Web Auth 2 for Flutter

This project is a continuation of flutter_web_auth by Linus Unnebäck with many new features and bug fixes.

A Flutter plugin for authenticating a user with a web service, even if the web service is run by a third party. Most commonly used with OAuth2, but can be used with any web flow that can redirect to a custom scheme.

In the background, this plugin uses ASWebAuthenticationSession on iOS 12+ and macOS 10.15+, SFAuthenticationSession on iOS 11, Chrome Custom Tabs on Android and opens a new window on Web. You can build it with iOS 8+, but it is currently only supported by iOS 11 or higher.

Usage

Add the following snippet to your pubspec.yaml and follow the Setup guide:

dependencies:
  flutter_web_auth_2: ^4.0.0-alpha.0

To authenticate against your own custom site:

import 'package:flutter_web_auth_2/flutter_web_auth_2.dart';

// Present the dialog to the user
final result = await FlutterWebAuth2.authenticate(url: "https://my-custom-app.com/connect", callbackUrlScheme: "my-custom-app");

// Extract token from resulting url
final token = Uri.parse(result).queryParameters['token'];

To authenticate the user using Google's OAuth2:

import 'package:flutter_web_auth_2/flutter_web_auth_2.dart';

import 'dart:convert' show jsonDecode;
import 'package:http/http.dart' as http;

// App specific variables
final googleClientId = 'XXXXXXXXXXXX-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com';
final callbackUrlScheme = 'com.googleusercontent.apps.XXXXXXXXXXXX-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';

// Construct the url
final url = Uri.https('accounts.google.com', '/o/oauth2/v2/auth', {
  'response_type': 'code',
  'client_id': googleClientId,
  'redirect_uri': '$callbackUrlScheme:/',
  'scope': 'email',
});

// Present the dialog to the user
final result = await FlutterWebAuth2.authenticate(url: url.toString(), callbackUrlScheme: callbackUrlScheme);

// Extract code from resulting url
final code = Uri.parse(result).queryParameters['code'];

// Construct an Uri to Google's oauth2 endpoint
final url = Uri.https('www.googleapis.com', 'oauth2/v4/token');

// Use this code to get an access token
final response = await http.post(url, body: {
  'client_id': googleClientId,
  'redirect_uri': '$callbackUrlScheme:/',
  'grant_type': 'authorization_code',
  'code': code,
});

// Get the access token from the response
final accessToken = jsonDecode(response.body)['access_token'] as String;

Note: To use multiple scopes with Google, you need to encode them as a single string, separated by spaces. For example, scope: 'email https://www.googleapis.com/auth/userinfo.profile'. Here is a list of all supported scopes.

Upgrading to 4.x

Version 4.0.0 introduced a new approach for Linux and Windows to authenticate users - using Webview APIs. Hence, you only need to change your code if you are targeting Linux or Windows. If you are fine with still using the old version, here is what you need to change:

• Pass useWebview: false into the options of your call to authenticate, like so:

  final result = await FlutterWebAuth2.authenticate(
    url: url,
    callbackUrlScheme: 'foobar',
    options: const FlutterWebAuth2Options(useWebview: false),
  );

If you want to use the new approach (default behaviour!), you need to do a bit more:

• Follow the "Getting started" guide of desktop_webview_window
• Make sure that your users know about the new requirements, as described here

Upgrading to 3.x

Version 3.0.0 featured a huge refactor which made it possible to maintain even more configuration possibilities. Even platform-specific ones! If you want to upgrade, you need to do the following:

• Follow the [Setup] within this README again for your platform and do it from scratch (there might be changes!)
• Dart SDK constraints have been updated to >=2.15.0.
• The configuration parameters have been removed from the authenticate function. Now, you have to pass a FlutterWebAuth2Options object with those options in it instead:
  • contextArgs: This is now called windowName within FlutterWebAuth2Options.
  • redirectOriginOverride: This is now called debugOrigin within FlutterWebAuth2Options.
  • preferEphemeral: This has been split into the two named parameters preferEphemeral (for iOS and MacOS) and intentFlags (for Android) within FlutterWebAuth2Options. The former works exactly the same. However, if you want the old behaviour using preferEphemeral on Android, use the ephemeralIntentFlags constant as value for intentFlags.

Upgrading from flutter_web_auth

If you used flutter_web_auth correctly (and without extra hackage) before, it should be sufficient to replace the following strings everywhere (yes, also in AndroidManifest.xml for example):

• FlutterWebAuth -> FlutterWebAuth2
• flutter_web_auth -> flutter_web_auth_2

If you are using versions >= 3.0.0, you also need to follow the migration guide(s) above!

If you are still unsure or something is not working as well as before, please open a new issue.

Setup

Setup is the same as for any Flutter plugin, with the following caveats:

Android

In order to capture the callback url, the following activity needs to be added to your AndroidManifest.xml. Be sure to replace YOUR_CALLBACK_URL_SCHEME_HERE with your actual callback url scheme.

<manifest>
  <application>

    <activity
      android:name="com.linusu.flutter_web_auth_2.CallbackActivity"
      android:exported="true">
      <intent-filter android:label="flutter_web_auth_2">
        <action android:name="android.intent.action.VIEW" />
        <category android:name="android.intent.category.DEFAULT" />
        <category android:name="android.intent.category.BROWSABLE" />
        <data android:scheme="YOUR_CALLBACK_URL_SCHEME_HERE" />
      </intent-filter>
    </activity>

  </application>
</manifest>

If you are using http or https as your callback scheme, you also need to specify a host etc. See c:geo as an example for this.

iOS

For "normal" authentication, just use this library as usual; there is nothing special to do!

To authenticate using Universal Links on iOS, use https as the provided callbackUrlScheme:

final result = await FlutterWebAuth2.authenticate(url: "https://my-custom-app.com/connect", callbackUrlScheme: "https");

Web

On the Web platform, an endpoint must be created that captures the callback URL and sends it to the application using the JavaScript postMessage() method. In the ./web folder of the project, create an HTML file named, e.g. auth.html with content:

<!DOCTYPE html>
<title>Authentication complete</title>
<p>Authentication is complete. If this does not happen automatically, please close the window.</p>
<script>
  function postAuthenticationMessage() {
    const message = {
      'flutter-web-auth-2': window.location.href
    };

    if (window.opener) {
      window.opener.postMessage(message, window.location.origin);
      window.close();
    } else if (window.parent && window.parent !== window) {
      window.parent.postMessage(message, window.location.origin);
    } else {
      localStorage.setItem('flutter-web-auth-2', window.location.href);
      window.close();
    }
  }

  postAuthenticationMessage();
</script>

This HTML file is designed to handle both traditional window-based and iframe-based authentication flows. The JavaScript code checks the context and sends the authentication response accordingly.

The redirect URL passed to the authentication service must be the same as the URL the application is running on (schema, host, port if necessary) and the path must point to the generated HTML file, in this case /auth.html. The callbackUrlScheme parameter of the authenticate() method does not take this into account, so it is possible to use a schema for native platforms in the code.

For the Sign in with Apple in web_message response mode, postMessage from https://appleid.apple.com is also captured, and the authorisation object is returned as a URL fragment encoded as a query string (for compatibility with other providers).

Additional parameters for the URL open call can be passed in the authenticate function using the windowName parameter from the options. The silentAuth parameter can be used to enable silent authentication within a hidden iframe, rather than opening a new window or tab. This is particularly useful for scenarios where a full-page redirect is not desirable. Setting this parameter to true allows for a seamless user experience by performing authentication in the background, making it ideal for token refreshes or maintaining user sessions without requiring explicit interaction from the user.

Windows and Linux

When using useWebview: false, there is a limitation that the callback URL scheme must start with http://localhost:{port}.

When specifying useWebview: true (which is the default behaviour), you need to make sure to follow desktop_webview_window's guide. Also be aware that your users might need to install a Webview API (which is preinstalled on Windows 11 and some Windows 10 and Linux installations). For details, see also desktop_webview_window's guide above.

Troubleshooting

When you use this package for the first time, you may experience some problems. These are some of the most common solutions:

General troubleshooting steps

1. Stop the application if it is running.
2. Run the following commands:
   • flutter clean
   • flutter pub upgrade
3. Rerun the application after executing the above commands. Sometimes, they work wonders!

Troubleshooting callbackUrlScheme

• callbackUrlScheme must be a valid schema string or else this library won't work
• A valid RFC 3986 URL scheme must consist of "a letter and followed by any combination of letters, digits, plus "+", period ".", or hyphen "-"
• scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
• This means you can not use underscore "_", space " " or uppercase "ABCDEF...". It must also not start with a number. See RFC3986#page-17
• Examples of VALID callbackUrlSchemes are callback-scheme, another.scheme, examplescheme
• Examples of INVALID callbackUrlSchemes are callback_scheme,1another.scheme, exampleScheme

Troubleshooting Flutter App

• You have to tell the FlutterWebAuth2.authenticate function what your callbackUrlScheme is.

• Example: If your callbackUrlScheme is valid-callback-scheme, your dart code will look like

  import 'package:flutter_web_auth_2/flutter_web_auth_2.dart';
  
  // Present the dialog to the user
  final result = await FlutterWebAuth2.authenticate(url: "https://my-custom-app.com/connect", callbackUrlScheme: "valid-callback-scheme");

Troubleshooting Android

• You will need to update your AndroidManifest.xml to include the com.linusu.flutter_web_auth_2.CallbackActivity activity, something like

  <manifest>
    <application>
  
      <!-- add the com.linusu.flutter_web_auth_2.CallbackActivity activity -->
      <activity
        android:name="com.linusu.flutter_web_auth_2.CallbackActivity"
        android:exported="true">
        <intent-filter android:label="flutter_web_auth_2">
          <action android:name="android.intent.action.VIEW" />
          <category android:name="android.intent.category.DEFAULT" />
          <category android:name="android.intent.category.BROWSABLE" />
          <data android:scheme="YOUR_CALLBACK_URL_SCHEME_HERE" />
        </intent-filter>
      </activity>
  
    </application>
  </manifest>

• Example of a valid AndroidManifest.xml with VALID callbackUrlScheme. In the example below our callbackUrlScheme is valid-callback-scheme.

  <manifest>
    <application>
      <activity
        android:name="com.linusu.flutter_web_auth_2.CallbackActivity"
        android:exported="true">
        <intent-filter android:label="flutter_web_auth_2">
          <action android:name="android.intent.action.VIEW" />
          <category android:name="android.intent.category.DEFAULT" />
          <category android:name="android.intent.category.BROWSABLE" />
          <data android:scheme="valid-callback-scheme" />
        </intent-filter>
      </activity>
  
    </application>
  </manifest>

• If you are targeting S+ (SDK version 31 and above) you need to provide an explicit value for android:exported. If you followed earlier installation instructions, this was not included. Make sure that you add android:exported="true" to the com.linusu.flutter_web_auth.CallbackActivity activity in your AndroidManifest.xml file.

  - <activity android:name="com.linusu.flutter_web_auth_2.CallbackActivity">
  + <activity
  +   android:name="com.linusu.flutter_web_auth_2.CallbackActivity"
  +   android:exported="true">

• If you want to have a callback URL with http or https scheme, you also need to specify a host etc. See c:geo as an example for this.

• There is also a known problem with task affinities and launch modes (see #113). In order to ensure that flutter_web_auth_2 works correctly, set android:launchMode="singleTop" and remove any android:taskAffinity entries. This configuration is guaranteed to work.

Troubleshooting OAuth redirects

• Your OAuth Provider must redirect to the valid callbackUrlScheme + ://. This mean if your callbackUrlScheme is validscheme, your OAuth Provider must redirect to validscheme://
• Example with PHP:

  <?php
  
  header("Location: validscheme://?data1=value1&data2=value2");

Troubleshooting HTML redirects

• If you are using HTML hyperlinks, it must be a valid callbackUrlScheme + ://. This means that if your callbackUrlScheme is customappname, your HTML hyperlink should be customappname://

• Example with HTML:

  <a href="customappname://?data1=value1&data2=value2">Go Back to App</a>

Troubleshooting passing data to app

• You can pass data back to your app by adding GET query parameters. This is done by adding a name=value type of data after your callbackUrlScheme + :// + ?

• Example to pass access-token to your app:

  my-callback-schema://?access-token=jdu9292s
  

• You can pass multiple dates by concatenating them with &:

  my-callback-schema://?data1=value1&data2=value2
  

• Example to pass access-token and user_id to your app:

  my-callback-schema://?access-token=jdu9292s&user_id=23
  

• You can get the data in your app through Uri.parse(result).queryParameters:

  // Present the dialog to the user
  final result = await FlutterWebAuth2.authenticate(url: "https://my-custom-app.com/connect", callbackUrlScheme: "valid-callback-scheme");
  // Extract token from resulting url
  String accessToken = Uri.parse(result).queryParameters['access-token'];
  String userId = Uri.parse(result).queryParameters['user_id'];

Cannot open keyboard on iOS

This seems to be a bug in ASWebAuthenticationSession and no workarounds have been found yet. Please see issue #120 for more info.

Error on macOS if Chrome is default browser

This seems to be a bug in ASWebAuthenticationSession and no workarounds have been found yet. Please see issue #136 for more info.