# Notification center messages Use Foundation’s notification center with Swift concurrency. ## Discussion In Swift, the [`Notification`](/documentation/Foundation/Notification) type is `nonisolated`, even in cases where it’s posted on a known isolation. To provide specific isolation information and better support Swift concurrency, `NotificationCenter` defines two message types. A [`NotificationCenter.MainActorMessage`](/documentation/Foundation/NotificationCenter/MainActorMessage) binds to the main actor, whereas a [`NotificationCenter.AsyncMessage`](/documentation/Foundation/NotificationCenter/AsyncMessage) uses an arbitrary isolation. Frameworks extend these types to define distinct messages, typically corresponding to an existing [`Notification.Name`](/documentation/Foundation/Notification/Name-swift.typealias), that declare instance properties for their values instead of using a `userInfo` dictionary. As a result, messages can conform to when they either don’t use properties or contain only sendable properties. If your project only needs to support Swift, you can just use the `Message` types. For projects with both Objective-C and Swift code, define a [`Notification`](/documentation/Foundation/Notification) as well as a corresponding `Message` type. ### Posting and observing messages You can post a message with the `post(_:subject:)` method, passing a message instance and optionally providing a subject. To receive messages, add an observer with the `addObserver(of:for:using:)` method. The overloads of this method allow you to observe either messages from a single object or from any object of a given type. Observation ends when you discard the token returned from `addObserver(of:for:using:)` or after an explicit call to [`removeObserver(_:)`](/documentation/Foundation/NotificationCenter/removeObserver(_:)-2gmm0). You can also receive messages as an with the `messages(of:for:bufferSize:)` methods. Several flavors of `addObserver(of:for:using:)` and `messages(of:for:bufferSize:)` take a [`NotificationCenter.MessageIdentifier`](/documentation/Foundation/NotificationCenter/MessageIdentifier), which frameworks may implement to provide a typed, ergonomic experience at the call point for convenience, as described in [SE-0299](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0299-extend-generic-static-member-lookup.md). ### Using messages with the SDK Many system frameworks — including Foundation, UIKit, and AppKit — adopt the [`NotificationCenter.MainActorMessage`](/documentation/Foundation/NotificationCenter/MainActorMessage) and [`NotificationCenter.AsyncMessage`](/documentation/Foundation/NotificationCenter/AsyncMessage) types to facilitate using their notifications in concurrency-safe Swift code. The following example shows how to observe Foundation’s [`TimeZone`](/documentation/Foundation/TimeZone) type for the [`systemTimeZoneDidChange`](/documentation/Foundation/NotificationCenter/MessageIdentifier/systemTimeZoneDidChange) message, which contains the property [`previousTimeZone`](/documentation/Foundation/TimeZone/SystemTimeZoneDidChangeMessage/previousTimeZone). ``` /// Note: Store `token` in a property for as long as you intend to observe. token = NotificationCenter.default.addObserver(of: TimeZone.self, for: .systemTimeZoneDidChange) { message in let identifier = message.previousTimeZone?.identifier ?? "(unknown)" print("Time zone changed from \(identifier).") } ``` For use with existing notification code, [`NotificationCenter.MainActorMessage`](/documentation/Foundation/NotificationCenter/MainActorMessage) and [`NotificationCenter.AsyncMessage`](/documentation/Foundation/NotificationCenter/AsyncMessage) define helper methods that conforming types implement to convert a message to a corresponding notification and vice versa. ## Topics ### Declaring a message [`NotificationCenter.MainActorMessage`](/documentation/Foundation/NotificationCenter/MainActorMessage) A protocol for creating types that you can post to a notification center and bind to the main actor. [`NotificationCenter.AsyncMessage`](/documentation/Foundation/NotificationCenter/AsyncMessage) A protocol for creating types that you can post to a notification center, which posts them to an arbitrary isolation. ### Using message identifiers [`NotificationCenter.MessageIdentifier`](/documentation/Foundation/NotificationCenter/MessageIdentifier) An optional identifier to associate a given message with a given type. [`NotificationCenter.BaseMessageIdentifier`](/documentation/Foundation/NotificationCenter/BaseMessageIdentifier) A type for use when defining optional Message identifiers. ### Observing concurrency-safe notifications [`addObserver(of:for:using:)`](/documentation/Foundation/NotificationCenter/addObserver(of:for:using:)-4d19x) Adds an observer to a center for messages delivered on the main actor with a given subject and identifier. [`addObserver(of:for:using:)`](/documentation/Foundation/NotificationCenter/addObserver(of:for:using:)-90os) Adds an observer to a center for messages delivered on the main actor with a given subject and identifier. [`addObserver(of:for:using:)`](/documentation/Foundation/NotificationCenter/addObserver(of:for:using:)-56bn4) Adds an observer to a center for messages delivered on the main actor with a given subject and message type. [`addObserver(of:for:using:)`](/documentation/Foundation/NotificationCenter/addObserver(of:for:using:)-twm3) Adds an observer to a center for messages delivered asynchronously with a given subject and identifier. [`addObserver(of:for:using:)`](/documentation/Foundation/NotificationCenter/addObserver(of:for:using:)-t1wr) Adds an observer to a center for messages delivered asynchronously with a given subject and message type. [`addObserver(of:for:using:)`](/documentation/Foundation/NotificationCenter/addObserver(of:for:using:)-64uw3) Adds an observer to a center for messages delivered asynchronously with a given subject and message type. [`removeObserver(_:)`](/documentation/Foundation/NotificationCenter/removeObserver(_:)-2gmm0) Stops the observation represented by the given observation token. [`NotificationCenter.ObservationToken`](/documentation/Foundation/NotificationCenter/ObservationToken) A unique token representing a single observer registration in a notification center. ### Receiving notifications as asynchronous sequences [`messages(of:for:bufferSize:)`](/documentation/Foundation/NotificationCenter/messages(of:for:bufferSize:)-4tof0) Returns an asynchronous sequence of messages produced by this center for a given subject and identifier. [`messages(of:for:bufferSize:)`](/documentation/Foundation/NotificationCenter/messages(of:for:bufferSize:)-1ub69) Returns an asynchronous sequence of messages produced by this center for a given subject type and identifier. [`messages(of:for:bufferSize:)`](/documentation/Foundation/NotificationCenter/messages(of:for:bufferSize:)-623kg) Returns an asynchronous sequence of messages produced by this center for a given subject and message type. ### Posting notification messages [`post(_:subject:)`](/documentation/Foundation/NotificationCenter/post(_:subject:)-87dbk) Posts a given main actor message to the notification center. [`post(_:)`](/documentation/Foundation/NotificationCenter/post(_:)-19s7b) Posts a given main actor message to the notification center. [`post(_:subject:)`](/documentation/Foundation/NotificationCenter/post(_:subject:)-5271w) Posts a given asynchronous message to the notification center. [`post(_:)`](/documentation/Foundation/NotificationCenter/post(_:)-7ia4j) Posts a given asynchronous message to the 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)