I have the following code:

class Auth with ChangeNotifier {
  String? _token;
  DateTime? _expiryDate;
  String? _userId;
  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();
  }
}

And the following errors:

The method 'isAfter' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').

The method 'cancel' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').

How can I fix this error messages?

CodePudding user response：

Change these lines of code to this:

        _expiryDate!.isAfter(DateTime.now()) &&

      _authTimer!.cancel();

The issue is that Dart knows that _expiryDate or _authTimer can be null. However, you assert by checking them both for not being null in your if statement that they cannot possibly be null at that point. Thus, you can add a !, which is a non-null assertion, basically saying 'I know this variable can have a value of null, but I'm sure that it can't be null at this point'.