Google Calendar API - skip prompt after user gives consent - flutter

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

Answer 1

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());
});