Biometric authentication in Flutter - Build, Publish & Deploy

Build & Publish - Biometric authentication

Performance Impact

Using biometric authentication in your Flutter app has minimal impact on frame rate and memory. The biometric check runs outside the main UI thread, so it won't block animations or cause jank. Battery usage is low since the biometric sensor activates only during authentication. However, frequent or unnecessary biometric prompts can annoy users and drain battery slightly.

Optimization Tips

To keep your app smooth at 60fps, trigger biometric authentication only when needed, such as on app launch or sensitive actions. Avoid calling it repeatedly or in rapid succession. Use Flutter's local_auth plugin efficiently by caching authentication state when possible. Also, handle errors gracefully to prevent UI freezes.

App Size and Startup Time

Adding biometric authentication via the local_auth plugin adds a small increase to your app bundle size (usually under 1MB). This is because native platform code is included for iOS and Android biometric APIs. Startup time impact is negligible since biometric code loads only when invoked, not at app launch.

iOS vs Android Differences

On iOS, biometric authentication uses Face ID or Touch ID via the LocalAuthentication framework. It requires user permission and device support checks. On Android, it uses the BiometricPrompt API supporting fingerprint, face, or iris depending on device. Android requires checking hardware availability and enrolled biometrics. Flutter's local_auth plugin abstracts these differences but you must handle platform-specific error codes and user flows.

Store Review Guidelines

• Apple App Store: Follow Apple's Human Interface Guidelines for biometric usage. Always ask user consent before biometric authentication. Do not use biometrics for tracking or unauthorized data collection. Ensure fallback authentication methods are available.
• Google Play Store: Comply with Google Play policies on user privacy and security. Clearly disclose biometric use in your privacy policy. Provide alternative authentication if biometrics fail or are unavailable.

Self-Check Question

Your app takes 5 seconds to load the biometric authentication screen. What's likely wrong?

• Biometric prompt is triggered too early, blocking UI thread.
• Heavy synchronous operations run before biometric call.
• Missing asynchronous handling causing UI freeze.
• Not caching authentication state, causing repeated checks.

Key Result

Biometric authentication in Flutter apps adds minimal performance overhead and small bundle size increase. Use the local_auth plugin efficiently, handle platform differences, and follow store guidelines for smooth user experience and successful publishing.

Practice

1. What is the main purpose of biometric authentication in a Flutter app?

easy

A. To send notifications to users

B. To change the app's theme colors automatically

C. To speed up app loading times

D. To secure the app using fingerprint or face recognition

Solution

1. Step 1: Understand biometric authentication

   Biometric authentication uses unique physical traits like fingerprint or face to secure access.

2. Step 2: Identify its purpose in apps

   It protects sensitive data by allowing only authorized users to enter.

3. Final Answer:

   To secure the app using fingerprint or face recognition -> Option D

4. Quick Check:

   Biometric authentication = Secure app access [OK]

Hint: Biometrics means using your body to unlock [OK]

Common Mistakes:

• Confusing biometric authentication with UI changes
• Thinking it improves app speed
• Assuming it sends notifications

2. Which Flutter package is commonly used to implement biometric authentication?

easy

A. local_auth

B. shared_preferences

C. provider

D. http

Solution

1. Step 1: Recall Flutter packages for biometrics

   The local_auth package provides access to fingerprint and face authentication.

2. Step 2: Compare with other packages

   http is for network calls, provider for state management, and shared_preferences for storing data.

3. Final Answer:

   local_auth -> Option A

4. Quick Check:

   Biometric package = local_auth [OK]

Hint: local_auth sounds like local device authentication [OK]

Common Mistakes:

• Choosing http for biometrics
• Confusing provider with authentication
• Picking shared_preferences for biometrics

3. What will the following Flutter code print if biometric authentication is available and successful?

final isAvailable = await auth.canCheckBiometrics;
if (isAvailable) {
  final didAuthenticate = await auth.authenticate(
    localizedReason: 'Please authenticate',
  );
  print(didAuthenticate);
} else {
  print('No biometrics');
}

medium

A. No biometrics

B. false

C. true

D. Exception error

Solution

1. Step 1: Check biometric availability

   The code checks if biometrics are available with canCheckBiometrics. If true, it proceeds.

2. Step 2: Authenticate and print result

   If authentication succeeds, didAuthenticate is true and printed.

3. Final Answer:

   true -> Option C

4. Quick Check:

   Available and success = true [OK]

Hint: Success prints true, failure prints false or message [OK]

Common Mistakes:

• Assuming it prints 'No biometrics' when available
• Expecting false on success
• Ignoring await causing errors

4. Identify the error in this Flutter biometric authentication snippet:

final didAuthenticate = await auth.authenticate(
  localizedReason: 'Scan fingerprint',
  useErrorDialogs: true,
  stickyAuth: true
);
if (didAuthenticate) {
  print('Access granted');
}

medium

A. No check if biometrics are available before authenticate

B. Incorrect parameter name localizedReason

C. Missing await keyword before authenticate

D. Missing else block after if

Solution

1. Step 1: Review biometric availability check

   The code calls authenticate without checking canCheckBiometrics first, which may cause errors on devices without biometrics.

2. Step 2: Validate other parts

   Await is present, parameter names are correct, and else block is optional.

3. Final Answer:

   No check if biometrics are available before authenticate -> Option A

4. Quick Check:

   Always check availability before authenticate [OK]

Hint: Check biometrics availability before authenticating [OK]

Common Mistakes:

• Skipping availability check
• Confusing parameter names
• Thinking else block is mandatory

5. You want to secure a Flutter app so it only allows access if biometric authentication succeeds. Which sequence of steps correctly implements this using local_auth? A) Call authenticate directly without checking availability. B) Check canCheckBiometrics, then call authenticate, then proceed if true. C) Skip biometric checks and use a password instead. D) Call authenticate and ignore its result.

hard

A. Call authenticate directly without checking availability

B. Check canCheckBiometrics, then authenticate, then proceed if true

C. Skip biometrics and use password only

D. Call authenticate and ignore its result

Solution

1. Step 1: Verify biometric availability

   Use canCheckBiometrics to ensure device supports biometrics before authenticating.

2. Step 2: Authenticate and check result

   Call authenticate and proceed only if it returns true, ensuring secure access.

3. Step 3: Avoid skipping checks or ignoring results

   Skipping availability or ignoring authentication result risks security.

4. Final Answer:

   Check canCheckBiometrics, then authenticate, then proceed if true -> Option B

5. Quick Check:

   Availability check + authenticate success = secure access [OK]

Hint: Always check availability before authenticating [OK]

Common Mistakes:

• Calling authenticate without availability check
• Ignoring authentication result
• Using password instead of biometrics when asked