Flutter default value for a Function

Question

I am currently learning a course on Flutter. They provide you with an old stub project (nullable). When I try to migrate to Flutter 2.12.0 or higher - null safety kicks in. I have a basic understanding on how it works - but this I cannot find anywhere on Google or StackOverFlow.

I have a custom Card widget. It requires a Color parameter. Also I need it to be able to receive a Widget child parameter as well as an onPress Function, but I don't want to make them required. Please, help. How do I create a default value for Widgets and Functions?

class ReusableCard extends StatelessWidget {
  ReusableCard({@required this.cardColor, this.cardChild, this.onPress});
  final Color cardColor;
  final Widget cardChild;
  final Function onPress;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onPress,
      child: Container(
        child: cardChild,
        margin: EdgeInsets.all(10.0),
        decoration: BoxDecoration(
          borderRadius: BorderRadius.circular(10.0),
          color: cardColor,
        ),
      ),
    );
  }
}

user_7110428 · Answer

You can make them nullable fields as follows

  ReusableCard({required this.cardColor, this.cardChild, this.onPress});
  final Color cardColor;
  final Widget? cardChild;
  final VoidCallback? onPress;

user_10438648 · Answer

just change

final Function onPress;

to

final Function() onPress;

user_14430639 · Answer

I think this is what you need: https://dart.dev/null-safety/understanding-null-safety#late-variables.

You would define your variables as

late final Widget cardChild; 
late final Function onPress;

and then initialize them in your constructor (or initState for State objects) before using them.

user_6618622 · Answer

For nullable function, use ?

class FooWidget extends StatelessWidget {
  final Function? onPressed; // <-- Nullable, use '?'

  FooWidget({
    this.onPressed,
  });

  // ... 
}

For non-nullable function, use required

class FooWidget extends StatelessWidget {
  final Function onPressed; // <-- Non-nullable

  FooWidget({
    required this.onPressed, // <-- Use 'required'
  });

  // ...
}

Flutter default value for a Function

4 Answers