How does whatsapp detects my newly added number using same app very quickly?

Question

How does Whatsapp detects any newly added number using the app very quickly, and shows an icon in contact details immediately. I am not asking how to do it but my question is how to do it as fast as whatsapp does ?

Answer 1

I would guess it is listening for contact changes or just checks each time it opens for any differences. (Can't comment yet so delete this if wrong)