Deleting Realm objects on a background thread blocks the main thread - ios

Question

I'm experiencing freezes on the main thread after deleting 10,000 objects out of 500,000 on a background thread. Insertions, however, don't cause this issue.
The trigger is the Results observer on the main thread.
Is this a bug in Realm or am I missing something?
Here is an example that produces the mentioned behavior:
AppDelegate
var realm: Realm!
var token: NotificationToken?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
realm = try! Realm()
token = realm.objects(Item.self).observe { change in
switch change {
case let .update(_, deletions, insertions, modifications):
print("deletions: \(deletions.count)")
print("insertions: \(insertions.count)")
print("modifications: \(modifications.count)")
default:
break
}
}
// addItemsAsync(count: 600000)
deleteItemsAsync(count: 10000)
return true
}
Adding items
func addItemsAsync(count: Int) {
DispatchQueue.global().async {
autoreleasepool {
let realm = try! Realm()
try! realm.write {
for i in 0..<count {
realm.create(Item.self, value: ["id": i])
}
}
}
}
}
Deleting items
func deleteItemsAsync(count: Int) {
DispatchQueue.global().async {
autoreleasepool {
let realm = try! Realm()
let itemsToDelete = realm.objects(Item.self).filter("id < \(count)")
try! realm.write {
realm.delete(itemsToDelete)
}
}
}
}
Item
class Item: Object {
#objc dynamic var id = 0
}
I also noticed that, unlike insertions, a deletion of this sort doesn't simply notify the observer with 10,000 deletions, but instead I get this here once the results are updated on the main thread:
deletions: 20000
insertions: 10000
modifications: 0
This is obviously due to re-ordering. But I would have expected that Realm updates the results in the background and then simply swaps it on the main thread (especially these kind of expensive operations).

Answer 1

Realm not thread safe. If you want to use in different thread you can use ThreadSafeReference.