.
DispatchQueue.main.sync {
// Check for changes to the displayed album itself
// (its existence and metadata, not its member assets).
if let albumChanges = changeInstance.changeDetails(for: assetCollection) {
// Fetch the new album and update the UI accordingly.
assetCollection = albumChanges.objectAfterChanges! as! PHAssetCollection
navigationController?.navigationItem.title = assetCollection.localizedTitle
}
// Check for changes to the list of assets (insertions, deletions, moves, or updates).
if let changes = changeInstance.changeDetails(for: fetchResult) {
// Keep the new fetch result for future use.
fetchResult = changes.fetchResultAfterChanges
if changes.hasIncrementalChanges {
// If there are incremental diffs, animate them in the collection view.
collectionView.performBatchUpdates({
// For indexes to make sense, updates must be in this order:
// delete, insert, reload, move
if let removed = changes.removedIndexes where removed.count > 0 {
collectionView.deleteItems(at: removed.map { IndexPath(item: $0, section:0) })
}
if let inserted = changes.insertedIndexes where inserted.count > 0 {
collectionView.insertItems(at: inserted.map { IndexPath(item: $0, section:0) })
}
if let changed = changes.changedIndexes where changed.count > 0 {
collectionView.reloadItems(at: changed.map { IndexPath(item: $0, section:0) })
}
changes.enumerateMoves { fromIndex, toIndex in
collectionView.moveItem(at: IndexPath(item: fromIndex, section: 0),
to: IndexPath(item: toIndex, section: 0))
}
})
} else {
// Reload the collection view if incremental diffs are not available.
collectionView.reloadData()
}
}
}
}