Set column width in flutter - user-interface

Question

I need to set a Column width in flutter, I have to do a layout with 3 sections, one should be 20% of the screen, the other one 60% and the last one 20%.
I know that those 3 columns should be into a row, but I don't know a way to set the size, when I do that, the 3 columns take the same size.
I will appreciate any feedback.

Answer 1

Instead of hard-coding the size, I would suggest using Flex like
Row(
children: <Widget>[
Expanded(
flex: 2, // 20%
child: Container(color: Colors.red),
),
Expanded(
flex: 6, // 60%
child: Container(color: Colors.green),
),
Expanded(
flex: 2, // 20%
child: Container(color: Colors.blue),
)
],
)
Which will produce like below,

Answer 2

Limiting the width of a Column could be
Limiting the width of Column itself, use SizedBox
SizedBox(
width: 100, // set this
child: Column(...),
)
2 (A). Limiting width of children inside Column, without hardcoding values
Row(
children: <Widget>[
Expanded(
flex: 3, // takes 30% of available width
child: Child1(),
),
Expanded(
flex: 7, // takes 70% of available width
child: Child2(),
),
],
)
2 (B). Limiting width of children inside Column, with hardcoding values.
Row(
children: <Widget>[
SizedBox(
width: 100, // hard coding child width
child: Child1(),
),
SizedBox(
width: 200, // hard coding child width
child: Child2(),
),
],
)

Answer 3

Just wrap it with a Row
Row( // this row has full width
children: [
Column(
children: [...]
)
])

Answer 4

This is not an answer to the original question but demonstrating a similar use case.
I have a container and I want to expand the width until certain value. If width gets bigger I want container to be always in the middle. This is useful when rendering forms especially on web and desktop applications.
import 'package:flutter/material.dart';
import 'dart:math' as math;
var index = 0;
Widget buildContainer() { // Just a placeholder with random colour
index++;
return Container(
height: 60,
margin: const EdgeInsets.only(right: 5),
color: Colors.primaries[math.Random().nextInt(Colors.primaries.length)],
child: Text("$index"),
);
}
Widget containers() {
return Row(
children: [
Expanded(child: buildContainer(),
flex: 2), // <- Control the width of each item. See other answers.
Expanded(child: buildContainer(), flex: 3,)
],
);
}
class FormLayout extends StatelessWidget {
const FormLayout({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return Center( //<- Centre the form
child: SizedBox(
width: 400, //<- Limit the width
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [containers()]),
),
);
}
}