Why is key required in constructor?

Question

I have created class which extend StatefulWidget

class RegistrationPage extends StatefulWidget {
  final String email;

  const RegistrationPage({Key key, required this.email}) : super(key: key);

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

The problem is android studio force me to put required before Key key. I googled some of examples how to pass values from screen to another screen and I have never seen that someone used required with Key. I do it within:

Navigator.push(
        context,
        new MaterialPageRoute(
          builder: (context) => RegistrationPage(email: email),
        ),
      );

so just to pass email value. I need to make Key nullable to make it work. Am I doing something wrong?

Key is expected to be nullable. The widget super constructors use a nullable key. — Christopher Moore, May 23 '21 at 17:22

score 4 · Accepted Answer · answered May 23 '21 at 17:21

4

Because you're using null-safe Dart and key can't be null because it has a non-nullable type Key.

Solutions:

Use required
```
FooPage({required Key key});
```
Make key nullable.
```
FooPage({Key? key});
```
Remove key altogether.
```
FooPage();
```

answered May 23 '21 at 17:21

CopsOnRoad

237,138
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Which one is better. I know that I need to add '`?' or `required` – traki111 May 23 '21 at 17:24
2

@traki111 If you are definitely going to use `key`, use `required`, if definitely not, remove the `key` and if you're not sure use `Key?`. I've provided all the three possible solutions. – CopsOnRoad May 23 '21 at 17:27

Milvintsiss · Answer 2 · 2021-05-23T17:25:13.783

2

I think your project is in null safety, with null safety a variable or object cannot be null execept if it is declared nullable. Try adding a ? after Key:

class RegistrationPage extends StatefulWidget {
  final String email;

  const RegistrationPage({Key? key, required this.email}) : super(key: key);

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

or you can simply delete the Key override:

class RegistrationPage extends StatefulWidget {
  final String email;

  const RegistrationPage({required this.email});

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

I suggest you to read https://dart.dev/null-safety/understanding-null-safety

edited May 23 '21 at 17:25

answered May 23 '21 at 17:23

Milvintsiss

1,420
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1

but it is fine if I remove Key? Everything will works like it should? – traki111 May 23 '21 at 17:23
2

@traki111 Yes unless you're planning on using keys, you don't need it. – Christopher Moore May 23 '21 at 17:24
yes, if you have no use for key it will work without any override – Milvintsiss May 23 '21 at 17:24
1

@traki111 please look at the link in my answer to understand more about null safety. All flutter projects from now will default to null safety. – Milvintsiss May 23 '21 at 17:27

score 2 · Answer 3 · answered May 23 '21 at 17:25

You're not doing anything wrong by making Key key nullable. The super constructors that you're passing the key to accept the nullable type.

So

const RegistrationPage({Key? key, required this.email}) : super(key: key);

is the norm as there is no reason to constrain the type by making it non-nullable and required.

If you have no need for keys with this widget, you can omit the super constructor and the key parameter completely.

Why is key required in constructor?

3 Answers3