Theme.of(context) as const - flutter

Question

I have several places with repetitive codes, for BoxDecoration
decoration: BoxDecoration(
color: Theme.of(context).primaryColor,
borderRadius: BorderRadius.circular(5),
boxShadow: [
BoxShadow(
color: Theme.of(context).hintColor.withOpacity(0.15),
offset: Offset(0, 3),
blurRadius: 10)
],
),
So I'd like to extract the BoxDecoration into separated dart file and reuse it.
Something like
static const boxDecoration = BoxDecoration(...);
then use it
final container = Container(
decoration: boxDecoration
)
But I get stuck on few things:
1. If I put it on file that only contains constants, I still need to access context, e.g. in Theme.of(context).primaryColor, which can only retrieved from build(Context) method
2. Then, I create a new stateless widget
import 'package:flutter/material.dart';
class FlutterTemplateStyle extends StatelessWidget {
static var boxDecoration;
#override
Widget build(BuildContext context) {
boxDecoration = BoxDecoration(
color: Theme.of(context).primaryColor,
borderRadius: BorderRadius.circular(5),
boxShadow: [
BoxShadow(
color: Theme.of(context).hintColor.withOpacity(0.15),
offset: Offset(0, 3),
blurRadius: 10)
],
);
return Container();
}
}
And try to access decoration: FlutterTemplateStyle.boxDecoration, which doesn't work. I suppose this is because build is never called.
How can I achieve this constant styling?
Thanks

Answer 1

You cannot use const with any method call because methods cannot be evaluated at compile time - which is the requirement for a const value.
If you know that your primaryColor is constant, will never change, and that you will never want to dynamically change the color of your decoration, you can specify the color without the theme: color: const Color(0xfef1d2e1).
Otherwise, you do not want to use const. You might want to use final if you never reassign your variable, however, that will not work if you need to access your context.
Having said that, you will probably just want to create a function that returns your decoration, so you can use it in multiple places:
BoxDecoration myAwesomeBoxDecoration(BuildContext context) => BoxDecoration(
color: Theme.of(context).primaryColor,
borderRadius: BorderRadius.circular(5),
boxShadow: [
BoxShadow(
color: Theme.of(context).hintColor.withOpacity(0.15),
offset: const Offset(0, 3),
blurRadius: 10,
),
],
);
Let me explain why I believe that the thought of making your BoxDecoration a constant is counterintuitive:
The color of your decoration depends on the theme of your app, which is something that is created entirely at runtime. This means that in order for your BoxDecoration to adapt to the Theme, it cannot be constant because it is adaptive.