main.dart

import 'dart:math';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'State Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<ItemData> items = [];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Sample App"),
        ),
        body: Container(
          padding: EdgeInsets.all(16),
          child: Stack(
            children: <Widget>[
              ListView.separated(
                  itemBuilder: (ctx, index) {
                    return ListItemStateless(
                      item: items[index],
                    );
                  },
                  separatorBuilder: (_, __) => Divider(),
                  itemCount: items.length),
              Align(
                alignment: Alignment.bottomLeft,
                child: RaisedButton(
                  onPressed: () {
                    setState(() {
                      items = List.generate(items.length+1, (index)=> ItemData("Hello $index"));
                    });
                  },
                  child: Text("Add"),
                ),
              ),
              Align(
                alignment: Alignment.bottomRight,
                child: RaisedButton(
                  onPressed: () {
                    setState(() {
                      items.clear();
                    });
                  },
                  child: Text("Reset"),
                ),
              )
            ],
          ),
        ));
  }
}

final List<Color> _colors = [
  Colors.red,
  Colors.amber,
  Colors.blue,
  Colors.green,
  Colors.lightGreen,
  Colors.yellow,
  Colors.grey,
  Colors.blueAccent,
  Colors.purpleAccent
];

final Random _random = Random();

class ListItemStateless extends StatefulWidget {
  final ItemData item;

  ListItemStateless({Key key, this.item}) : super(key: key);

  @override
  _ListItemStatelessState createState() => _ListItemStatelessState();
}

class _ListItemStatelessState extends State<ListItemStateless> {
  final Color _color = _colors[_random.nextInt(1000) % _colors.length];

  @override
  Widget build(BuildContext context) {
    String text = "$this";
    return SizedBox(
      width: double.infinity,
      height: 50,
      child: Container(color: _color, child: Center(child: Text("${widget.item.title} $text"))),
    );
  }
}


class ItemData{
  final String title;

  ItemData(this.title);
}