在空上调用获取器“分类”

Question

我大约两周大了，我不明白这个错误的真正含义。我将在下面添加堆栈：

List<DropdownMenuItem> showDropdownMenus() {
return CLASSIFICATION.map((item){
  return DropdownMenuItem(
    value: item,
    child: Text(item),
  );
}).toList();

}

错误堆栈：

Answer 1

您的分类可能未正确初始化。

请查看以下示例

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

List<Map> _jsonApi1 = [
  {"id": 0, "name": "default 1"}
];
List<Map> _jsonApi2 = [
  {"id": 1, "name": "second 2"},
  {"id": 2, "name": "third 3"}
];
List<Map> _myJson = new List.from(_jsonApi1)..addAll(_jsonApi2);

class _MyHomePageState extends State<MyHomePage> {
  String _mySelection;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: SafeArea(
        child: Column(
          children: <Widget>[
            Container(
              height: 500.0,
              child: new Center(
                child: new DropdownButton<String>(
                  isDense: true,
                  hint: new Text("${_jsonApi1[0]["name"]}"),
                  value: _mySelection,
                  onChanged: (String newValue) {
                    setState(() {
                      _mySelection = newValue;
                    });

                    print(_mySelection);
                  },
                  items: _myJson.map((Map map) {
                    return new DropdownMenuItem<String>(
                      value: map["id"].toString(),
                        child: new Text(
                          map["name"],
                        ),
                    );
                  }).toList(),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}