Updating UICollectionView after deleting a Realm object - ios

Question

When I try to delete an item from a Realm database I am unable to update a UICollection View appropriately.
Lets assume a Realm container children of type List<Child>:
var children = realm.objects(Parent).first!.children
When I want to remove this child from the database by:
try! realm.write {
realm.delete(children[indexPath.row])
}
updating the collectionView by collectionView.deleteItemsAtIndexPaths([indexPath]) gives the following error:
Got error: *** Terminating app due to uncaught exception 'RLMException', reason: 'Object has been deleted or invalidated.'
The only way I get the collectionView updated is by using collectionView.reloadData(), but that is not what I want since the animation of a cell deletion is missing.
However, when I only remove a child from this container at indexPath.row (without removing it from the database) by:
try! realm.write {
children.removeAtIndex(indexPath.row)
}
updating the collectionView with collectionView.deleteItemsAtIndexPaths([indexPath]) works without problems.
What would be the best way to update a UICollectionView after removing an item from the database?

Answer 1

The error you're facing appears when you keep accessing an object, which was already deleted. So, you're storing likely somewhere a reference to your object, which is fine per se, but keep accessing it after it was invalidated.
That could happen e.g. in your custom subclass of UICollectionViewCell. I'd recommend to implement a setter on your cell and pull from that method the property values into your view components. You can even use KVO in your cell to update these. (We've an example based on ReactKit for that up in our repo.) You can't though keep accessing the properties when the object might be already deleted at a later point in time, e.g. if your cell needs to be drawn or layout when it is faded out.
I'd recommend to subscribe to fine-grained notifications for the list you're using to fill your collection view's cells and only propagate updates in that way to the collection view. In that way you can make sure that your items will be removed with a nice animation as requested and it is automatically taken care of. All put together this could look like seen below. Over at our repo, you'll find a complete runnable sample.
class Cell: UICollectionViewCell {
#IBOutlet var label: UILabel!
func attach(object: DemoObject) {
label.text = object.title
}
}
class CollectionViewController: UICollectionViewController {
var notificationToken: NotificationToken? = nil
lazy var realm = try! Realm()
lazy var results: Results<DemoObject> = {
self.realm.objects(DemoObject)
}()
// MARK: View Lifecycle
override func viewDidLoad() {
super.viewDidLoad()
// Observe Notifications
notificationToken = results.addNotificationBlock { [weak self] (changes: RealmCollectionChange) in
guard let collectionView = self?.collectionView else { return }
switch changes {
case .Initial:
// Results are now populated and can be accessed without blocking the UI
collectionView.reloadData()
break
case .Update(_, let deletions, let insertions, let modifications):
// Query results have changed, so apply them to the UITableView
collectionView.performBatchUpdates({
collectionView.insertItemsAtIndexPaths(insertions.map { NSIndexPath(forRow: $0, inSection: 0) })
collectionView.deleteItemsAtIndexPaths(deletions.map { NSIndexPath(forRow: $0, inSection: 0) })
collectionView.reloadItemsAtIndexPaths(modifications.map { NSIndexPath(forRow: $0, inSection: 0) })
}, completion: { _ in })
break
case .Error(let error):
// An error occurred while opening the Realm file on the background worker thread
fatalError("\(error)")
break
}
}
}
deinit {
notificationToken?.stop()
}
// MARK: Helpers
func objectAtIndexPath(indexPath: NSIndexPath) -> DemoObject {
return results[indexPath.row]
}
// MARK: UICollectionViewDataSource
override func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return results.count
}
override func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
let object = objectAtIndexPath(indexPath)
try! realm.write {
realm.delete(object)
}
}
override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
let object = objectAtIndexPath(indexPath)
let cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath) as! Cell
cell.attach(object)
return cell
}
}