---
title: NotificationCenter.MainActorMessage
framework: foundation
role: symbol
role_heading: Protocol
path: foundation/notificationcenter/mainactormessage
---

# NotificationCenter.MainActorMessage

A protocol for creating types that you can post to a notification center and bind to the main actor.

## Declaration

```swift
protocol MainActorMessage : SendableMetatype
```

## Overview

Overview You post types conforming to  MainActorMessage to a notification center using post(_:subject:) and observe them with addObserver(of:for:using:). The notification center delivers MainActorMessage types synchronously when posted. For types that post on an arbitrary isolation, use NotificationCenter.AsyncMessage. Each MainActorMessage is associated with a specific Subject type. For example, a MainActorMessage associated with the type Event could use the following declaration: struct EventDidStart: NotificationCenter.MainActorMessage {     typealias Subject = Event } MainActorMessage 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 MainActorMessage 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 MainActor 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.MainActorMessage: struct EventDidFinish: NotificationCenter.MainActorMessage {     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 MainActorMessage type receives information even if the poster used the Notification equivalent, and vice versa.

## Topics

### Declaring the message name and subject

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

### Converting between messages and notifications

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

## Relationships

### Inherits From

- [SendableMetatype](swift/sendablemetatype.md)

### Conforming Types

- [Date.SystemClockDidChangeMessage](foundation/date/systemclockdidchangemessage.md)
- [FileManager.UbiquityIdentityDidChangeMessage](foundation/filemanager/ubiquityidentitydidchangemessage.md)
- [Locale.CurrentLocaleDidChangeMessage](foundation/locale/currentlocaledidchangemessage.md)
- [NSExtensionContext.DidBecomeActiveMessage](foundation/nsextensioncontext/didbecomeactivemessage.md)
- [NSExtensionContext.DidEnterBackgroundMessage](foundation/nsextensioncontext/didenterbackgroundmessage.md)
- [NSExtensionContext.WillEnterForegroundMessage](foundation/nsextensioncontext/willenterforegroundmessage.md)
- [NSExtensionContext.WillResignActiveMessage](foundation/nsextensioncontext/willresignactivemessage.md)
- [TimeZone.SystemTimeZoneDidChangeMessage](foundation/timezone/systemtimezonedidchangemessage.md)
- [UndoManager.CheckpointMessage](foundation/undomanager/checkpointmessage.md)
- [UndoManager.DidCloseUndoGroupMessage](foundation/undomanager/didcloseundogroupmessage.md)
- [UndoManager.DidOpenUndoGroupMessage](foundation/undomanager/didopenundogroupmessage.md)
- [UndoManager.DidRedoChangeMessage](foundation/undomanager/didredochangemessage.md)
- [UndoManager.DidUndoChangeMessage](foundation/undomanager/didundochangemessage.md)
- [UndoManager.WillCloseUndoGroupMessage](foundation/undomanager/willcloseundogroupmessage.md)
- [UndoManager.WillRedoChangeMessage](foundation/undomanager/willredochangemessage.md)
- [UndoManager.WillUndoChangeMessage](foundation/undomanager/willundochangemessage.md)
- [UserDefaults.SizeLimitExceededMessage](foundation/userdefaults/sizelimitexceededmessage.md)

## See Also

### Declaring a message

- [NotificationCenter.AsyncMessage](foundation/notificationcenter/asyncmessage.md)