Based on my read of the documentation, it seems like you could automatically subscribe every new user to a topic such as "appWideAlerts" upon sign in. Then use this as your app wide notification system.
Based on the publish/subscribe model, FCM topic messaging allows you
to send a message to multiple devices that have opted in to a
particular topic. You compose topic messages as needed, and FCM
handles routing and delivering the message reliably to the right
devices.
For example, users of a local weather forecasting app could opt in to
a "severe weather alerts" topic and receive notifications of storms
threatening specified areas. Users of a sports app could subscribe to
automatic updates in live game scores for their favorite teams.
Some things to keep in mind about topics:
- Topic messaging supports unlimited topics and subscriptions for each app.
- Topic messaging is best suited for content such as news, weather, or other publicly available information.
- Topic messages are optimized for throughput rather than latency. For fast, secure delivery to single devices or small groups of
devices, target messages to registration tokens, not topics.
- If you need to send messages to multiple devices per user, consider device group messaging for those use cases.