title: Flutter: Why Future.then() doesn't work on class variable?
body: <p>Code I show you is the simplified code which I'm troubled in.</p>
  
  <p>My expected result is [1,2,3,4,5,6], but app says [1,2,3].</p>
  
  <p>I know "loadMoreInterger()" should be in "initState()", but for some reason I have to put it in Widget build() {"HERE"}.</p>
  
  <p>I wonder if why doesn't it work, and the solution for correct result.....
  I really appreciate for your help :)</p>
  
  <pre><code>import 'package:flutter/material.dart';
  import 'dart:async';
  
  void main() =&gt; runApp(MyApp());
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: MyHomePage(title: 'Flutter Demo Home Page'),
      );
    }
  }
  
  class MyHomePage extends StatefulWidget {
    MyHomePage({Key key, this.title}) : super(key: key);
    final String title;
    @override
    _MyHomePageState createState() =&gt; _MyHomePageState();
  }
  
  // ↓↓↓↓↓↓↓↓↓↓↓WHERE I CANNOT UNDERSTAND↓↓↓↓↓↓↓↓↓↓↓
  class _MyHomePageState extends State&lt;MyHomePage&gt; {
    List&lt;int&gt; intList = [1,2,3];
  
    Future&lt;List&lt;int&gt;&gt; loadMoreInteger() async {
      print('Future');
      return [4,5,6];
    }
  
    @override
    Widget build(BuildContext context) {
  
      loadMoreInteger().then((value) {
        intList.addAll(value); // why doesn't it work?
      });
      print("console: $intList");
  
      return Scaffold(
        body: Center(
          child: Text("display: $intList")
        )
      );
    }
  }
  
  //Expected result: [1,2,3,4,5,6]
  //Actual result: [1,2,3]
  </code></pre>