Realm Swift: Object has been deleted or invalidated - ios

Question

I have a tableViewController (ArticlesVC) that displays an array of Articles that are fetched from an API. Each of these tableViewCell has a "more action" button that allows the user to save the article for later reading in the SavedVC. Note that only saved articles are written to Realm DB, the displaying of articles in the ArticlesVC are not written to Realm DB.
If the user has saved the article, I would show "Remove saved article", otherwise, I show "Save for later". I query Realm to conduct this check.
However, if I delete a saved article (or all saved articles) from Realm from SavedVC and go back to the ArticlesVC and start swiping the tableView, it crashes with the above error. This crash happens on an intermittent basis, making it pretty hard to pinpoint.
Code
//At ArticlesVC, where articles are fetched via API
func getArticles() {
AF.request(apiUrl).responseJSON { (response) in
//Error checks and decoding ...
self.articles = try decoder.decode(Article.self, from: data)
}
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath) as! ArticleCell
cell.article = articles[indexPath.row]
cell.moreButton.tag = indexPath.row
cell.moreButton.addTarget(self, action: #selector(moreButtonTapped(sender:)), for: .touchUpInside)
return cell
}
#objc func moreButtonTapped(sender: UIButton) {
let buttonTag = sender.tag
let article = articles[buttonTag]
let alert = UIAlertController(title: "More", message: nil, preferredStyle: .actionSheet)
let action = getAction(article: article)
alert.addActions([action, .cancelAction()])
//present alert
}
func getAction(article: Article) -> UIAlertAction {
do {
let realm = try Realm()
let articleInRealm = realm.objects(Article.self).filter("id == %#", article.id)
if articleInRealm.isInvalidated {
return UIAlertAction(title: "Save for later", style: .default, handler: {(_) in
self.saveArticle(article: article)
})
} else {
if articleInRealm.count == 0 {
return UIAlertAction(title: "Save for later", style: .default, handler: {(_) in
self.saveArticle(article: article)
})
} else {
return UIAlertAction(title: "Remove saved article", style: .destructive, handler: {(_) in
self.deleteArticle(article: articleInRealm)
})
}
}
} catch {
Log("Err getting article: \(error.localizedDescription)")
return UIAlertAction(title: "Save for later", style: .default, handler: {(_) in
self.saveArticle(article: article)
})
}
}
func saveArticle(article: Article) {
do {
let realm = try Realm()
try realm.write {
realm.add(article, update: .modified)
}
} catch {
Log("Err saving article: \(error.localizedDescription)")
}
}
func deleteArticle(article: Results<Article>) {
do {
let realm = try Realm()
try realm.write {
realm.delete(article)
}
} catch {
Log("Err saving article: \(error.localizedDescription)")
}
}
//At SavedVC, which is another VC in the tabBarController.
//ArticlesVC is in tab1 and SavedVC is in tab2
var notificationToken: NotificationToken? = nil
var articles: Results<Article>?
override func viewDidLoad() {
super.viewDidLoad()
subscribeToRealmNotifications()
}
fileprivate func subscribeToRealmNotifications() {
let realm = try! Realm()
let results = realm.objects(Article.self)
notificationToken = results.observe { [weak self] (changes: RealmCollectionChange) in
guard let tableView = self?.tableView else { return }
switch changes {
case .initial:
self?.articles = results
tableView.reloadData()
case .update(_, _, _, _):
tableView.reloadData()
case .error(let error):
fatalError("Realm notif: \(error.localizedDescription)")
}
}
}
My guess is that because I have deleted the articles in SavedVC but ArticlesVC is still pointing to those Realm objects which no longer exist and therefore crashed.
I have read multiple SO posts that suggest to do a check on obj.isInvalidated, which I did and have included a condition for it. But this still crashes.
Other attempts includes:
using realm.create() instead of realm.add()

Answer 1

Here's the issue: You've saving an unmanaged object (article) into realm
func getAction(article: Article)
...
self.saveArticle(article: article)
which then makes it a managed object.
If it's deleted from Realm,
realm.delete(article)
it will also be removed from the results object that contains it
articles[indexPath.row]
Which will then make your app crash as it's scrolling as the dataSource and tableView are out of sync.
I have a feeling you don't want it to actually be removed from the master list of articles.
My suggestion is to save any articles you want to read later by their id which will have no effect on the master list of articles as they are added and removed from realm.
In other words your master list of articles stays the same but in Realm, you have a table of id's which are strings for example. So when you want to track what they want to read
self.saveArticleId(article: article.id)
and then when it's time to delete it from their saved reading list
self.deleteArticleBy(articleId: artical.id)