How can I migrate NotificationCenter code to Swift 6 concurrency?

You can use the notification center API that returns an AsyncSequence, notifications(named:object:). Then just consume the AsyncSequence in a main actor task. Rather than capturing the unsendable NSObject, capture just its ObjectIdentifier, since that's all you need for the identity comparison. private func registerObserver( _ name: NSNotification.Name, ignoreIfSentFrom ignoredObject: NSObject, block: @Sendable @MainActor @escaping (Notification) -> () ) { let ignoredObjectID = ObjectIdentifier(ignoredObject) let task = Task { @MainActor in let stream = NotificationCenter.default.notifications(named: name, object: nil).filter { ($0.object as? NSObject).map(ObjectIdentifier.init) != ignoredObjectID } for await notification in stream { block(notification) } } // the 'task' in this case has a similar role to an observer token // you can store it in an array and cancel it once you don't need it anymore // observerTokens.append(task) }

I would also consider simply returning the async sequence to let the caller decide how to consume it.

private func notifications( _ name: NSNotification.Name, ignoreIfSentFrom ignoredObject: NSObject ) -> some AsyncSequence<Notification, Never> { let ignoredObjectID = ObjectIdentifier(ignoredObject) return NotificationCenter.default.notifications(named: name, object: nil).filter { ($0.object as? NSObject).map(ObjectIdentifier.init) != ignoredObjectID } }

This way you don't force the caller to provide a main actor isolated callback. The caller can decide where it wants to be isolated (if at all).