Why is CustomPaint affected by whether it is under a Scaffold? - flutter

Question

App〈 MaterialApp〈 MyWidget : all is good
Whether I draw the figure on a canvas with a small or a large scale—by replacing v = 100000.0 with v = 1000.0 in the following code, the image doesn't change, as expected.
(If you are not familiar with the idea of using a SizedBox inside a FittedBox, it is explained here.)
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
const v = 10000.0;
class Painter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
final mypaint = Paint()
..style = PaintingStyle.stroke
..strokeWidth = 0.012 * v
..color = Colors.blue;
canvas.drawCircle(
Offset(1.5 * v, 1 * v),
1 * v, mypaint);
}
#override
bool shouldRepaint(Painter oldDelegate) => false;
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(
title: "My App",
home: FittedBox(
child: SizedBox(
width: 3 * v,
height: 2 * v,
child: CustomPaint(
painter: Painter()
),
),
),
);
}
}
App〈 Scaffold〈 MaterialApp〈 MyWidget : My chosen size in SizedBox is ignored; why?
But if I put MaterialApp inside a Scaffold, I still get the image on the left for v = 10000.0, but with v = 100.0, I get instead the image on the right.
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
const v = 10000.0;
class Painter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
final mypaint = Paint()
..style = PaintingStyle.stroke
..strokeWidth = 0.012 * v
..color = Colors.blue;
canvas.drawCircle(
Offset(1.5 * v, 1 * v),
1 * v, mypaint);
}
#override
bool shouldRepaint(Painter oldDelegate) => false;
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(
title: "My App",
home: Scaffold(
appBar: AppBar(
title: const Text('My Title'),
),
body: FittedBox(
child: SizedBox(
width: 3 * v,
height: 2 * v,
child: CustomPaint(
painter: Painter()
),
),
),
),
);
}
}
How do I get a scale invariant CustomPaint with a Scaffold?

Answer 1

The custom paint is independant of the size. It takes the size sent from it's caller. If you set the sizedbox's size to 200x200 it will paint in that space. Now 10k or 1k will give same result since there is no space left on the device to paint something since width of device is less than 1k. But 10, 100 and 1k will show different sized circles. The only factor that defines the scale here would be the size of the SizedBox
The draw circle can also be written as
canvas.drawCircle(Offset(size.height / 2, size.width / 2), size.width/2);
Now if you pass a sized box of 100x100 it will draw a circle of diameter 100