I have developed an Android application that is supposed to be used for individual use, but also for two users to share information between their respective phones. Unfortunately, I have not managed to figure out how to solve the latter. I've searched a lot, for example, how to share information from one app to another, but I mostly found examples of how to do something as easy as sending data between individual activities within one android application (not passing data to other users).
I want users to be able to:
For the login part, I’ve looked at Facebook’s SDK which seems rather easy to implement. But once logged in using Facebook, I am not sure how to use it to send data between users. However, I do not want non-Android users to e.g. be able to receive an invite from one of my users. I’m looking for something really simple but still intuitive, user-friendly, and easy-to-integrate. I do not want to host a server or a database myself.
I believe that your best bet is to use Firebase Cloud Messaging
While it does require some setup and technically involves hosting. It is one of the most popular and useful services for building apps based on peer to peer mobile messaging and friend/follower structures.
It sounds like something that would directly solve your problems with what you are trying to achieve. It is also free in many use cases!