Automatically Keeping Users Logged In on Mobile

Modern mobile applications often require persistent user authentication. In other words, once the user logs in, they should remain authenticated across app restarts—without needing to re-enter credentials each time. Below, I’ll discuss how I build an “auto-login” feature, including some best practices around tokens, local storage, and secure checks.

The Business Logic

Token-Based Authentication

When a user logs in, the server returns an authentication token (e.g., a JWT or a session token). Typically:

1. Login: User submits email/password to a secure API endpoint.
2. Token Issuance: Server responds with a short-lived token and (in some cases) a refresh token.
3. Storage: The app persists these credentials in local storage (e.g., Keychain on iOS, Keystore on Android, or a library such as flutter_secure_storage in Flutter).
4. Auto-Login: On each app launch, the application reads these tokens from storage and checks validity, possibly refreshing them if needed.

If the tokens are valid, we skip the sign-in screen; otherwise, the app requires new credentials.

Architectural Overview

• Network Layer: Responsible for injecting the stored token into every outgoing request (e.g., using an interceptor).
• State Manager: Tracks if the user is logged in. If tokens are missing or expired, we prompt for login.
• Token Renewal: Optionally handle silent token refresh behind the scenes if the server supports that.

One important detail is to never hardcode your secret keys in the client, and always use SSL/TLS to prevent man-in-the-middle attacks.

Example Code Snippet (Flutter-Dart)

Below is a condensed snippet showing the typical flow: reading an existing token, calling an API to check if it’s valid, and storing user info in your model. This is adapted from an internal codebase I wrote (and I’ve expanded or refined it to illustrate best practices). The actual production version is more extensive.

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'my_auth_api.dart';

class AuthService {
  Future<bool> checkAutoLogin() async {
    final prefs = await SharedPreferences.getInstance();
    final storedToken = prefs.getString('app_token') ?? '';
    if (storedToken.isEmpty) {
      return false; // no token found
    }

    // Attempt to verify or refresh the token
    try {
      bool valid = await MyAuthApi.checkIsLogin(storedToken);
      if (!valid) {
        // token is invalid => force re-login
        await prefs.remove('app_token');
        return false;
      } else {
        // fetch user info to keep things updated
        final userInfo = await MyAuthApi.getUserInfo(storedToken);
        // store user info in memory, e.g., global singleton or provider
        // ...
        return true;
      }
    } catch (e) {
      // in case of network error or server error
      await prefs.remove('app_token');
      return false;
    }
  }
}

1. SharedPreferences: This snippet uses SharedPreferences for simplicity, but in production, you might prefer an encrypted store for security.
2. checkIsLogin: A server call to validate the token or determine if the user is effectively logged in.
3. User Info: If valid, we retrieve user metadata (like name, avatar, roles) and store it in memory (or in a global data model).

By hooking checkAutoLogin() into your app’s initialization phase, you can automatically guide the user to either their home screen or a login screen.

Handling Expired Tokens

If the server sets short-lived tokens, you typically use a refresh token or re-prompt for credentials. A common pattern is:

• On every request, if the token is expired, the client tries to exchange the refresh token for a new access token (completely invisible to the user).
• If the refresh token itself has expired, you redirect to login.

Security Precautions

• Use Secure Storage: On iOS, Keychain is recommended; on Android, the Keystore. Flutter wrappers like flutter_secure_storage can help.
• Rotate Tokens: Invalidate tokens server-side after a reasonable lifespan.
• Logout: Provide a reliable logout flow that clears tokens from local storage.
• SSL/TLS: Always use HTTPS to prevent interception of token traffic.

Conclusion

Automatic login significantly enhances user experience by skipping repetitive credential prompts. The implementation focuses on safely storing tokens, verifying them on app launch, and refreshing them in the background if needed. By carefully combining secure local storage with a robust server-side authentication flow, you can deliver a streamlined experience without sacrificing security.

In my own projects, I’ve integrated these steps into dedicated methods, ensuring a consistent approach for all user-facing screens. The principle is universal: persist tokens securely, validate them gracefully, and degrade to manual login only if absolutely necessary.