Expanded and Flexible in Flutter

Introduction

Expanded and Flexible help you control how widgets grow and share space inside a row or column. They make your app look neat on different screen sizes.

You want buttons or boxes to fill available space evenly in a row.

You want one widget to take more space than others inside a column.

You want to avoid overflow errors when screen size changes.

You want to create responsive layouts that adjust nicely on phones and tablets.

Syntax

Flutter

Expanded(
  flex: 1, // optional, default is 1
  child: Widget(),
)

Flexible(
  flex: 1, // optional, default is 1
  fit: FlexFit.loose, // or FlexFit.tight
  child: Widget(),
)

Expanded forces the child to fill the available space.

Flexible lets the child be smaller if it wants, depending on the fit.

Examples

Two containers share the row space equally.

Flutter

Row(
  children: [
    Expanded(
      child: Container(color: Colors.red),
    ),
    Expanded(
      child: Container(color: Colors.blue),
    ),
  ],
)

The green container takes twice the space of yellow. Yellow can be smaller if it wants.

Flutter

Row(
  children: [
    Flexible(
      flex: 2,
      fit: FlexFit.tight,
      child: Container(color: Colors.green),
    ),
    Flexible(
      flex: 1,
      fit: FlexFit.loose,
      child: Container(color: Colors.yellow, width: 50),
    ),
  ],
)

Sample App

This app shows a row with two colored boxes. The red box uses Expanded with flex 2, so it fills two parts of the space. The blue box uses Flexible with flex 1 and loose fit, so it only takes 50 pixels wide even if more space is available.

Flutter

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Expanded and Flexible Example')),
        body: Row(
          children: [
            Expanded(
              flex: 2,
              child: Container(color: Colors.red, height: 100),
            ),
            Flexible(
              flex: 1,
              fit: FlexFit.loose,
              child: Container(color: Colors.blue, width: 50, height: 100),
            ),
          ],
        ),
      ),
    );
  }
}

Important Notes

Use flex to control how much space each widget takes relative to others.

Expanded is like Flexible with fit: FlexFit.tight.

Flexible with FlexFit.loose lets the child be smaller than the available space.

Summary

Expanded makes a widget fill available space tightly.

Flexible lets a widget be flexible in size, either tight or loose.

Use these widgets inside Row or Column to create responsive layouts.

Practice

1. What does the Expanded widget do inside a Row or Column in Flutter?

easy

A. It makes the child widget scrollable.

B. It makes the child widget ignore available space and keep its size.

C. It makes the child widget fill all the available space tightly.

D. It adds padding around the child widget.

Solution

1. Step 1: Understand Expanded behavior

   The Expanded widget forces its child to fill the available space tightly inside a Row or Column.

2. Step 2: Compare with other options

   Other options describe unrelated behaviors like padding or scrolling, which Expanded does not do.

3. Final Answer:

   It makes the child widget fill all the available space tightly. -> Option C

4. Quick Check:

   Expanded = fills space tightly [OK]

Hint: Expanded fills space tightly inside Row or Column [OK]

Common Mistakes:

• Confusing Expanded with Flexible
• Thinking Expanded adds padding
• Assuming Expanded makes widget scrollable

2. Which of the following is the correct syntax to use Flexible with a child widget in Flutter?

easy

A. Flexible(child: Text('Hello'))

B. Flexible(Text('Hello'))

C. Flexible = Text('Hello')

D. Flexible: Text('Hello')

Solution

1. Step 1: Recall Flexible widget syntax

   The correct syntax requires the child widget passed as a named parameter: Flexible(child: ...).

2. Step 2: Identify incorrect syntax

   Options B, C, and D miss the named parameter or use invalid assignment syntax.

3. Final Answer:

   Flexible(child: Text('Hello')) -> Option A

4. Quick Check:

   Flexible uses child: parameter [OK]

Hint: Flexible requires child: parameter for its widget [OK]

Common Mistakes:

• Omitting child: label
• Using assignment instead of constructor
• Passing child without parentheses

3. Consider this Flutter code inside a Row:

Row(
  children: [
    Expanded(child: Container(width: 50, color: Colors.red)),
    Expanded(flex: 2, child: Container(width: 50, color: Colors.blue)),
  ],
)

What will be the relative widths of the red and blue containers on screen?

medium

A. Red fills 1/3, blue fills 2/3 of the row width

B. Red fills half the row, blue fills the other half

C. Red and blue both have fixed width 50 pixels

D. Red fills all space, blue is ignored

Solution

1. Step 1: Understand Expanded and Expanded with flex

   Expanded takes available space tightly with default flex=1. Expanded(flex:2) takes twice the space tightly.

2. Step 2: Calculate relative widths

   Total flex = 1 (first Expanded) + 2 (second Expanded) = 3. Red gets 1/3, blue gets 2/3 of the row width. The width: 50 is overridden by the tight constraints.

3. Final Answer:

   Red fills 1/3, blue fills 2/3 of the row width -> Option A

4. Quick Check:

   Flex ratio 1:2 means widths 1/3 and 2/3 [OK]

Hint: Sum flex values, divide space proportionally [OK]

Common Mistakes:

• Ignoring flex values
• Thinking width property overrides flex
• Assuming equal widths without flex

4. This Flutter code throws an error. What is the problem?

Row(
  children: [
    Flexible(flex: 1),
    Container(color: Colors.green),
  ],
)

medium

A. flex property must be greater than 1.

B. Container cannot be inside a Row.

C. Row requires at least three children.

D. Flexible widget is missing a child widget.

Solution

1. Step 1: Check Flexible widget usage

   Flexible requires a child widget to display. Here, it is missing.

2. Step 2: Validate other options

   Container can be inside Row, flex can be 1, and Row can have any number of children.

3. Final Answer:

   Flexible widget is missing a child widget. -> Option D

4. Quick Check:

   Flexible must have child [OK]

Hint: Flexible always needs a child widget [OK]

Common Mistakes:

• Omitting child in Flexible
• Thinking flex must be >1
• Assuming Row needs 3 children

5. You want two buttons side by side in a Row. The first button should take 1/4 of the width, the second button 3/4. Which Flutter widget setup achieves this best?

hard

A. Row(children: [Expanded(child: Button1), Expanded(child: Button2)])

B. Row(children: [Flexible(flex: 1, child: Button1), Flexible(flex: 3, child: Button2)])

C. Row(children: [Button1, Button2]) with fixed widths

D. Column(children: [Flexible(flex: 1, child: Button1), Flexible(flex: 3, child: Button2)])

Solution

1. Step 1: Understand flex distribution in Row

   Using Flexible with flex values 1 and 3 divides the Row width into 4 parts, assigning 1/4 and 3/4 respectively.

2. Step 2: Evaluate other options

   Row(children: [Expanded(child: Button1), Expanded(child: Button2)]) gives equal widths (1/2 each). Row(children: [Button1, Button2]) with fixed widths is less flexible. Column(children: [Flexible(flex: 1, child: Button1), Flexible(flex: 3, child: Button2)]) uses Column, which stacks vertically, not side by side.

3. Final Answer:

   Row(children: [Flexible(flex: 1, child: Button1), Flexible(flex: 3, child: Button2)]) -> Option B

4. Quick Check:

   Flexible flex ratio 1:3 divides width correctly [OK]

Hint: Use Flexible with flex ratio matching desired width parts [OK]

Common Mistakes:

• Using Expanded for unequal widths
• Using Column instead of Row
• Setting fixed widths instead of flex