我有一个关于Dart和Flutter中的Oauth2的（可能是？）小问题（我使用的是标准的Oauth2包：https://pub.dev/packages/oauth2）。具体来说，我需要一些作为Authorization流的一部分传回的值，但它们不是标准值，因此它不会作为客户端的一部分返回。
具体地说，作为我的流的一部分，我运行：

final returnValue = await authClient.authenticate(
        authorizationUrl: authorizationUrl,
        redirectUri: redirectUri!,
      );

没问题。然后我跑：

client = await grant
          .handleAuthorizationResponse(Uri.parse(returnValue).queryParameters);

同样，正常工作时，返回一个我可以使用的客户端。但是，在此函数之后，handleAuthorizationResponse调用_handleAuthorizationResponse。函数_handleAuthorizationResponse以以下形式结束：

var response =
        await _httpClient!.post(tokenEndpoint, headers: headers, body: body);
    // print(response.headers);
    // print(response.body);

    var credentials = handleAccessTokenResponse(
        response, tokenEndpoint, startTime, _scopes, _delimiter,
        getParameters: _getParameters);
    return Client(credentials,
        identifier: identifier,
        secret: secret,
        basicAuth: _basicAuth,
        httpClient: _httpClient,
        onCredentialsRefreshed: _onCredentialsRefreshed);

这就是我的问题所在。这个响应的主体中有一些我需要的字段。它们不是标准的，所以它们不会作为Client的一部分被传回。在不重写这些函数的情况下，有没有其他方法可以访问这些值？
为了回答一个问题，我们在FHIR上使用了SMART启动（这是一个启动框架，基本上只是一个oauth2 Package 器）--这是医疗保健领域的标准。

{
  "need_patient_banner": true,
  "smart_style_url": "https://smart.argo.run/smart-style.json",
  "patient": "87a339d0-8cae-418e-89c7-8651e6aab3c6",
  "token_type": "Bearer",
  "scope": "launch/patient patient/Observation.rs patient/Patient.rs",
  "expires_in": 3600,
  "access_token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJuZWVkX3BhdGllbnRfYmFubmVyIjp0cnVlLCJzbWFydF9zdHlsZV91cmwiOiJodHRwczovL3NtYXJ0LmFyZ28ucnVuLy9zbWFydC1zdHlsZS5qc29uIiwicGF0aWVudCI6Ijg3YTMzOWQwLThjYWUtNDE4ZS04OWM3LTg2NTFlNmFhYjNjNiIsInRva2VuX3R5cGUiOiJiZWFyZXIiLCJzY29wZSI6ImxhdW5jaC9wYXRpZW50IHBhdGllbnQvT2JzZXJ2YXRpb24ucnMgcGF0aWVudC9QYXRpZW50LnJzIiwiY2xpZW50X2lkIjoiZGVtb19hcHBfd2hhdGV2ZXIiLCJleHBpcmVzX2luIjozNjAwLCJpYXQiOjE2MzM1MzIwMTQsImV4cCI6MTYzMzUzNTYxNH0.PzNw23IZGtBfgpBtbIczthV2hGwanG_eyvthVS8mrG4",
  "refresh_token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJjb250ZXh0Ijp7Im5lZWRfcGF0aWVudF9iYW5uZXIiOnRydWUsInNtYXJ0X3N0eWxlX3VybCI6Imh0dHBzOi8vc21hcnQuYXJnby5ydW4vL3NtYXJ0LXN0eWxlLmpzb24iLCJwYXRpZW50IjoiODdhMzM5ZDAtOGNhZS00MThlLTg5YzctODY1MWU2YWFiM2M2In0sImNsaWVudF9pZCI6ImRlbW9fYXBwX3doYXRldmVyIiwic2NvcGUiOiJsYXVuY2gvcGF0aWVudCBwYXRpZW50L09ic2VydmF0aW9uLnJzIHBhdGllbnQvUGF0aWVudC5ycyBvZmZsaW5lX2FjY2VzcyIsImlhdCI6MTYzMzUzMzg1OSwiZXhwIjoxNjY1MDY5ODU5fQ.Q41QwZCEQlZ16M7YwvYuVbUP03mRFJoqRxL8SS8_ImM"
}

所以它有accessToken的典型值（expires_in，token_type等），但它也有'patient'这样的值，这些都是我需要的值。
在我的示例中，最终的凭据（client.credentials看起来像这样）：

{
    "accessToken": "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiJ1cm46b2lkOmZoaXIiLCJjbGllbnRfaWQiOiIzZmE1Y2FmOS04YTk4LTQ4MjgtOTJkZS01OWU2NmJjYjIwNjQiLCJlcGljLmVjaSI6InVybjplcGljOk9wZW4uRXBpYy1jdXJyZW50IiwiZXBpYy5tZXRhZGF0YSI6IjVYWnFRU0lrSk9PNi1XRkhpVXBiMlg2ak5MQzJ1aDFQdWtaSHVWSHkzcTdJMTBBT1BfYXR5V0tEb19LMlRFRF9ic21TNk9UTmZiVGtISnY1dnFvM1RtTVduRlk2RDBPSlE2WkRhd1NJWkk4WDh0Xy1XT1pYWEs1WjFrcTNBNm9mIiwiZXBpYy50b2tlbnR5cGUiOiJhY2Nlc3MiLCJleHAiOjE2NTQ2NDEzMDIsImlhdCI6MTY1NDYzNzcwMiwiaXNzIjoidXJuOm9pZDpmaGlyIiwianRpIjoiMDg1ZTdhNTYtY2I0OS00Zjg3LWFiYmEtZDg0M2ZmODI2YmQ2IiwibmJmIjoxNjU0NjM3NzAyLCJzdWIiOiJlYjRHaWE3RnlpanRQbVhrcnRqUnBQdzMifQ.rAKweImVE86oF3ciZDGhDysrYY9-XV6fBbyzqkQiJxHg-V-zImW414m3X5wKcP9B0J1MMdJCwg5DTpcbd0iU-N3SXRVXxBO2BqTcMAGLr-jlepnqBfu1Esg0nAI9jVasSWhz6tXFcLWOoCocg1hLcMfY875xnszwztJiJieDhumKZSStcsQM4KR9lUQZdJ3-U6IXV7wn3kaD4GQBSPZ0OkUe2d8zdCpjcbGCO-wWNdfe_sQDd7k7MbBJ1ryFRtd45GSzhKFa3Cch8kWTo3bGPlzzFuvhX_kbX1WtqTXaeB2G-o49lT4RJldnZi62L51VtS69_M15EsQtmMRHg6WMEA",
    "refreshToken": null,
    "idToken": "eyJhbGciOiJSUzI1NiIsImtpZCI6InRvVzlqTVVTTi81L0wzaXdhUUdkVG1ORHVodnAvSmNBWlZIL2NPSjZPckU9IiwidHlwIjoiSldUIn0.eyJhdWQiOiIzZmE1Y2FmOS04YTk4LTQ4MjgtOTJkZS01OWU2NmJjYjIwNjQiLCJleHAiOjE2NTQ2MzgwMDIsImZoaXJVc2VyIjoiaHR0cHM6Ly9maGlyLmVwaWMuY29tL2ludGVyY29ubmVjdC1maGlyLW9hdXRoL2FwaS9GSElSL1I0L1BhdGllbnQvZXJYdUZZVWZ1Y0JaYXJ5VmtzWUVjTWczIiwiaWF0IjoxNjU0NjM3NzAyLCJpc3MiOiJodHRwczovL2ZoaXIuZXBpYy5jb20vaW50ZXJjb25uZWN0LWZoaXItb2F1dGgvb2F1dGgyIiwic3ViIjoiZXJYdUZZVWZ1Y0JaYXJ5VmtzWUVjTWczIn0.hh33_q4f3tnioB7Iq6jY07-m5i_OsaqUt_kg_ZnPMGPKK8AnYVk3Tps2XTdUzUIHizFRWlGmAT_E0F283LBmVPTrbtD_X6EwqmUbTBrWj94RyvE-k3ofoEo-CwbSJZXu8MrQTb3DzpRKTGo7D1sI5E4UqnKQhPWFmhwCjMXpbdRy6bddb14fdWZzjS_Ffq4OsNRIalnePR8z1zNtSy14_RCiSh8o2elkj3p1AOmSXeD9-nZ91Z646lt4C5oP9gwN7OhmBovQRuDYaql1tz1aHOhilIsBZc1jMxEZJ65cekmFy6HZ4rME23xg-EQHu7XhKWOpOjovbMPwapSlC-eUcA",
    "tokenEndpoint": "https://fhir.epic.com/interconnect-fhir-oauth/oauth2/token",
    "scopes": [
        "patient/Patient.read",
        "patient/Questionnaire.read",
        "patient/QuestionnaireResponse.Read",
        "fhirUser",
        "launch/patient",
        "openid",
        "profile"
    ],
    "expiration": 1654641291566
}

我尝试过解码accessToken和idToken，但它们都没有“patient”这样的字段。

{
    "access_token": "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.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.AJOd9g8YAJp91n0qY3Hg9F2sNpo26VMYKpNKR5y7CIV8zrADh2whv2WRm8gi-cIeS6XUR6UzXyzXVJ9Ips5FgFdIZ4yQI_HXxH9r8aeF6VS6jT-ZQygtzWnVYeyJvu-1b3YpbgdCd3KTrnWLwhU3vqUmil2L8gJzWG473ihXDz-7ezsJBBl9R-c5Ap_L6WF6Ox8lHH6mgwbZHeKr0U0aYne-QLM7mylsPC5BC_WlUOwMnEJ73DKjF2E0X6wMCP7jMieJxhpkTIDRwKQbuGwLjtneS-Efu69NHGsxSP_m3aN652rdh9-b5WyIsT-DqjPHxHTtbxGQI-WthHOhnLaDkQ",
    "token_type": "Bearer",
    "expires_in": 3600,
    "scope": "patient/Patient.read patient/Questionnaire.read patient/QuestionnaireResponse.Read fhirUser launch/patient openid profile",
    "__epic.dstu2.patient": "TnOZ.elPXC6zcBNFMcFA7A5KZbYxo2.4T-LylRk4GoW4B",
    "id_token": "eyJhbGciOiJSUzI1NiIsImtpZCI6InRvVzlqTVVTTi81L0wzaXdhUUdkVG1ORHVodnAvSmNBWlZIL2NPSjZPckU9IiwidHlwIjoiSldUIn0.eyJhdWQiOiIzZmE1Y2FmOS04YTk4LTQ4MjgtOTJkZS01OWU2NmJjYjIwNjQiLCJleHAiOjE2NTQ2NDQzMDgsImZoaXJVc2VyIjoiaHR0cHM6Ly9maGlyLmVwaWMuY29tL2ludGVyY29ubmVjdC1maGlyLW9hdXRoL2FwaS9GSElSL1I0L1BhdGllbnQvZXJYdUZZVWZ1Y0JaYXJ5VmtzWUVjTWczIiwiaWF0IjoxNjU0NjQ0MDA4LCJpc3MiOiJodHRwczovL2ZoaXIuZXBpYy5jb20vaW50ZXJjb25uZWN0LWZoaXItb2F1dGgvb2F1dGgyIiwic3ViIjoiZXJYdUZZVWZ1Y0JaYXJ5VmtzWUVjTWczIn0.wxlvguGhAZdWJiSpX1-jzANXk0hFhLeIPFS5BlnIJLLZg8ibvpzLutQr2Z7Rg_d07_amI4gGbNigso9gvPbN5e1jjDGZkU2QYUbcLZbwkTcxXfVWOsyAADOZZrqx0J1yrGIeA4V4EfqQ4xBym_e8CeEjGP9L4ouRBKK6AHR5N5Mmdo_I4_RoPr-mCR2e2Q_of7tYFuhcl8mHaT6brbn-ZoEuAMgAQztF-7SBpDSvRB1C4HzV6mk-Hql0jNhZ0WefZe_ve0gB3exdWDjCLClpRRjt_MRaFTYGPqiZuyJF-dEFEqNar1Y5BRjQmUdJbDWj8ecfWaldigXNVAvNthbs4g",
    "patient": "erXuFYUfucBZaryVksYEcMg3"
}

所以你可以理解为什么我需要访问这些信息。除了扩展类和重写函数之外，我还有什么办法可以访问它吗？