Conversations.im does just that, uses FCM to get the messages in the background. They recently open sourced push server that interacts with FCM: https://github.

Conversations.im does just that, uses FCM to get the messages in the background. They recently open sourced push server that interacts with FCM: https://github.com/iNPUTmice/p2 https://github.com/iNPUTmice/p2