Flutter Text Widget doesn't allow non nullable string param-CodePudding

This is a code that throws the error:

class DogName extends StatelessWidget {
  final String name;

  const DogName(this.name);

  @override
  Widget build(BuildContext context) {
    return const DecoratedBox(
      decoration: BoxDecoration(
          color: Colors.lightBlueAccent
      ),
      child: Padding(
        padding: EdgeInsets.all(8.0),
        child: Text(name),
      ),
    );
  }
}

and the error is the following

Error

Could you please, I'm just refresing my Flutter knowledge

thanks

CodePudding user response：

You should write your widget as follow

class DogName extends StatelessWidget {

  const DogName(this.name);
  final String name;

  @override
  Widget build(BuildContext context) {
    return DecoratedBox(
      decoration: const BoxDecoration(
          color: Colors.lightBlueAccent
      ),
      child: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Text(name),
      ),
    );
  }
}

Basically you can't define you Text widget as a constant because the name property isn't constant.

CodePudding user response：

To make nullable data use dataType?. Another issue is you are reading name on runtime, therefore DecoratedBox cant be const because of name variable.

class DogName extends StatelessWidget {
  final String name;

  const DogName(this.name);

  @override
  Widget build(BuildContext context) {
    return DecoratedBox(
      decoration: BoxDecoration(
          color: Colors.lightBlueAccent
      ),
      child: Padding(
        padding: EdgeInsets.all(8.0),
        child: Text(name),
      ),
    );
  }
}

But it would be better if you use named constructor with key,

class DogName extends StatelessWidget {
  final String name;
  const DogName({
    Key? key,
    required this.name,
  }) : super(key: key);

 

  @override
  Widget build(BuildContext context) {
    return DecoratedBox(
      decoration: BoxDecoration(color: Colors.lightBlueAccent),
      child: Padding(
        padding: EdgeInsets.all(8.0),
        child: Text(name),
      ),
    );
  }
}

Now if you want nullable, you need to provide default value means when name will get null.

class DogName extends StatelessWidget {
  final String? name;
  const DogName({
    Key? key,
    this.name,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return DecoratedBox(
      decoration: BoxDecoration(color: Colors.lightBlueAccent),
      child: Padding(
        padding: EdgeInsets.all(8.0),
        child: Text(name ?? "default value"),
      ),
    );
  }
}

More about null-safety

CodePudding user response：

The problem is in the const keyword at line 6. The name variable here is not const

Change your class to

class DogName extends StatelessWidget {
  final String name;

  const DogName(this.name);

  @override
  Widget build(BuildContext context) {
    return DecoratedBox(
      decoration: const BoxDecoration(color: Colors.lightBlueAccent),
      child: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Text(name),
      ),
    );
  }
}