Web Auth 2 for Flutter

This project is a continuation of flutter_web_auth by Linus Unneb├Ąck 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: ^2.0.0

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 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 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>

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>
  if (window.opener) {
    window.opener.postMessage({
      'flutter-web-auth-2': window.location.href
    }, window.location.origin);
  } else {
    localStorage.setItem('flutter-web-auth-2', window.location.href);
  }
  window.close();
</script>

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 authorization object is returned as a URL fragment encoded as a query string (for compatibility with other providers).

If you want to pass additional parameters to the URL open call, you can do so in the authenticate function using the parameter contextArgs.

Windows and Linux

There is still a limitation that the callback URL scheme must start with http://localhost:{port}.

If you have any experience in removing this limitation, please let me know!

Troubleshooting

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

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">
    

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.

Libraries

flutter_web_auth_2