Should the Push Notification Guide direct the user to subscribe to FCM token updates?

[dminkovsky]

In the Push Notification Guide, it says the user can get the FCM token by querying InstanceID:

Messaging.messaging().apnsToken = deviceToken
InstanceID.instanceID().instanceID { (result, error) in
	if let error = error {
		NotificationCenter.default.post(name: Notification.Name(CAPNotifications.DidFailToRegisterForRemoteNotificationsWithError.name()), object: error)
	} else if let result = result {
		NotificationCenter.default.post(name: Notification.Name(CAPNotifications.DidRegisterForRemoteNotificationsWithDeviceToken.name()), object: result.token)
	}
}

However, in the FCM docs about setting up an iOS client, it says you can also monitor the token to find out when it changes:

https://firebase.google.com/docs/cloud-messaging/ios/client?authuser=1#monitor-token-refresh

Practically, does it make a difference? Should the docs direct the user to this second approach?

[dminkovsky]

Also maybe this is all beyond the scope of these docs, but in terms of FCM token lifecycle, not only can you monitor as mentioned above, but on server-side message sending you have to watch out for the error messaging/registration-token-not-registered in case a token has become invalid. Per the docs, this error is returned when:

The provided registration token is not registered. A previously valid registration token can be unregistered for a variety of reasons, including: 

- The client app unregistered itself from FCM.
- The client app was automatically unregistered. This can happen if the user uninstalls the application or, on iOS, if the APNS Feedback Service reported the APNS token as invalid.
- The registration token expired. For example, Google might decide to refresh registration tokens or the APNS token may have expired for iOS devices.
- The client app was updated, but the new version is not configured to receive messages.

For all these cases, remove this registration token and stop using it to send messages.