How does FCM notification work?

If the device comes back online FCM will notify the app that pending messages were deleted: When the device gets back online, FCM will call onDeletedMessages() on Android . When the app is opened, FCM will trigger FIRMessagingMessagesDeletedNotification on iOS to notify the app that pending messages were deleted.

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What is FCM push notification?

Using FCM, you can notify a client app that new email or other data is available to sync . You can send notification messages to drive user re-engagement and retention. For use cases such as instant messaging, a message can transfer a payload of up to 4000 bytes to a client app.

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