Standardizing sizes across application, but still use `const` - flutter

Question

I'd like to standard size across a flutter application to comfort to a 4 pt grid. Here's one example of how this could be done:
class Spacing {
const Spacing(double val) : points = val * 4;
final double points;
}
class PtPadding extends Padding {
PtPadding({Spacing padding, Widget child}) : super(padding: padding.points, child: child);
}
PtPadding(padding: Spacing(4), child: Text('Hello'));
// or just with regular old Padding
Padding(padding: Spacing(4).points, child: Text('Hello'));
This is great, but it seems I forgo the ability to const my specialized PtPadding forces developers to use Spacing. On the other hand, just using Spacing in a constructor and accessing the points, prevents any widget from being "const"able. So it seems like I have to take a performance hit if I want to implement this spacing in my system.
I could have a class with static const members that point to doubles, but then I'm restrained to the sizes available (ie I can only have so many static members) and I also don't get the benefits of type restrictions.
I'm wondering if anyone else has thoughts in how I might approach this.
For what it's worth, I understand why Spacing(4).points is not a const (methods inherently aren't consts), but not sure how to get around this.

Answer 1

The problem is, you are extending Padding. Widgets are not made to be extended. Instead, you should use composition.
class Spacing {
const Spacing(double val) : points = val * 4;
final double points;
}
class PtPadding extends StatelessWidget {
const PtPadding({
Key key,
#required this.padding,
this.child,
}) : super(key: key);
final Spacing padding;
final Widget child;
#override
Widget build(BuildContext context) {
return Padding(
padding: EdgeInsets.all(padding.points),
child: child,
);
}
}