Google Calendar API - skip prompt after user gives consent

Question

I have a flutter app that needs access to multiple user's calendars.

I can get access for one user working, but the access grant is only temporary and the user gets re-prompted every time. I want to develop something like the firebase google authentication and many other Oauth examples: You get prompted once to grant access and then the access is granted offline until you revoke the access grant.

How can I request access with the remembered credentials? Here is my code:

final ClientId _userAccountCredentials = new ClientId(
      "XXXXX.apps.googleusercontent.com",
      "");

void prompt(String url) async {
    if (await canLaunch(url)) {
      await launch(url);
    } else {
      throw 'Could not launch $url';
    }
  }

  void getCalendarEventsOnline() {
    clientViaUserConsent(_userAccountCredentials, scopes, prompt).then((AuthClient client) {
      var calendar = calendarapi.CalendarApi(client);
      var calEvents = calendar.events.list("primary");
      calEvents.then((calendarapi.Events events) {
        /// some logic within
      });
      client.close();
    });
  }

I tried using a service account, but impersonating a user works only for a Gsuite account

Asking each user to add the service account to their calendar is error-prone, clumsy and unacceptable for non-technical users (think your grandparents), given that many popular services can do this directly

Solution

I discovered that the client contains all the information I need for future calls:

debugPrint(' access token: ' + client.credentials.accessToken.data +' refresh token ' + client.credentials.refreshToken);
// store the tokens in the apps key store

When my user makes a new call a second or a year from now, I retrieve the tokens from storage, create new credentials and make my calls:

AccessCredentials _newCredentials = AccessCredentials(fromStorage.credentials.accessToken, 
fromStrorage.credentials.refreshToken, _scopes );

var _newClient = new http.Client();
AccessCredentials _accessCredentials = await refreshCredentials( _clientID, newCredentials , _newClient);
_newClient = authenticatedClient(_newClient, _accessCredentials);
// the code below was just for me to test this out with my API scopes. replace with your code

var calendar = cal.CalendarApi(_newClient);
String calendarId = "---some string---";
cal.FreeBusyRequest _request = cal.FreeBusyRequest.fromJson(
    {
      'items': [
        {'id': calendarId, 'busy': 'Active'}
      ],
      'timeMin': (new DateTime(2020, 11, 17)).toIso8601String()+'Z',
      'timeMax': (new DateTime(2020, 11, 19)).toIso8601String()+'Z'
    });
  debugPrint('request: ' + _request.toJson().toString());
  cal.FreeBusyResponse response = await calendar.freebusy.query(_request);
  debugPrint(response.toJson().toString());
});