Flutter FractionallySizedBox example

This article gives you a practical Flutter example of implementing FractionallySizedBox, a widget that sizes its child to a fraction of the total available space. Here’s its constructor:

FractionallySizedBox({
  Key? key, 
  AlignmentGeometry alignment = Alignment.center, 
  double? widthFactor, 
  double? heightFactor, 
  Widget? child
})

Example

This sample app contains a yellow box with a height of 500 and a width of double.infinity. The child of the yellow box is a FractionallySizedBox whose child is a purple rectangular. The width of the purple one is 80% of the width of the yellow box, and its height is 30% of the yellow box.

Screenshot:

The code (for the yellow box and its descendants):

Container(
          width: double.infinity,
          height: 500,
          color: Colors.yellow,

          // implement FractionallySizedBox
          child: FractionallySizedBox(
            widthFactor: 0.8,
            heightFactor: 0.3,
            alignment: Alignment.center,
            // the purple box
            child: Container(
              color: Colors.purple,
            ),
          ),
)

The full source code in main.dart (with boilerplate):

// main.dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Hide the debug banner
      debugShowCheckedModeBanner: false,
      title: 'KindaCode.com',
      theme: ThemeData(primarySwatch: Colors.indigo),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text('KindaCode.com'),
          backgroundColor: Colors.indigo,
        ),
        // this Container constructs the yellow box
        body: Container(
          width: double.infinity,
          height: 500,
          color: Colors.yellow,

          // implement FractionallySizedBox
          child: FractionallySizedBox(
            widthFactor: 0.8,
            heightFactor: 0.3,
            alignment: Alignment.center,
            // the purple box
            child: Container(
              color: Colors.purple,
            ),
          ),
        ));
  }
}

That’s it. Further reading:

• Flutter: SliverGrid example
• Create a Custom NumPad (Number Keyboard) in Flutter
• Flutter: GridPaper example
• Flutter: Creating Custom Back Buttons
• Best Libraries for Making HTTP Requests in Flutter
• Flutter: Creating an Auto-Resize TextField

You can also take a tour around our Flutter topic page and Dart topic page to see the latest tutorials and examples.