in_collection_flow.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Column(
          children: [
            // you could use [if] statement inside the list
            // if(someting == true)
              // otherWidget(),
            // use [for] loop inside the list
            for(var i=0;i<3;i++) ...[ // use [spread ...] to render more than one widget by iteration
              SizedBox(height: 6),
              Row(
                children: [
                  for(var j=0; j<3; j++) ...[
                    SizedBox(width: 6),
                    MyReusableWidget(i:i,j:j),
                  ],
                ],
              ),
            ]
          ],
        ),
      ),
    );
  }
}

class MyReusableWidget extends StatelessWidget {
  MyReusableWidget({
    required this.i,
    required this.j,
  });
  
  final int i;
  final int j;
  
  @override
  Widget build(BuildContext context) {
    return Container(
      alignment: Alignment.center,
      padding: EdgeInsets.all(8.0),
      color: Colors.green,
      child:  Text(
        'i:$i j:$j',
      ),
    );
  }
}