Flutter: login through a webview

You can use my plugin flutter_inappwebview, which is a Flutter plugin that allows you to add inline WebViews or open an in-app browser window and has a lot of events, methods, and options to control WebViews.

You can use onLoadStart or onLoadStop events to detect URL changes. For example, you can get the token:

• from the url
• from cookies
• from localStorage

Full example:

import 'dart:async';

import 'package:flutter/material.dart';

import 'package:flutter_inappwebview/flutter_inappwebview.dart';

Future main() async {
  runApp(new MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {

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

  @override
  void dispose() {
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: InAppWebViewPage()
    );
  }
}

class InAppWebViewPage extends StatefulWidget {
  @override
  _InAppWebViewPageState createState() => new _InAppWebViewPageState();
}

class _InAppWebViewPageState extends State<InAppWebViewPage> {
  InAppWebViewController webView;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
            title: Text("InAppWebView")
        ),
        body: Container(
            child: Column(children: <Widget>[
              Expanded(
                child: Container(
                  child: InAppWebView(
                    initialUrl: "https://myUrl.com",
                    initialHeaders: {},
                    initialOptions: InAppWebViewWidgetOptions(
                      inAppWebViewOptions: InAppWebViewOptions(
                        debuggingEnabled: true,
                      ),
                      androidInAppWebViewOptions: AndroidInAppWebViewOptions(
                        domStorageEnabled: true,
                        databaseEnabled: true,
                      ),
                    ),
                    onWebViewCreated: (InAppWebViewController controller) {
                      webView = controller;
                    },
                    onLoadStart: (InAppWebViewController controller, String url) {

                    },
                    onLoadStop: (InAppWebViewController controller, String url) async {
                      if (url.startsWith("https://myUrl.com/auth-response")) {
                        // get your token from url
                        RegExp regExp = new RegExp("access_token=(.*)");
                        String token = regExp.firstMatch(url)?.group(1);
                        print(token);

                        // or using CookieManager
                        CookieManager cookieManager = CookieManager.instance();
                        Cookie token = await cookieManager.getCookie(url: "https://myUrl.com/auth-response", name: "access_token");
                        print(token.value);

                        // or using javascript to get access_token from localStorage
                        String token = await controller.evaluateJavascript(source: "localStorage.getItem('access_token')");
                        print(token);
                      }
                    },
                  ),
                ),
              ),
            ]))
    );
  }
}

In my app I use instagram implicit authentification, which implies to login user in webview and get token from redirect url. I use flutter_webview_plugin Next code builds WebviewScaffold with login url. And it listen for url changes. So when response is redirected to my redirectUrl it parses url to get token. Then you need to save token for following requests in app.

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_webview_plugin/flutter_webview_plugin.dart';

class LoginScreen extends StatefulWidget {
  @override
  _LoginScreenState createState() => new _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  final flutterWebviewPlugin = new FlutterWebviewPlugin();

  StreamSubscription _onDestroy;
  StreamSubscription<String> _onUrlChanged;
  StreamSubscription<WebViewStateChanged> _onStateChanged;

  String token;

  @override
  void dispose() {
    // Every listener should be canceled, the same should be done with this stream.
    _onDestroy.cancel();
    _onUrlChanged.cancel();
    _onStateChanged.cancel();
    flutterWebviewPlugin.dispose();
    super.dispose();
  }

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

    flutterWebviewPlugin.close();

    // Add a listener to on destroy WebView, so you can make came actions.
    _onDestroy = flutterWebviewPlugin.onDestroy.listen((_) {
      print("destroy");
    });

    _onStateChanged =
        flutterWebviewPlugin.onStateChanged.listen((WebViewStateChanged state) {
      print("onStateChanged: ${state.type} ${state.url}");
    });

    // Add a listener to on url changed
    _onUrlChanged = flutterWebviewPlugin.onUrlChanged.listen((String url) {
      if (mounted) {
        setState(() {
          print("URL changed: $url");
          if (url.startsWith(Constants.redirectUri)) {
            RegExp regExp = new RegExp("#access_token=(.*)");
            this.token = regExp.firstMatch(url)?.group(1);
            print("token $token");

            saveToken(token);
            Navigator.of(context).pushNamedAndRemoveUntil(
                "/home", (Route<dynamic> route) => false);
            flutterWebviewPlugin.close();
          }
        });
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    String loginUrl = "someservise.com/auth";

    return new WebviewScaffold(
        url: loginUrl,
        appBar: new AppBar(
          title: new Text("Login to someservise..."),
        ));
  }
}