Question

我是飞镖/扑扑的新手。我正在更改通过 flutter create 命令创建的默认应用。这是代码（我想显示来自EventSource的信息）：

import 'dart:async';
import 'package:w3c_event_source/event_source.dart';
import 'package:flutter/material.dart';
import 'blocs/main_bloc.dart';

void main() async {
    runApp(MyApp());
    final events = EventSource(Uri.parse("https://example.com/broadcast/"));
    final subscription = events.events.listen((MessageEvent message) {
      var myhomepagestate = MyHomePageState();
      myhomepagestate.eventsource_message = '${message.data}';
      //print('${message.data}');
    });  
  
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'EXAMPLE',
      home: MyHomePage(title: 'EXAMPLE'),
    );
  }
}

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

  @override
  MyHomePageState createState() => MyHomePageState();
}

class MyHomePageState extends State<MyHomePage> {
  var _myeventsource = "";
  
  void set eventsource_message(message) {
    //if (!mounted) return;
    setState(() {
      _myeventsource = message;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'EventSource:',
            ),
            Text(
              '$_myeventsource',
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

此命令错误：

setState(() {
  _myeventsource = message;
});

错误是：未处理的异常：构造函数中调用了setState（）：MyHomePageState＃8034c（生命周期状态：已创建，未安装任何小部件）

我搜索了一天，找不到解决方案。有人可以暗示我应该研究什么吗？我可以只命名元素，然后像HTML DOM一样直接更改元素吗？

谢谢。

Answer 1

您正在实例化一个MyHomePageState小部件，该小部件将不需要执行。而是，您必须实例化MyHomePage。但是，由于MyHomePage仍未安装在任何地方，因此无法在这种情况下使用。您只是实例化它并尝试设置该值。

从主要方法中删除此部分：

final events = EventSource(Uri.parse("https://example.com/broadcast/"));
final subscription = events.events.listen((MessageEvent message) {
  var myhomepagestate = MyHomePageState();
  myhomepagestate.eventsource_message = '${message.data}';
  //print('${message.data}');
});

像这样修改MyHomePageState（添加initState和dispose方法。它们就像构造函数和析构函数一样）：

class MyHomePageState extends State<MyHomePage> {
  String _myeventsource = "";
  
  /**
  void set eventsource_message(message) {
    //if (!mounted) return;
    setState(() {
      _myeventsource = message;
    });
  }
  **/
 
 // Init state is called once when the widget is mounted. It's more like a constructor for your stateful widget which gets called by flutter itself when the widget is mounted.
 // You don't need to call this method explicitly.
 // Note that it won't be called again on hot reload. Refer to the documentation for more about it
 @override
 void initState() {
     super.initState();
     final events = EventSource(Uri.parse("https://example.com/broadcast/"));
     events.events.listen((MessageEvent message) {
         setState(() => _myeventsource = message.data);
     }
 }
 
 // And this method is like a destructor.
 // Called automatically by flutter when this widget is removed.
 @override
 void dispose() {
     // Remember to remove the listeners here
     super.dispose();
 }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'EventSource:',
            ),
            Text(
              '$_myeventsource',
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

Flutter Dart setState无法正常工作：未处理的异常：在构造函数中调用了setState（）：...（生命周期状态：已创建，没有窗口小部件，未安装）

1 个答案: