Strategies and Practices for Forcing onTokenRefresh() in Firebase FCM

Introduction

During the migration from Google Cloud Messaging (GCM) to Firebase Cloud Messaging (FCM), developers often face a critical issue: the onTokenRefresh() method is automatically called upon app installation, when users are not yet logged in, preventing token association with user IDs. Based on high-scoring answers from Stack Overflow, this article delves into this problem and offers practical solutions.

FCM Token Generation Mechanism

According to Firebase documentation, the FCM SDK generates a registration token for the client app instance upon initial startup. This token is used to send notifications to specific devices. Since token generation is independent of user login status, it triggers onTokenRefresh() immediately after installation. This necessitates handling the timing between token generation and user login.

Core Solution: Token Saving and Delayed Use

To address token generation before user login, it is recommended to save the token to local storage, such as SharedPreferences, in the onTokenRefresh() method. The token can then be retrieved from storage after user login and registered with the server. This approach avoids the overhead of forcing token regeneration while ensuring data persistence.

Here is a code snippet demonstrating how to save the token in onTokenRefresh():

private void saveTokenToPrefs(String token) {
    SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
    SharedPreferences.Editor editor = preferences.edit();
    editor.putString("registration_id", token);
    editor.apply();
}

After user login, the token can be retrieved from SharedPreferences:

private String getTokenFromPrefs() {
    SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
    return preferences.getString("registration_id", null);
}

Method to Force onTokenRefresh()

If it is necessary to force onTokenRefresh() under specific conditions, such as after user login, this can be achieved by deleting the current token instance. This involves creating an IntentService that calls FirebaseInstanceId.getInstance().deleteInstanceId() to reset the instance ID and revoke all tokens. Subsequently, calling FirebaseInstanceId.getInstance().getToken() will trigger onTokenRefresh().

Below is a complete code example implementing this functionality:

public class DeleteTokenService extends IntentService {
    public static final String TAG = DeleteTokenService.class.getSimpleName();

    public DeleteTokenService() {
        super(TAG);
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        try {
            String originalToken = getTokenFromPrefs();
            Log.d(TAG, "Token before deletion: " + originalToken);
            FirebaseInstanceId.getInstance().deleteInstanceId();
            saveTokenToPrefs("");
            String tokenCheck = getTokenFromPrefs();
            Log.d(TAG, "Token deleted. Proof: " + tokenCheck);
            Log.d(TAG, "Getting new token");
            FirebaseInstanceId.getInstance().getToken();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void saveTokenToPrefs(String token) {
        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
        SharedPreferences.Editor editor = preferences.edit();
        editor.putString("registration_id", token);
        editor.apply();
    }

    private String getTokenFromPrefs() {
        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
        return preferences.getString("registration_id", null);
    }
}

Migration to onNewToken() Method

It is important to note that FirebaseInstanceIdService and its onTokenRefresh() method are deprecated. Firebase recommends using the onNewToken() method, which is part of FirebaseMessagingService. Developers should update their code to use the new method to ensure app compatibility and performance.

Here is an example of how to implement onNewToken():

public class MyFirebaseMessagingService extends FirebaseMessagingService {
    @Override
    public void onNewToken(String token) {
        super.onNewToken(token);
        Log.e("NEW_TOKEN", token);
        saveTokenToPrefs(token);
    }

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        super.onMessageReceived(remoteMessage);
    }
}

Conclusion

By saving tokens to local storage and using them after user login, developers can elegantly handle the timing issue between FCM token generation and user login status. The method to force onTokenRefresh() offers additional flexibility but should be used cautiously to avoid unnecessary token refreshes. Meanwhile, migrating to onNewToken() is crucial for keeping applications up-to-date. The code examples and strategies provided in this article aim to help developers efficiently address common challenges in FCM migration.