如何在Flutter Web中捕获OAuth2令牌？

如何解决如何在Flutter Web中捕获OAuth2令牌？

This dependency应该支持Web，但是缺少用于侦听回调和检索令牌的实现。经过三天的研究后，一些人建议它可以使用带有onMessage和postMessage函数的dart：html库工作。

这是我的设置：

import 'dart:html' as html;
import 'package:http/http.dart' as http;
import 'package:flutter/material.dart';
import 'package:oauth2/oauth2.dart' as oauth2;
import 'package:url_launcher/url_launcher.dart';

class AuthYoutube {
var httpClient = http.Client();
  Future<oauth2.Client> getClient() async {
    var grant = new oauth2.AuthorizationCodeGrant(
        CLIENT_ID,AUTH_DOMAIN,AUTH_TOKEN,secret: secret);
    var authorizationUrl =
        grant.getAuthorizationUrl(REDIRECT_URI,scopes: scopes);

    html.window.open(authorizationUrl.toString(),"open");
    Completer<LinkedHashMap<dynamic,dynamic>> completer = Completer();
    html.window.onMessage.listen((event) async {
      completer.complete(event.data as LinkedHashMap<dynamic,dynamic>);
    });
    Uri responseUri;
    completer.future.then((value) {
      responseUri = Uri.parse(value.toString());
      return grant.handleAuthorizationResponse(responseUri.queryParameters);
    });
  }

} 并将单独的路由设置为回调URL：

class Special extends StatelessWidget {
  static String route = "/special";
  @override
  Widget build(BuildContext context) {
    final loc = Uri.parse(html.window.location.href);
    final code = loc.queryParameters["code"];
    print("code $code");
    html.window.postMessage(code,html.window.location.origin);
    return Text("All done");
  }
}

最后发生的是，身份验证令牌显示在最后一步的地址栏中。我可以在?code=之后在其中看到它，但是它没有打印出来。 listen方法在整个流程中被触发5到6次（在另一个窗口中单击“允许”）。在控制台中，我仅收到此错误：

Uncaught (in promise) Error: Bad state: Future already completed
    at Object.throw_ [as throw] (errors.dart:212)
    at _AsyncCompleter.new.complete (future_impl.dart:43)
    at authYT.dart:41
    at Generator.next (<anonymous>)
    at runBody (async_patch.dart:84)
    at Object._async [as async] (async_patch.dart:123)
    at authYT.dart:40
    at Object._checkAndCall (operations.dart:324)
    at Object.dcall (operations.dart:329)
    at html_dart2js.dart:37246