1. @override
Future<AuthToken> fetchTokenByRefreshToken(AuthServer server, String refreshToken)

Returns a AuthToken for an refreshToken.

This method must return an instance of AuthToken if one exists for refreshToken. Otherwise, it must return null. server is the AuthServer requesting the AuthToken.

Source 

@override
Future<AuthToken> fetchTokenByRefreshToken(
    AuthServer server, String refreshToken) async {
  var query = new Query<ManagedToken>(context)
    ..where.refreshToken = refreshToken;
  var token = await query.fetchOne();

  return token?.asToken();
}