Deleting Realm objects on a background thread blocks the main thread

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.