Demo of spacer widget

SpacerDemo.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.red,
      padding: EdgeInsets.all(16),
      child: Column(
        children: [
          Text(
            "Using Containers",
            style: TextStyle(fontSize: 30, color: Colors.white),
          ),
          Row(
            children: [
              GreenRectangle(),
              Container(width: 20),
              GreenRectangle(),
              Container(width: 20),
              GreenRectangle(),
              Container(width: 20),
              GreenRectangle(),
              Container(width: 20),
              GreenRectangle(),
            ],
          ),
          Container(height: 30),
          Text(
            "Using Spacer",
            style: TextStyle(fontSize: 30, color: Colors.white),
          ),
          Row(
            children: [
              GreenRectangle(),
              Spacer(),
              GreenRectangle(),
              Spacer(),
              GreenRectangle(),
              Spacer(),
              GreenRectangle(),
              Spacer(),
              GreenRectangle(),
            ],
          ),
        ],
      ),
    );
  }
}

class GreenRectangle extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.green,
      height: 50,
      width: 50,
    );
  }
}