Flutter setState((){}) 无效

Question

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

class DemoWidget extends StatefulWidget {
  @override
  _DemoWidgetState createState() => _DemoWidgetState();
}

class _DemoWidgetState extends State<DemoWidget> {
  @override
  Widget build(BuildContext context) {
    var _demoText = "";

    getFromServer() async {
      var response = await http.read("http://xxx.xxx.xx.xx/xxx.json");
      setState(() {
        _demoText = response;
      });
    }

    return Scaffold(
      appBar: AppBar(
        title: Text("DemoWidgetPage"),
      ),
      body: Column(
        children: <Widget>[
          FlatButton(onPressed: getFromServer, child: Text("GetFromServer")),
          Text(_demoText),
        ],
      ),
    );
  }
}

即便点击了 GetFromServer ，下方的 Text(_demoText) 却不改变。

Answer 1

将变量 _demoText 放在 build() 方法外面，作为类的实例变量而不是方法的实例变量。

class _DemoWidgetState extends State<DemoWidget> {
    var _demoText = "";   // <================

  @override
  Widget build(BuildContext context) {

    getFromServer() async {
      var response = await http.read("http://xxx.xxx.xx.xx/xxx.json");
      setState(() {
        _demoText = response;
      });
    }

    return Scaffold(
      appBar: AppBar(
        title: Text("DemoWidgetPage"),
      ),
      body: Column(
        children: <Widget>[
          FlatButton(onPressed: getFromServer, child: Text("GetFromServer")),
          Text(_demoText),
        ],
      ),
    );
  }
}

Answer 2

// response 的值打出来看看，是不是由于报错没执行下面的setState，或者值没改变
var response = await http.read("http://xxx.xxx.xx.xx/xxx.json");