I'm making an Android app, and I already have a contact list implemented, it is working fine. The problem is: my app is showing to the user all contacts they have added on their mobile. I want to show only the contacts that have installed my app.
I think it is possible! I have an example: Whatsapp, they only show users who have their app installed.
In WhatsApp each user registers with his phone number. The app is syncing the contacts on the device with the known list from the WhatsApp-Servers. For this it is sending all contacts to the WhatsApp-Server. The server replies which contacts are known. Then for each contact you can show which contact has installed the app.