How to organize a mobile chat?

Question

Android

Good afternoon!
The task is to organize a messenger for mobile devices. What is the best way to do this? With what technologies? Interested, so to speak, in the principle of communication between the server and the application (socket or are there any tools from Google / Apple that are made specifically for this)?
It is possible to adjust the backend to any needs of mobile devices. Another important feature is its own backend, that is, the use of third-party services is not required.
Please point me to the right path.
PS: there is an understanding that this task is not entirely trivial.

Answer 1

There are ready-made chats here:
https://www.cocoacontrols.com/search?utf8=%E2%9C%9...
And a year ago I did it on:
quickblox.com/developers/SimpleSample-chat_users-ios

Answer 2

look towards XMPP, look towards open source ready-made implementations for the server.
e.g. openfire

Answer 3

ICQ Navigator
(GooglePlay)
Client - OpenSource

Answer 4

Possibly based on https://core.telegram.org/