null safety on widget parameter

Question

Before null-safety, I can pass widget, if check it is null, will show other widget. On null-safety environment, is there a way if I didn't pass widget image or note ?

class Message extends StatelessWidget {
  const Message({
    Key key,
    this.image,
    this.title = "",
    this.subtitle = "",
    this.note,
    this.onTap
  }) : super(key: key);

  final Widget image;
  final String title;
  final String subtitle;
  final Widget note;
  final Function onTap

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onTap,
      child: Column(
        children:[
          image ?? Container(),
          if(note != null) note
        ]
      )
    );
  }
};

Answer 1

You can declare your image and note as nullable types. If you need to make any of the field mandatory, you can mark it as required .

class Message extends StatelessWidget {
  const Message({
    Key? key,
    this.image,
    this.title = "",
    this.subtitle = "",
    this.note,
    required this.onTap,
  }) : super(key: key);

  final Widget? image;
  final String title;
  final String subtitle;
  final Widget? note;
  final VoidCallback onTap;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onTap,
      child: Column(
        children:[
          image ?? Container(),
          note ?? Container(),
        ]
      )
    );
  }
}

Answer 2

• Use ? if the child can be null .

   class FooWidget extends StatelessWidget { final Widget? child; // <-- Nullable, use '?' FooWidget({ this.child, }); //... }

• Use required if the child can't be null .

   class FooWidget extends StatelessWidget { final Widget child; // <-- Non-nullable FooWidget({ required this.child, // <-- Use 'required' }); //... }