ios - Objective C:: How to check if the user already installed your app? -

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i new ios (iphone , ipad) development.

i developing app. in app, @ 1 point, need retrieve contact lists, , question is, after user's contact list, possible check each user have/have not installed app? if yes, how? if no, how viber works?

they have database phone numbers. , guess send phonenumbers in contacts server , return use viber. i'm guessing, never installed app.

from faq:

you don't have register! know phone number, , friends, why bother yet username , password? viber uses phone number "identity" , lets make free viber phone calls of friends have viber - using phone number.


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