Why StatefulWidgets need keys

flutter_key_example.dart

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

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

Color randomColor() {
  Random r = Random();
  return Color(0xFF000000 + r.nextInt(0x00FFFFFF)); 
}

class MyApp extends StatefulWidget {
  @override
  createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: TileList(),
    );
  }
}

class TileList extends StatefulWidget {
  TileList({Key key}) : super(key: key);
  
  @override
  createState() => TileListState();
}

class TileListState extends State<TileList> {
  List<Tile> tiles;
    
  @override
  void initState() {
    super.initState();
    tiles = List.generate(10, (i) => Tile());
  }
  
  void removeTile() {
    setState(() {
      // Remove the third tile
      tiles.removeAt(2);
      print(tiles.length);
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Tiles'),
      ),
      body: ListView.builder(
        itemCount: tiles.length,
        itemBuilder: (context, i) {
          return tiles[i];
        }
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: removeTile,
        tooltip: 'Increment',
        child: Icon(Icons.remove),
      ),
    );
  }
}

class Tile extends StatefulWidget {
  Tile({Key key}) : super(key: key);
  
  @override
  createState() => TileState();
}

class TileState extends State<Tile> {
  Color color;
    
  @override
  void initState() {
    super.initState();
    color = randomColor();
  }
  
  @override
  Widget build(BuildContext context) {
    return Container(
      height: 150,
      color: color,
    );
  }
}