FR: Add API for checking subscription status of a given topic

[js]

In order to show users a list of subscribed topics (eg UI with the ability to remove active subscriptions) we have to do one of two things today:

• implement a shadow storage of active subscriptions (in Firebase DB or elsewhere), with all the potential for sync-misshaps
• implement the instance ID API and have the app query our server, which then queries Googles servers, resulting in more latency, server maintenance/dev overhead

I sorely miss a Messaging.messaging().isSubscribed(to: topic) like API in the FCM SDK for checking whether a topic is currently subscribed to by the current user

[rsattar]

Hi @js! Agreed it would be nice to have that in the SDK. There is a REST API to request this, it currently requires your API Key to perform that check, which the SDK doesn't have.

[hixfield]

+1

[FDREAL]

+1