How to use @required annotation on Flutter constructor parameters?

Question

When I annotate a constructor parameter with @required IntelliJ shows an error:

Annotation must be either a const variable reference or const constructor invocation

Can anyone suggest what I'm doing wrong?

class StatusBar extends StatelessWidget {
  final String text;

  const StatusBar({Key key, @required this.text})
      : assert(text != null),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    //...
  }
}

score 61 · Accepted Answer · edited Apr 17 '18 at 17:54

61

Annotations need to be imported

Adding at the top of your file

import 'package:flutter/foundation.dart';

should fix it.

Annotations the DartAnalyzer understands are provided by the meta package.

To make it easier for Flutter developers, the Flutter team decided to add the meta package to the Flutter SDK and re-export it in flutter/foundation.dart. The annotations by flutter are therefore exactly the same as these provided by the meta package and you can as well add meta to your dependencies in pubspec.yaml and import annotations from there if you prefer. If you want to reuse code between for example AngularDart and Flutter that is the preferred way because code that imports from package:flutter/... can't be used in Dart web applications.

edited Apr 17 '18 at 17:54

Randal Schwartz

39,428
4
43
70

answered Apr 17 '18 at 17:48

Günter Zöchbauer

623,577
216
2,003
1,567

1

Great answer to avoid issues when using dart repositories for web and android both. – SouravOrii Jan 07 '19 at 11:30
i am using android studio. why it is does not shown error if i does not pass a variable – Parth Godhani Jul 22 '19 at 05:31
1

important to note that when you migrate to null-safety, you don't use the required annotation as in '@required' but instead use the required keyword – bangeboss Jan 15 '22 at 08:39

score 30 · Answer 2 · answered Jul 07 '18 at 13:34

30

Please import package "meta" at the beginning of the source file.

// @required is defined in the meta.dart package
import 'package:meta/meta.dart';

answered Jul 07 '18 at 13:34

madeinQuant

1,721
1
18
29

score 5 · Answer 3 · edited May 31 '21 at 19:23

5

Does your code include the following code?

import 'package:meta/meta.dart';

If your code contains the code above and you get errors, check pubspec.yaml file:

dependencies:
  meta: ^1.4.0
  flutter:
    sdk: flutter

Pay attention to the meta section from the above sample.

If the error persists, try the following on the CLI:

pub upgrade

edited May 31 '21 at 19:23

Alex

5,510
8
35
54

answered Jun 10 '19 at 01:50

booiljoung

776
6
10

Your solution adds new value at all. First, `import 'package:meta/meta.dart';` solution is already provided by someone, second what you have written extra has no meaning when it comes to importing `meta.dart` package because you're not using `meta` in `pubspec.yaml` file. Not sure how you got 2 upvotes. – iDecode May 21 '20 at 08:35

score 2 · Answer 4 · answered Aug 18 '20 at 06:05

I found that this problem can happen if your class has a variable called required

class TextFieldInputWidget extends StatefulWidget {
  final String title;
  final bool required;

  const TextFieldInputWidget({@required this.title, this.required = false});

  @override
  _TextFieldInputWidget createState() => _TextFieldInputWidget();
}

Just change the variable name to something else, like "require"

score 0 · Answer 5 · answered Nov 11 '21 at 07:18

0

If you have null safety enabled, the required keyword is an actual keyword and not a @ notation:

DateTime startDate;
  YourWidget(required this.startDate);

answered Nov 11 '21 at 07:18

stan

229
3
10

How to use @required annotation on Flutter constructor parameters?

5 Answers5