最好的方法是使用FutureBuilder。
从FutureBuilder文档中：
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}'); } },)另一件事是您要在State.build方法之外构建窗口小部件并保存窗口小部件本身，这是一种反模式。您实际上应该每次都在build方法中构建窗口小部件。
您可以在没有FutureBuilder的情况下使它起作用，但是您应该保存http调用的结果（经过适当处理），然后在构建函数中使用数据。
看到这一点，但请注意，使用FutureBuilder是实现此目的的更好方法，我只是向您提供此知识。
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.depre(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(), ), ), ); } }}
