---
title: Flutter SDK
---

# Flutter SDK

Flutter plugin built on the native iOS / Android SDKs, exposing a single Dart `CaptchalaClient` for both mobile platforms (with Linux / macOS / Windows desktop support via embedded WebView).

## Demo on GitHub

::: tip 📦
[Captcha-La/flutter-demo](https://github.com/Captcha-La/flutter-demo) — full runnable example with every integration step.
:::

## Install

Add the package from pub.dev:

```yaml
# pubspec.yaml
dependencies:
  captchala: ^1.3.2
  http: ^1.2.0          # used by the demo's token-fetch helpers
```

```bash
flutter pub get
flutter run               # auto-picks the foreground device
# or:
flutter run -d <device-id>
```

For iOS / macOS the first build runs `pod install` automatically. Make sure `ios/Podfile` declares `platform :ios, '13.0'` (or higher).

## Quick start

```dart
import 'package:flutter/material.dart';
import 'package:captchala/captchala.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({super.key});
  @override
  State<LoginScreen> createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  String _status = 'Tap to verify';
  bool _verifying = false;

  Future<void> _runVerify() async {
    setState(() { _verifying = true; _status = 'Fetching server_token...'; });

    // 1. Fetch a one-shot server_token from YOUR backend.
    final initialToken = await fetchServerTokenFromYourBackend();

    // 2. Build config from your business action.
    final config = CaptchalaConfig(
      appKey: 'YOUR_APP_KEY',
      action: 'login',                  // login, register, pay, …
      lang: 'en',                       // en, zh-CN, zh-TW, ja, ko, ms, vi, id
      theme: 'light',                   // 'light' | 'dark'
      enableVoice: true,
      enableOfflineMode: true,
      maskClosable: false,
      serverToken: initialToken,
    );

    // 3. Initialize, register callbacks, then call verify().
    final client = await CaptchalaClient.instance.initialize(config);
    client.setCallbacks(
      onSuccess: (r) async {
        // Send r.passToken to YOUR backend for validation.
        if (!mounted) return;
        setState(() { _status = 'OK: ${r.passToken}'; _verifying = false; });
      },
      onFail:    (e) { if (mounted) setState(() { _status = 'fail: ${e.code}'; _verifying = false; }); },
      onError:   (e) { if (mounted) setState(() { _status = 'error: ${e.code} ${e.message}'; _verifying = false; }); },
      onClose:   () { if (mounted && _verifying) setState(() { _status = 'closed'; _verifying = false; }); },
      onServerTokenExpired: () => fetchServerTokenFromYourBackend(),
    );
    await client.verify();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(mainAxisSize: MainAxisSize.min, children: [
          ElevatedButton(
            onPressed: _verifying ? null : _runVerify,
            child: Text(_verifying ? 'Verifying...' : 'Verify with CAPTCHA'),
          ),
          const SizedBox(height: 12),
          Text(_status, style: const TextStyle(fontFamily: 'monospace')),
        ]),
      ),
    );
  }
}
```

## API surface

| Symbol | Purpose |
| --- | --- |
| `CaptchalaClient.instance` | Singleton accessor for the plugin. |
| `CaptchalaConfig(...)` | Plain Dart record: `appKey`, `action`, `lang`, `theme`, `enableVoice`, `enableOfflineMode`, `maskClosable`, `serverToken`. |
| `await client.initialize(config)` | Apply config. Returns the same client instance so you can chain `setCallbacks`. |
| `setCallbacks(...)` | Register `onSuccess`, `onFail`, `onError`, `onClose`, `onServerTokenExpired` callbacks. |
| `await client.verify()` | Open the native CAPTCHA view (Android: Activity over Flutter; iOS: UIViewController over Flutter). |
| `CaptchalaResult` | Provided to `onSuccess`. Fields: `passToken`, `challengeId`, `ttl`, `isOffline`, `isClientOnly`. |

## Server-side validation

Forward `result.passToken` (or `result.token`) to your backend and validate it against the CaptchaLa API. Never expose `X-App-Secret` in client code.

```bash
POST https://apiv1.captcha.la/v1/validate
X-App-Key: YOUR_APP_KEY
X-App-Secret: YOUR_APP_SECRET
Content-Type: application/json

{ "pass_token": "<result.passToken>" }
```

See the [API Reference](../api-reference) for the full validation endpoint and `X-App-Key` / `X-App-Secret` flow.

## Troubleshooting

- **iOS pod install fails**  
  In `ios/Podfile` set `platform :ios, '13.0'` (or higher). Run `flutter clean && flutter pub get && cd ios && pod install` after switching the plugin version.

- **Android `minSdkVersion` mismatch**  
  Bump `android/app/build.gradle` to `minSdkVersion 21`. The native Android SDK will refuse to compile below that.

- **Callbacks fire on background isolate**  
  Always guard `setState` with `if (mounted)` before mutating UI state inside callbacks. The demo wraps every callback that updates state with this check.

- **Desktop targets (Linux / Windows / macOS)**  
  Linux requires GTK3 + WebKitGTK + libcurl. Windows needs the Edge WebView2 Runtime (Win 10 1809+). macOS desktop uses the system WebView automatically.

## Requirements

- Flutter 3.10+ / Dart 3.0+
- Android: `minSdkVersion 21` (Android 5.0+)
- iOS: 13.0+ with CocoaPods
- Desktop (optional): GTK3 + WebKitGTK on Linux, WebView2 Runtime on Windows 10 1809+