---
title: NotificationCenter.AsyncMessage
framework: foundation
role: symbol
role_heading: Protocol
path: foundation/notificationcenter/asyncmessage
---

# NotificationCenter.AsyncMessage

A protocol for creating types that you can post to a notification center, which posts them to an arbitrary isolation.

## Declaration

```swift
protocol AsyncMessage : Sendable
```

## Overview

Overview You post types conforming to AsyncMessage to a notification center using post(_:subject:) and observe them with addObserver(of:for:using:). The notification center delivers AsyncMessage types asynchronously when posted. Asynchronous delivery isn’t suitable for messages with time-critical deliveries, such as a message that must have its observers called before a certain action takes place. For types that post on the main actor, use NotificationCenter.MainActorMessage. Each AsyncMessage is associated with a specific Subject type. For example, an AsyncMessage associated with the type Event could use the following declaration: struct EventDidStart: NotificationCenter.AsyncMessage {     typealias Subject = Event } AsyncMessage can use an optional NotificationCenter.MessageIdentifier type for context-aware observer registration: extension NotificationCenter.MessageIdentifier where Self == NotificationCenter.BaseMessageIdentifier<EventDidStart> {     static var didStart: Self { .init() } } With this identifier, observers can receive information about a specific instance by registering for this message with a NotificationCenter: let observerToken = NotificationCenter.default.addObserver(of: importantEvent, for: .didStart) Or an observer can receive information about any instance with: let observerToken = NotificationCenter.default.addObserver(of: Event.self, for: .didStart) The notification center ties observation the lifetime of the returned NotificationCenter.ObservationToken and automatically de-registers the observer if the token goes out of scope. You can also remove observation explicitly: NotificationCenter.default.removeObserver(observerToken) Notification Interoperability AsyncMessage includes optional interoperability with Notification, enabling posters and observers of both types to pass information. It does this by offering a makeMessage(_:) method that collects values from a Notification‘s userInfo and populates properties on a new message. In the other direction, a makeNotification(_:) method collects the message’s defined properties and loads them into a new notification’s userInfo dictionary. For example, if there exists a Notification posted on an arbitrary isolation identified by the Notification.Name "eventDidFinish" with a userInfo dictionary containing the key "duration" as an NSNumber, an app could post and observe the notification with the following NotificationCenter.AsyncMessage: struct EventDidFinish: NotificationCenter.AsyncMessage {     typealias Subject = Event     static var name: Notification.Name { Notification.Name("eventDidFinish") }

var duration: Int

static func makeNotification(_ message: Self) -> Notification {         return Notification(name: Self.name, userInfo: ["duration": NSNumber(message.duration)])     }

static func makeMessage(_ notification: Notification) -> Self? {         guard let userInfo = notification.userInfo,               let duration = userInfo["duration"] as? Int         else {             return nil         }

return Self(duration: duration)     } } With this definition, an observer for this AsyncMessage type receives information even if the poster used the Notification equivalent, and vice versa.

## Topics

### Declaring the message name and subject

- [name](foundation/notificationcenter/asyncmessage/name.md)
- [Subject](foundation/notificationcenter/asyncmessage/subject.md)

### Converting between messages and notifications

- [makeMessage(_:)](foundation/notificationcenter/asyncmessage/makemessage(_:).md)
- [makeNotification(_:)](foundation/notificationcenter/asyncmessage/makenotification(_:).md)

## Relationships

### Inherits From

- [Sendable](swift/sendable.md)
- [SendableMetatype](swift/sendablemetatype.md)

### Conforming Types

- [Bundle.DidLoadMessage](foundation/bundle/didloadmessage.md)
- [Calendar.CalendarDayChangedMessage](foundation/calendar/calendardaychangedmessage.md)
- [FileHandle.ConnectionAcceptedMessage](foundation/filehandle/connectionacceptedmessage.md)
- [FileHandle.DataAvailableMessage](foundation/filehandle/dataavailablemessage.md)
- [FileHandle.ReadCompletionMessage](foundation/filehandle/readcompletionmessage.md)
- [FileHandle.ReadToEndOfFileCompletionMessage](foundation/filehandle/readtoendoffilecompletionmessage.md)
- [HTTPCookieStorage.CookiesChangedMessage](foundation/httpcookiestorage/cookieschangedmessage.md)
- [NSBundleResourceRequest.LowDiskSpaceMessage](foundation/nsbundleresourcerequest/lowdiskspacemessage.md)
- [NSMetadataQuery.DidFinishGatheringMessage](foundation/nsmetadataquery/didfinishgatheringmessage.md)
- [NSMetadataQuery.DidStartGatheringMessage](foundation/nsmetadataquery/didstartgatheringmessage.md)
- [Port.DidBecomeInvalidMessage](foundation/port/didbecomeinvalidmessage.md)
- [Process.DidTerminateMessage](foundation/process/didterminatemessage.md)
- [ProcessInfo.PowerStateDidChangeMessage](foundation/processinfo/powerstatedidchangemessage.md)
- [ProcessInfo.ThermalStateDidChangeMessage](foundation/processinfo/thermalstatedidchangemessage.md)
- [UserDefaults.DidChangeMessage](foundation/userdefaults/didchangemessage.md)

## See Also

### Declaring a message

- [NotificationCenter.MainActorMessage](foundation/notificationcenter/mainactormessage.md)