问使用oauth2_client flutter软件包的coinbase上的OAuth invalid_grant错误

EN

import 'package:oauth2_client/oauth2_client.dart';
import 'package:meta/meta.dart';

/// Implements an OAuth2 client against GitHub
///
/// In order to use this client you need to first create a new OAuth2 App in the GittHub Developer Settings (https://github.com/settings/developers)
///
class GitHubOAuth2Client extends OAuth2Client {
  GitHubOAuth2Client(
      {@required String redirectUri, @required String customUriScheme})
      : super(
            authorizeUrl: 'https://github.com/login/oauth/authorize',
            tokenUrl: 'https://github.com/login/oauth/access_token',
            redirectUri: redirectUri,
            customUriScheme: customUriScheme) {
    accessTokenRequestHeaders = {'Accept': 'application/json'};
  }
}

void _oauthMethod() async {
    //clientID
    String cID = 'x';

    //clientSecret
    String cSecret = 'y';

    OAuth2Client client = GitHubOAuth2Client(
        redirectUri: 'my.app://oauth2redirect', customUriScheme: 'my.app');

    AccessTokenResponse tknResp = await client.getTokenWithAuthCodeFlow(
        clientId: cID, clientSecret: cSecret, scopes: ['repo']);

    http.Response resp = await http.get('https://api.github.com/user/repos',
        headers: {'Authorization': 'Bearer ' + tknResp.accessToken});
   
  }

class MyOAuth2Client extends OAuth2Client {
  MyOAuth2Client(
      {@required String redirectUri, @required String customUriScheme})
      : super(
            authorizeUrl:
                'https://www.coinbase.com/oauth/authorize', //Your service's authorization url
            tokenUrl:
                'https://api.coinbase.com/oauth/token', //Your service access token url
            redirectUri: redirectUri,
            customUriScheme: customUriScheme) {
    this.accessTokenRequestHeaders = {'Accept': 'application/json'};
  }
}

void _coinbaseAuth() async {
    String cID = 'x';
    String cSecret = 'y';
    MyOAuth2Client client = MyOAuth2Client(
        redirectUri: 'my.app://oauth2redirect', customUriScheme: 'my.app');

    AccessTokenResponse tknResp = await client.getTokenWithAuthCodeFlow(
        clientId: cID, clientSecret: cSecret, scopes: ['wallet:user:read']);

    print(tknResp);


    //code fails 
    //http.Response resp =
    //        await http.get('https://api.coinbase.com/v2/user', headers: {
    //      'Authorization': 'Bearer ' + tknResp.accessToken,
    //      'Content-Type': 'application/json',
    //      'Charset': 'utf-8'
    //    });


  }