title: Restoring user data after app uninstall and reinstall in Android and iOS
body: <p>I am developing a mobile app which involves Guest Login.
  I want the users to easily restore the Guest Login. For example,</p>
  
  <ol>
  <li>Install my app.</li>
  <li>Use my app with Guest Login, without manual ID and password creation.</li>
  <li>Uninstall my app.</li>
  <li>Install my app again.</li>
  <li>Use my app with the existing Guest Login again, without manual ID and password.</li>
  </ol>
  
  <p>Even if the user's mobile app is not associated to Google nor Apple ID, I want the users can restore his or her Guest Login. Of course, resetting his or her mobile device is an exception.</p>
  
  <p>I think Apple KeyChain Services, Google KeyStore API, Android <code>FileBackupHelper</code>, Android AAID may be the solution, but I am not sure of it.</p>
  
  <p>Of course, I know that other ways than Guest Login such as Facebook or Google login, but my question is for the people without those login accounts.</p>