Login using Provider

Question

I am trying to create a Login using Provider pattern. This is what I've tried so far...

First this is my UserRepository:

enum Status { Uninitialized, Authenticated, Authenticating, Unauthenticated }
class UserRepository with ChangeNotifier{
  User user;
  Status _status =Status.Uninitialized;

  Status get status => _status;
  User get authUser => user;

  Future<bool> signIn(String email, String password) async {
    try{
    var body = jsonEncode({
      'email': email,
      'password': password
    });

    var res = await http.post(('http://192.168.178.35:8000/auth/login'),
        body: body,
        headers: {
          "Accept": "application/json",
          "content-type": "application/json"
        });
    var response = jsonDecode(res.body);

    if(response.status == 200)
    {
      notifyListeners();
      return true;
    }
    }
    catch(e)
    {
      _status = Status.Unauthenticated;
      notifyListeners();
      return false;
    }
  }
}

And this should be my entry page (HomePage):

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<UserRepository>(     
    child: Consumer(
        builder: (context, UserRepository user, _) {
          switch (user.status) {
            case Status.Uninitialized:
              return Login();
            case Status.Unauthenticated:
            case Status.Authenticating:
              return Login();
            case Status.Authenticated:
              return DriverDashboard(user: user.user);
          }
        },
      ),
    );
  }
}

And after I try to run the app I get this error:

Does anyone have any idea why is this happening? Is this correct way how to implement ChangeNotifierProvider?

I am trying to follow this tutorial https://medium.com/flutter-community/flutter-firebase-login-using-provider-package-54ee4e5083c7 ...

However, except Firebase I use custom created RESTful API as a backend.

Answer 1

Although, this error message is not intuitive, I guess you might return a null in the builder function.

    class HomePage extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return ChangeNotifierProvider<UserRepository>(     
        child: Consumer(
            builder: (context, UserRepository user, _) {
              //NOTICE HERE: What If your user is null?
              switch (user.status) {
                case Status.Uninitialized:
                  return Login();
                case Status.Unauthenticated:
                case Status.Authenticating:
                  return Login();
                case Status.Authenticated:
                  return DriverDashboard(user: user.user);
              }
              // return SplashPageHere();
            },
          ),
        );
      }
    }

You should give a splash page, or something that return Widget for builder.

Edit: Here is the example to use ChangeNotifierProvider

    ChangeNotifierProvider<AppState>(
              builder: (_) => AppState(),
              child: MyHomePage(),
            ));

And this is the code you post from medium:

      @override
      Widget build(BuildContext context) {
        return ChangeNotifierProvider(
          builder: (_) => UserRepository.instance(),
          child: Consumer(
            builder: (context, UserRepository user, _) {
              switch (user.status) {
                case Status.Uninitialized:
                  return Splash();
                case Status.Unauthenticated:
                case Status.Authenticating:
                  return LoginPage();
                case Status.Authenticated:
                  return UserInfoPage(user: user.user);
              }
            },
          ),
        );
      }

ChangeNotifierProvider should contain a builder and child, That's why you always have null for your user...