Flutter，异步调用后渲染小部件

我想呈现一个需要HTTP调用来收集一些数据的小部件.

得到以下代码(简化)

import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:convert';

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

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

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  var asyncWidget;

  @override
  initState() {
    super.initState();

    loadData().then((result) {
      print(result);
      setState(() {
       asyncWidget = result;
      });
    });
  }

  loadData() async {
    var widget = new AsyncWidget();
    return widget.build();
  }

  @override
  Widget build(BuildContext context) {

    if(asyncWidget == null) {
      return new Scaffold(
        appBar: new AppBar(
          title: new Text("Loading..."),
        ),
      );
    } else {
      return new Scaffold(
        appBar: new AppBar(
          title: new Text(widget.title),
        ),
        body: new Center(
          child: this.asyncWidget,
        ),
      );
    }
  }
}

class MyRenderer {

  MyRenderer();

  Widget render (List data) {

    List<Widget> renderedWidgets = new List<Widget>();

    data.forEach((element) {
      renderedWidgets.add(new ListTile(
        title: new Text("one element"),
        subtitle: new Text(element.toString()),
      ));
    });
    var lv = new ListView(
      children: renderedWidgets,
    );
    return lv;
  }
}

class MyCollector {

  Future gather() async {

    var response = await // do the http request here;

    return response.body;
  }
}

class AsyncWidget {

  MyCollector collector;
  MyRenderer renderer;

  AsyncWidget() {
    this.collector = new MyCollector();
    this.renderer = new MyRenderer();
  }

  Widget build() {

    var data = this.collector.gather();
    data.then((response) {
      var responseObject = JSON.decode(response);
      print(response);
      return this.renderer.render(responseObject);
    });
    data.catchError((error) {
      return new Text("Oups");
    });
  }
}

我的代码是这样工作的:使用异步数据的小部件需要一个收集器(进行http调用)和一个呈现器，后者将用http数据呈现小部件. 我在initState()上创建了这个小部件的一个实例，然后进行异步调用.

我发现一些文档说我们应该使用setState()方法用新数据更新小部件，但这对我不起作用.

但是，当我放入一些日志(log)时，我看到HTTP调用已经完成，并且调用了setState()方法，但是小部件没有更新.