Question

我在让Dropdownbutton小部件在Flutter（1.0）中工作时遇到问题，它不断抛出错误，

以下是我的代码：

import 'package:flutter/material.dart';

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

class Myapp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new Myappstate();
  }
}

class Myappstate extends State<Myapp> {
  String value = '';
  List<String> values = [];
  void init() {
    values.addAll(['one', 'two']);
    value = values.elementAt(0);
  }

  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("drop down demo"),
      ),
      body: new Container(
        child: new Column(
          children: <Widget>[
            new DropdownButton(
              items: values.map((String val) {
                return new DropdownMenuItem(
                  value: val,
                  child: Text(val),
                );
              }).toList(),
              onChanged: null,
            )
          ],
        ),
      ),
    );
  }
}

我收到以下错误：

Launching lib\main.dart on Android SDK built for x86 in debug mode...
Built build\app\outputs\apk\debug\app-debug.apk.
I/flutter ( 4352): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 4352): The following assertion was thrown building Myapp(state: Myappstate#1abe5):
I/flutter ( 4352): MediaQuery.of() called with a context that does not contain a MediaQuery.
I/flutter ( 4352): No MediaQuery ancestor could be found starting from the context that was passed to MediaQuery.of().
I/flutter ( 4352): This can happen because you do not have a WidgetsApp or MaterialApp widget (those widgets introduce
I/flutter ( 4352): a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.
I/flutter ( 4352): The context used was:
I/flutter ( 4352):   Scaffold(dirty, state: ScaffoldState#cda06(lifecycle state: initialized, tickers: tracking 1
I/flutter ( 4352):   ticker))

我正在尝试添加这个非常基本的小部件，但是由于某种原因，我未能找到问题所在。

感谢我能找到的任何帮助。

Answer 1

您必须包装MaterialApp并将初始数据加载到initState中。

请稍作更改检查此代码：

import 'package:flutter/material.dart';

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

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

class MyHomeapp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new MyHomeappstate();
  }
}

class MyHomeappstate extends State<MyHomeapp> {
  String value = '';
  List<String> values = [];
  @override
  void initState() {
    super.initState();
    setState(() {
      values.addAll(['one', 'two']);
      value = values.elementAt(0);
    });
  }

  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("drop down demo"),
      ),
      body: new Container(
        child: new Column(
          children: <Widget>[
            new DropdownButton(
              hint: Container(
                child: Text('Select option'),
              ),
              items: values.map((String val) {
                return new DropdownMenuItem(
                  value: val,
                  child: Text(val),
                );
              }).toList(),
              onChanged: (val) {
                debugPrint('selected option: $val');
              },
            )
          ],
        ),
      ),
    );
  }
}

颤振中的下拉按钮问题

1 个答案: