RealmSwift RealmCollectionChange tableView crash - swift

Question

Hy, I'm trying to couple RealmNotifications with updating a tableView but for some reason this keeps generating multiple crashes on the tableView because of inconsistency between the number of sections that exist and what the realm notification has sent. This is the code I have for observing any changes on the Results<T>:
do {
let realm = try Realm()
sections = realm.objects(AssetSections.self).filter("isEnabled = true AND (assets.#count > 0 OR isLoading = true OR banners.#count > 0 OR tag == 'My Tools')").sorted(byKeyPath: "sort_order", ascending: true)
guard let sections = sections else { return }
// Watch on the asset sections
notificationToken = sections.observe { [weak self] (change: RealmCollectionChange) in
switch change {
case .initial: break
case .error(let error):
self?.handle(error: error)
case .update(_, let deletions, let insertions, let modifications):
self?.updatedModel.onNext((insertions: insertions, modifications: modifications, deletions: deletions))
}
}
} catch { }
The above code occurs on a ViewModel and a ViewController is observing those changes like so:
vm.updatedModel
.subscribe(onNext: { [weak self] (insertions, modifications, deletions) in
guard let `self` = self else { return }
self.tableView.beginUpdates()
self.tableView.insertSections(insertions, animationStyle: .none)
self.tableView.deleteSections(deletions, animationStyle: .none)
self.tableView.reloadSections(modifications, animationStyle: .none)
self.tableView.endUpdates()
})
.disposed(by: disposeBag)
I'm working with sections instead of Rows because this is a tableView with multiple sections and just one row per section.
The crash I'm getting is if I do a pull to refresh which does multiple network calls which in turn makes multiple changes to the objects. The funny thing is I can always almost replicate the crash if I scroll down rapidly during a pull to refresh. The error is the following:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'attempt to delete section 14, but there are only 14 sections before the update'
The way I get the numberOfSections for the tableView is the following:
var numberOfSections: Int {
return sections?.count ?? 0
}
My suspicion is that since the notifications are getting delivered on the next runLoop of the Main Thread and since I'm making the thread busy by scrolling and messing with the UI by the time I get a notification and the tableView reacts to it, it's already out of sync. But I'm not exactly sure if this is the problem or if it is how to solve it.
Thank you
Edit
One way to avoid this is just .reloadData() on the tableView but it's a perfomance hit especially on big datasets, and I can't use the default tableView animations. To diminish the perfomance hit of calling .reloadData() multiple times I'm using debounce.
vm.updatedModel
.debounce(1.0, scheduler: MainScheduler.instance)
.subscribe(onNext: { [weak self] (insertions, modifications, deletions) in
guard let `self` = self else { return }
self.tableView.reloadData()
})
.disposed(by: disposeBag)

Answer 1

Great question, and it looks like the Realm documentation does exactly as you do (with rows rather than sections) but doesn't address this. I can't see an obvious answer, but best I can do is these possible workarounds (although tbh, neither is great).
Rework the update code to pull together results, then do a single commit when they are complete.
Make a static copy of sections to use locally, and update it within the subscription. i.e. copy the RealmCollection to an array, which then is not a dynamic view into the realm. This will stay synchronised to your updates.
Best I can do. Otherwise, I can't see how you can guarantee synchronisation between the dynamic query and the notification.