Get position and size of Widget during build - flutter

Question

How can I get the size and position of a widget during the build() method? In the following code, I want to draw rectangle in the CustomPaint widget that is centered and is a certain percentage of the remaining area of the viewport, which is everything below below the Text widget, marked (A). I can get the width from MediaQuery, but how would I get the remaining height after (A)? I would have to know the position of the Text widget.
This answer does not work because Flutter throws an exception when attempting to retrieve the RenderBox during build().
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text('App')),
body: Column(children: [
Text('Test'), // (A) Get position of this
CustomPaint(
painter: DrawArea(MediaQuery.of(context).size),
size: Size(1000, 800),
),
]),
);
}
}
class DrawArea extends CustomPainter {
final Size viewportSize;
DrawArea(this.viewportSize);
#override
void paint(Canvas canvas, Size size) {
var paint = Paint()
..color = Colors.black
..strokeWidth = 1
..style = PaintingStyle.stroke;
double yOffset = (this.viewportSize.width - this.viewportSize.width * .95) / 2;
canvas.drawRect(Rect.fromLTWH(yOffset, 0, this.viewportSize.width * .95, 800), paint);
}
#override
bool shouldRepaint(CustomPainter oldDelegate) => true;
}

Answer 1

The second argument of the paint method is the size of the CustomPaint widget. You just need to make sure your widget gets the right size.
To make the CustomPaint take the rest of the space in the Column, you will need to wrap it in an Expanded widget:
body: Column(children: [
Text('Test'),
Expanded(
child: CustomPaint(
painter: DrawArea(),
size: Size.infinite, // make it as large as possible
),
),
]),
Then you can just use the passed size parameter in your custom painter:
class DrawArea extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
var paint = Paint()
..color = Colors.black
..strokeWidth = 1
..style = PaintingStyle.stroke;
double xOffset = size.width * 0.025
double yOffset = size.height * 0.025
canvas.drawRect(Rect.fromLTWH(xOffset, yOffset, size.width * .95, size.height * .95), paint);
}
#override
bool shouldRepaint(CustomPainter oldDelegate) => true;
}