Signing out of flutter app with sqflite database - flutter

Question

I am trying to write the code for signing out of a flutter app logging in (The database of the map is made with sqflite).
However I am getting the following error message:
flutter: NoSuchMethodError: The method 'notify' was called on null.
Receiver: null
Tried calling: notify(Instance of 'AuthState')
Have provided code of required files below.
I am trying to incorporate a sign out function in the home_screen.dart file but I feel I am missing a link between the auth.dart, auth_provider.dart, login_screen.dart and home_screen.dart files. The codes of the required files for the issue are as follows:
File: database_helper.dart
import 'dart:async';
import 'dart:io' as io;
import 'package:path/path.dart';
import 'package:better_login/user.dart';
import 'package:sqflite/sqflite.dart' ;
import 'package:path_provider/path_provider.dart';
class DatabaseHelper {
static final DatabaseHelper _instance = new
DatabaseHelper.internal();
factory DatabaseHelper() => _instance;
static Database _db;
Future<Database> get db async {
if(_db != null)
return _db;
_db = await initDb();
return _db;
}
DatabaseHelper.internal();
initDb() async {
io.Directory documentsDirectory = await
getApplicationDocumentsDirectory();
String path = join(documentsDirectory.path, "main.db");
var theDb = await openDatabase(path, version: 1, onCreate:
_onCreate);
return theDb;
}
void _onCreate(Database db, int version) async {
// When creating the db, create the table
await db.execute(
"CREATE TABLE User(username TEXT,password TEXT)");
print("Created tables");
}
Future<int> saveUser(User user) async {
var dbClient = await db;
int res = await dbClient.insert("User", user.toMap());
return res;
}
Future<int> deleteUsers() async {
var dbClient = await db;
int res = await dbClient.delete("User");
return res;
}
Future<bool> isLoggedIn() async {
var dbClient = await db;
var res = await dbClient.query("User");
return res.length > 0? true: false;
}
}
File:auth.dart
import 'package:better_login/database_helper.dart';
enum AuthState{ LOGGED_IN, LOGGED_OUT }
abstract class AuthStateListener {
void onAuthStateChanged(AuthState state);
}
class AuthStateProvider {
static final AuthStateProvider _instance = new
AuthStateProvider.internal();
List<AuthStateListener> _subscribers;
factory AuthStateProvider() => _instance;
AuthStateProvider.internal() {
_subscribers = new List<AuthStateListener>();
initState();
}
void initState() async {
var db = new DatabaseHelper();
var isLoggedIn = await db.isLoggedIn();
if(isLoggedIn)
notify(AuthState.LOGGED_IN);
else
notify(AuthState.LOGGED_OUT);
}
void subscribe(AuthStateListener listener) {
_subscribers.add(listener);
}
void dispose(AuthStateListener listener) {
for(var l in _subscribers) {
if(l == listener)
_subscribers.remove(l);
}
}
void notify(AuthState state) {
_subscribers.forEach((AuthStateListener s) =>
s.onAuthStateChanged(state));
}
}
File: auth_provider.dart
import 'package:flutter/material.dart';
import 'package:better_login/auth.dart';
class AuthProvider extends InheritedWidget {
const AuthProvider({Key key, Widget child, this.auth}) : super(key: key, child: child);
final AuthStateListener auth;
#override
bool updateShouldNotify(InheritedWidget oldWidget) => true;
static AuthProvider of(BuildContext context) {
return context.inheritFromWidgetOfExactType(AuthProvider);
}
}
File: home_screen.dart
import 'package:flutter/material.dart';
import 'package:better_login/auth.dart';
import 'package:better_login/login_screen.dart';
import 'package:better_login/login_screen_presenter.dart';
import 'package:better_login/auth_provider.dart';
class HomeScreen extends StatelessWidget {
HomeScreen({this.authStateListener, this.authStateProvider});
final AuthStateListener authStateListener;
final AuthStateProvider authStateProvider;
void _signOut() async {
try{
authStateProvider.notify(AuthState.LOGGED_OUT);
authStateListener.onAuthStateChanged(AuthState.LOGGED_OUT);
}catch(e){
print(e);
}
}
#override
Widget build(BuildContext context) {
// TODO: implement build
return new Scaffold(
appBar: new AppBar(
title: new Text("Home"),
actions: <Widget>[
new IconButton(icon: new Icon(Icons.exit_to_app), onPressed: (){_signOut();}),
],
),
body: new Center(
child: new Text("Welcome home!"),
),
);
}
}