# addObserver(of:for:using:) Adds an observer to a center for messages delivered on the main actor with a given subject and message type. ``` func addObserver(of subject: Message.Subject? = nil, for messageType: Message.Type, using observer: @escaping @MainActor (Message) -> Void) -> NotificationCenter.ObservationToken where Message : NotificationCenter.MainActorMessage, Message.Subject : AnyObject ``` ## Parameters `subject` The subject to be observed. Specify a metatype to observe all values for a given type. `messageType` The message type to be observed. `observer` A closure to execute when receving a message. ## Return Value A token representing the observation registration with the given notification center. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)