It uses Google Cloud Messaging. There needs to be a server in between and I believe GCM is free. Consider a situation where Bobs phone is switched off. Alice sends a message and turns off their phone. Now, Bob turns in their phone. How do we get the message Alice sent to Bob?
Give me some open source code that I can run on my own server to manage the notifications for me and for my friends, and allow me to tell the client app on my phone to use it.
That's way better than forcing me to rely on a specific Google server and protocol, which further requires proprietary software on the client.
