You just don’t know. You can take the device identification string, which is supposedly identical for the same application token string, and track whether the token has changed for a specific device. Thus, you can update the device token with a new one, but there is no way to find out if your application has been removed from the device.
Update: You're right. You can get this information from the feedback service, however there is a reservation. To maintain a constant connection with feedback, you must install at least one application with messaging enabled. I would call these clients potentially nonexistent, but I would not delete these people from the client database. Who knows why your push message may be rejected. Perhaps your device is in a contradictory state and you permanently delete your subscriber. If you have 10k zombie clients, this is a pocket change for the database. If you have lost 100 thousand customers, you have many serious problems.
bioffe
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