Question

我无法理解以下代码段中Widget BuildContext和Builder BuildContext之间的差异：

@override
  Widget build(BuildContext context) {

和

new Builder(
  builder: (BuildContext context){

使用Widget BuildContext SnackBar不会出现在用户界面中，但日志中会显示错误，表明Scaffold.of() Context在使用时没有脚手架Builder context一切正常。

Scaffold.of(context).showSnackBar(
  new SnackBar(content: new Text('Processing Data')));

编辑：main.dart文件：

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'APP'),
    );
  }
}

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> {
  final formKey = new GlobalKey<FormState>();
  String username;
  @override
  //This context
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        body: new Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          mainAxisSize: MainAxisSize.max,
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            new Flexible(
              child: new Container(
                child: new Center(
                  child: new Text(widget.title),
                ),
              ),
              flex: 1,
            ),
            new Form(
              key: formKey,
              child:
              new Row(children: <Widget>[
                new Flexible(child:
                  new Container(
                    margin: EdgeInsets.zero,
                    child: new TextFormField(
                      decoration:
                      new InputDecoration(
                        hintText: 'Username',
                        labelText: "1",
                        labelStyle: new TextStyle(color: new Color.fromARGB(255, 0, 0, 0)),
                      ),
                      validator: (val) => val.isEmpty? 'Username can\'t be empty.' : null,
                      ),
                  ),
                  flex: 1,
                ),
                new Flexible(child:
                  new Container(
                    margin: EdgeInsets.only(
                      left: 8.0,
                      right: 8.0,
                    ),
                    child: new TextFormField(
                      decoration: new InputDecoration(
                        border: InputBorder.none,
                        hintText: "Password",

                      ),
                      validator: (val) => val.isEmpty? 'Password can\'t be empty.' : null,
                      obscureText: true,
                    ),
                  ),
                  flex: 1,
                ),
                new Container(
                  child:
                  new Builder(
                    //And this context
                    builder: (BuildContext context){
                    return RaisedButton(
                      child: new Text("Sign in"),
                      onPressed: (){
                        if (formKey.currentState.validate()) {
                          // If the form is valid, display a snackbar. In the real world, you'd
                          // often want to call a server or save the information in a database
                          Scaffold.of(context).showSnackBar(
                          new SnackBar(content: new Text('Processing Data')));
                        }
                      },
                    );
                    },
                  ),
                ),
              ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Answer 1

如果您使用Scaffold属性引用SnackBar出现在key的当前final GloabalKey<ScaffoldState> _key = GlobalKey<ScaffoldState>();，则会更好。

Scaffold( key:_key, ... )

_key.currentState.showSnackBar(mySnackBar)

显示小吃店

context

请注意，在使用需要访问此上下文的窗口小部件时，每个构建器方法都会获得context以便能够引用它。在您的代码中，build混淆了，如果您将context方法(Buildcontext scaffoldContext)更改为Scaffold.of(scaffoldContext)之类的独特内容，然后使用function tr() { var vysledok = document.getElementById('vysledok_body'); var text= document.getElementById('source').value; var languageFrom = document.getElementById("src").value; var languageTo = document.getElementById("dst").value; vysledok.innerHTML=""; var xmlhttp = new XMLHttpRequest(); xmlhttp.open('GET', "https://.... someurl.php? from=" + languageFrom + "&to=" + languageTo + "&text=" + text, true); xmlhttp.onreadystatechange = function() { if (xmlhttp.readyState == 4) { if(xmlhttp.status == 200) { var obj = JSON.parse(xmlhttp.responseText); console.log(obj.ip); vysledok.innerHTML = "<p class='bdr'>"+obj.text+"</p>"; } if(xmlhttp.status == 500) { //console.log('nejde'); tr(); } } }; xmlhttp.send(null); }我不是确定这是否有效，因为我经常在Flutter看到这种混乱。

无论如何，上面的解决方案更清洁了。

在颤动中使用哪种上下文？有什么不同？

1 个答案: