Google allows to perform app-invites and also track how well they improve your app installations:
https://www.youtube.com/watch?v=UfdCNYXMC9M
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Answers:
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The Firebase analytics integration has been streamlined from the legacy integration that required a tracking Id. Now with Firebase only setup and a call to getInvitation() are needed. No additional code or tracking ids required. Remember it takes up to 24 hours to see the results in the console. However, you can see the messages being sent immediately which is a great indication that it's working. Just enable verbose logging as described in Analytics Log Events documentation.
The Firebase and android developer documentation for appinvites are identical.
Yes, you must call getInvitation() to get analytics tracking, and do the setup as described above.
The iOS and Android equivalence apps should be in the same project. Just move them to a single project in the Firebase console. You can just remove one from it's existing project and add it to the other to make a combined project. The change should be reflected instantly.