Information about a downstream message received by the app.

Firebase Messaging lets you reliably deliver messages at no cost.

To send or receive messages, the app must get a registration token. This token authorizes an app server to send messages to an app instance.

In order to handle incoming Messaging messages, set the UNUserNotificationCenter‘s delegate property and implement the appropriate methods.

This class is used to automatically populate a notification with an image if it is specified in the notification body via the image parameter. Images and other rich content can be populated manually without the use of this class. See the UNNotificationServiceExtension type for more details.