I am following a flutter tutorial uses the following codes

class Auth with ChangeNotifier {
  late String _token;
  late DateTime _expiryDate;
  late String _userId;
  late Timer _authTimer;

  String get token {
    if (_expiryDate != null &&
        _expiryDate.isAfter(DateTime.now()) &&
        _token != null) {
      return _token;
    }
    return null;
  }

  Future<void> logout() async {
    _token = null;
    _userId = null;
    _expiryDate = null;
    if (_authTimer != null) {
      _authTimer.cancel();
      _authTimer = null;
    }
    notifyListeners();
    final prefs = await SharedPreferences.getInstance();
    // prefs.remove('userData');
    prefs.clear();
  }
}

But as I am using the new version of flutter it gives me the following errors:

A value of type 'Null' can't be returned from the function 'token' because it has a return type of 'String'.

Or

A value of type 'Null' can't be assigned to a variable of type 'Timer'.
Try changing the type of the variable, or casting the right-hand type to 'Timer'.

Also

A value of type 'Null' can't be assigned to a variable of type 'DateTime'.
Try changing the type of the variable, or casting the right-hand type to 'DateTime'.

I don't know how to fix this to make code run?

CodePudding user response：

Based on your declaration of method your return type is String and you returned Null instead of string. You should return empty string instead of Null.

String? get token {
   if (_expiryDate != null &&
       _expiryDate.isAfter(DateTime.now()) &&
       _token != null) {
    return _token;
   }
   return '';
}

for another one you should assign value based on your data type not null in every case. you should create nullable variables;

class Auth with ChangeNotifier {
  String? _token;
  DateTime? _expiryDate;
  String? _userId;
  Timer? _authTimer;

Future<void> logout() async {
  _token = null;
  _userId = null;
  _expiryDate = null;
  if (_authTimer != null) {
    _authTimer?.cancel();
    _authTimer = null;
  }
  notifyListeners();
  final prefs = await SharedPreferences.getInstance();
  // prefs.remove('userData');
  prefs.clear();
}

CodePudding user response：

You should return the empty string '' instead of Null.

Like it says, you can't return Null.

CodePudding user response：

replace null with empty string in return type like this.

String get token {
    if (_expiryDate != null &&
        _expiryDate.isAfter(DateTime.now()) &&
        _token != null) {
      return _token;
    }
    return '';
  }

CodePudding user response：

From memory, try changing the function type to String? get token {...} that way you're telling dart that the function can return either a String or null.