最好的方法是使用FutureBuilder。
来自FutureBuilder文档：

new FutureBuilder<String>(
  future: _calculation, // a Future<String> or null
  builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
    switch (snapshot.connectionState) {
      case ConnectionState.none: return new Text('Press button to start');
      case ConnectionState.waiting: return new Text('Awaiting result...');
      default:
        if (snapshot.hasError)
          return new Text('Error: ${snapshot.error}');
        else
          return new Text('Result: ${snapshot.data}');
    }
  },
)

另一件事是，您在www.example.com方法之外构建小部件State.build，并保存小部件本身，这是一个反模式，您应该每次都在build方法中构建小部件。
您可以在没有FutureBuilder的情况下使用它，但是您应该保存http调用的结果（经过适当的处理），然后在构建函数中使用这些数据。
请注意，使用FutureBuilder是一种更好的方法，我只是提供这些内容供您学习。

import 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';

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> {
  List data;

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

    new Future<String>.delayed(new Duration(seconds: 5), () => '["123", "456", "789"]').then((String value) {
      setState(() {
        data = json.decode(value);
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    if (data == 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: new ListView(
            children: data
                .map((data) => new ListTile(
                      title: new Text("one element"),
                      subtitle: new Text(data),
                    ))
                .toList(),
          ),
        ),
      );
    }
  }
}

完整示例

异步调用后rander小工具的最佳方式是使用**FutureBuilder()**

class _DemoState extends State<Demo> {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<String>(
      future: downloadData(), // function where you call your api
      builder: (BuildContext context, AsyncSnapshot<String> snapshot) {  // AsyncSnapshot<Your object type>
        if( snapshot.connectionState == ConnectionState.waiting){
            return  Center(child: Text('Please wait its loading...'));
        }else{
            if (snapshot.hasError)
              return Center(child: Text('Error: ${snapshot.error}'));
            else
              return Center(child: new Text('${snapshot.data}'));  // snapshot.data  :- get your object which is pass from your downloadData() function
        }
      },
    );
  }
  Future<String> downloadData()async{
    //   var response =  await http.get('https://getProjectList');    
    return Future.value("Data download successfully"); // return your response
  }
}

在future builder中，它调用future函数以等待结果，一旦产生结果，它就调用我们构建小部件的builder函数。

异步快照有3种状态：

1. connectionState.none --在此状态下，将来为空
2. connectionState.waiting -- [将来]不为空，但尚未完成
3. connectionState.done -- [future]不为空，并且已完成。如果future成功完成，则[AsyncSnapshot.data]将设置为future完成时的值。如果future完成时出现错误，则[AsyncSnapshot.hasError]将为true