Dart: const class or const var? - flutter

Question

Newb question.
For performance, there's much difference between the class below (widget to build TextField),
or a const that doesn't rebuild anything? I heard in Flutter as everything are widgets, when building for example func vs class, always build a class to replace whatever you are building on your code. I thought this could be the same.
class RoundedBox extends StatelessWidget {
const RoundedBox({
this.text,
this.onChanged,
});
final String text;
final Function onChanged;
#override
Widget build(BuildContext context) {
return TextField(
onChanged: onChanged,
decoration: InputDecoration(
hintText: text,
hintStyle: TextStyle(
color: Colors.grey,
),
contentPadding: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
enabledBorder: OutlineInputBorder(
borderSide: BorderSide(color: Colors.lightBlueAccent, width: 1.0),
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
focusedBorder: OutlineInputBorder(
borderSide: BorderSide(color: Colors.lightBlueAccent, width: 2.0),
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
),
);
}
}
Or just a simple const var?:
const kTextFieldDecoration = InputDecoration(
hintText: 'Enter a value',
hintStyle: TextStyle(
color: Colors.grey,
),
contentPadding: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
enabledBorder: OutlineInputBorder(
borderSide: BorderSide(color: Colors.lightBlueAccent, width: 1.0),
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
focusedBorder: OutlineInputBorder(
borderSide: BorderSide(color: Colors.lightBlueAccent, width: 2.0),
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
);

Answer 1

In this case you will get better performance if you reuse the InputDecoration defined as const.
But this is equivalent(notice the const before InputDecoration). It's a good practice to use const for Widgets that does not change.
#override
Widget build(BuildContext context) {
return TextField(
onChanged: onChanged,
decoration: const InputDecoration(
hintText: text,
hintStyle: TextStyle(
color: Colors.grey,
),
contentPadding: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
enabledBorder: OutlineInputBorder(
borderSide: BorderSide(color: Colors.lightBlueAccent, width: 1.0),
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
focusedBorder: OutlineInputBorder(
borderSide: BorderSide(color: Colors.lightBlueAccent, width: 2.0),
borderRadius: BorderRadius.all(Radius.circular(32.0)),
),
),
);
}
}