Quick start

1. Create the plugin

final auth = FlutterLocalAuthentication();

The object is cheap to create and has no state of its own. Keep one instance around, for example as a field of your State.

2. Check that the user can authenticate

bool canAuthenticate;
try {
  canAuthenticate = await auth.canAuthenticate();
} on Exception catch (error) {
  debugPrint('Could not check the support for Local Authentication. $error');
  canAuthenticate = false;
}

canAuthenticate is false when the device has no biometric hardware, or when the user did not enroll any biometrics. In that case hide the option, or allow the device credential. To know the exact reason use getAvailability.

3. Authenticate

try {
  await auth.authenticate();
  // The user is authenticated.
} on AuthenticationException catch (error) {
  // The user canceled, is locked out, failed to authenticate...
  debugPrint('Not authenticated: ${error.reason.name}');
}

authenticate returns true when the user is authenticated, and throws an AuthenticationException otherwise. Its reason tells what happened. See error handling.

Complete example

import 'package:flutter/material.dart';
import 'package:flutter_local_authentication/flutter_local_authentication.dart';

class UnlockButton extends StatefulWidget {
  const UnlockButton({super.key, required this.onUnlocked});

  final VoidCallback onUnlocked;

  @override
  State<UnlockButton> createState() => _UnlockButtonState();
}

class _UnlockButtonState extends State<UnlockButton> {
  static const _method = AuthenticationMethod.biometricsOrDeviceCredential;

  final _auth = FlutterLocalAuthentication();
  bool _canAuthenticate = false;

  @override
  void initState() {
    super.initState();
    _checkSupport();
  }

  Future<void> _checkSupport() async {
    final canAuthenticate = await _auth.canAuthenticate(method: _method);
    if (!mounted) return;
    setState(() => _canAuthenticate = canAuthenticate);
  }

  Future<void> _unlock() async {
    try {
      await _auth.authenticate(method: _method);
      widget.onUnlocked();
    } on AuthenticationException catch (error) {
      if (error.isCanceled || !mounted) return;
      ScaffoldMessenger.of(context).showSnackBar(
        const SnackBar(content: Text('Could not verify your identity')),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return FilledButton(
      onPressed: _canAuthenticate ? _unlock : null,
      child: const Text('Unlock'),
    );
  }
}

A runnable app lives in the example folder of the repository.