Android: Subscribe to Firebase Cloud Messaging(FCM) Topic

Question

According to Firebase cloud messaging documentation, for subscribing a user to a topic I need to call

FirebaseMessaging.getInstance().subscribeToTopic("news");

1. In my application, I need all users to be subscribed to my cloud messaging topic. Since return value is void, the question is how can I understand that subscription was successful?
2. Is it a bad practice to call subscribeToTopic each time my application starts?

Answer 1

Waitting for answers