app/lib/data/auth.dart

import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';

String getSession(userId, sessionToken) {
  var sessionData = {
    "user_id": userId,
    "session_token": sessionToken
  };
  var sessionRaw = jsonEncode(sessionData).toString();
  var sessionEncoded = base64.encode(utf8.encode(sessionRaw));
  return sessionEncoded;
}

Future<void> addSession(userId, sessionToken) async {
  final prefs = await SharedPreferences.getInstance();

  String session = getSession(userId, sessionToken);

  await prefs.setString('session', session);
}

Future<void> removeSession() async {
  final prefs = await SharedPreferences.getInstance();

  await prefs.remove('session');
}