If you install Twitter, or Vine, equally, then you log in, then uninstall the app, then install it again, it somehow appears already logged in. I've got several ideas about how this might work, but they all are dismissed by various reasons:
- associate a unique cookie with the device itself, but that seems to contradict the App Store TOS (I don't know; I've written "seems", so I'm not stating it), and it will sustain after the device has been sold, so this approach is hardly viable;
- store the login data somewhere in the cloud, but, as far as I know, all iCloud records get wiped along with app uninstallation;
- use OS keyring, but I doubt there is such an API.
For one, Telegram messenger always installs fresh so you have to log in again after each installation.
That said, how does Twitter or Vine do this?