import 'package:flutter/material.dart';

​

void main() {

  runApp(ResponsiveApp());

}

​

class ResponsiveApp extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    return MaterialApp(

      home: Scaffold(

        appBar: AppBar(

          title: Text('Responsive Web App'),

        ),

        body: LayoutBuilder(

          builder: (BuildContext context, BoxConstraints constraints) {

            if (constraints.maxWidth > 600) {

              return _buildWideLayout();

            } else {

              return _buildNarrowLayout();

            }

          },

        ),

      ),

    );

  }

​

  Widget _buildWideLayout() {

    return Row(

      children: [

        Expanded(

          flex: 1,

          child: Container(

            color: Colors.blue,

            height: double.infinity,

            child: Center(

              child: Text(

                'Sidebar',

                style: TextStyle(color: Colors.white, fontSize: 30),

              ),

            ),

          ),

        ),

        Expanded(

          flex: 2,

          child: Container(

            color: Colors.white,

            height: double.infinity,

            child: Center(

              child: Text(

                'Main Content',

                style: TextStyle(fontSize: 30),

              ),

            ),

          ),

        ),

      ],

    );

  }

​

  Widget _buildNarrowLayout() {

    return Column(

      children: [

        Container(

          color: Colors.blue,

          height: 100,

          child: Center(

            child: Text(

              'Sidebar',

              style: TextStyle(color: Colors.white, fontSize: 30),

            ),

          ),

        ),

        Expanded(

          child: Container(

            color: Colors.white,

            height: double.infinity,

            child: Center(

              child: Text(

                'Main Content',

                style: TextStyle(fontSize: 30),

              ),

            ),

          ),

        ),

      ],

    );

  }

}