SharedPreferences empty after restarting service / device - firebase

Question

As I am working with Google Firebase for Push Notifications, I want to save the Instance Token to the SharedPreferences. Unfortunately, whenever the token gets refreshed and want to check the previous one from SharedPreferences, they are empty...
Is it because I am using a Service here?
public class MyFirebaseIIDService : FirebaseInstanceIdService
{
public override void OnTokenRefresh()
{
var sharedPreferences = PreferenceManager.GetDefaultSharedPreferences(this);
var sharedPreferencesEditor = sharedPreferences.Edit();
// Get Firebase Instance Token
var refreshedToken = FirebaseInstanceId.Instance.Token;
// Check if a Firebase Instance Token has been registered before and unregister it
var oldToken = sharedPreferences.GetString("FirebaseInstanceToken", null);
if (oldToken != null) // is ALWAYS null :(
{
// Unregister old token...
}
// Save the Firebase Instance Token locally
sharedPreferencesEditor.PutString("FirebaseInstanceToken", refreshedToken);
sharedPreferencesEditor.Apply();
// At this point, the SharedPreferences have to token saved.
// Next time, the app reaches this point, it is gone...
}
}
Sorry for the syntax confusion, I use Xamarin, so this is C# but it should not make any difference.

Answer 1

I don't know how it work in xamarin but in native android getSharedPreferences from service context maybe wrong. You should use only applicationContext or MODE_MULTI_PROCESS when open shared preferences.
You can see similar question here.
Xamarin and C# also have this mode when your open some file, so i think exactly the same with preferences. Try some like this instead of using GetDefaultSharedPreferences:
ISharedPreferences prefs = Application.Context.GetSharedPreferences ("PREF_NAME", FileCreationMode.MultiProcess);

Answer 2

Did you uninstall your app to refresh you token? If yes I think you can not get the old token because the SharedPreferences is cleared when you uninstall your app.
I have tried java code to save the token :
public void onTokenRefresh() {
// Get updated InstanceID token.
String refreshedToken = FirebaseInstanceId.getInstance().getToken();
Log.d("Mike", "Refreshed token: " + refreshedToken);
SharedPreferences mSharedPreferences = getSharedPreferences("hello",0);
String oldToken = mSharedPreferences.getString("Token",null);
if(oldToken == null)
{
Log.d("Mike", "oldToken: " + null);
}
SharedPreferences.Editor mEditor = mSharedPreferences.edit();
mEditor.putString("Token", refreshedToken);
mEditor.commit();
// TODO: Implement this method to send any registration to your app's servers.
//sendRegistrationToServer(refreshedToken);
}
At first time you install your app you will get the token and save it in the SharedPreferences, next time you open your app and show the old token in the textview then you can find your token has been saved , And do not uninstall your app:
TextView tv1 = (TextView)findViewById(R.id.tv1);
SharedPreferences mSharedPreferences = getSharedPreferences("hello",0);
String oldToken = mSharedPreferences.getString("Token",null);
tv1.setText(oldToken);
It works. But when you uninstall your app the textview shows null.

Answer 3

I solved the problem and it turned out, that my code was working correctly but behaved strange on my test device.
After re-building the application, SharedPreferences have been cleared, although I checked the Preserve application data/cache on device between deploys option in Visual Studio. That was because my physical testing device was rooted and did not accept this option.
When trying on an unrooted device, everything worked as expected.