How does apps like whatsapp show contacts that are only using the app?
问题 In apps that use phone authentication like whatsapp: When any user opens to check his/her contacts, whatsapp will show only the contacts from your phone that are using whatsapp (like they filter your contacts). My thoughts on implementing something similar: I can save all users in a database (when they authenticate with my app), now when I load contacts in an activity I check to see if the contact exist in the database, and if the user exist then I add him/her to the list. Problem: If I used